Carbon footprint assessment of face masks in the context of the COVID-19 pandemic: Based on different protective performance and applicable scenarios

Abstract

Widespread concerns have been raised about the huge environmental burden caused by massive consumption of face masks in the context of the COVID-19 pandemic. However, most of the existing studies only focus on the environmental impact associated with the product itself regardless of the actual usage scenarios and protective performance of products, resulting in unrealistic conclusions and poor applicability. In this context, this study integrated the product performance into the existing carbon footprint assessment methodology, with focus on the current global concerns regarding climate change. Computational case studies were conducted for different mask products applicable to the scenarios of low-, medium- and high-risk levels. The results showed that reusable cotton masks and disposable medical masks suitable for low-risk settings have a total carbon footprint of 285.484 kgCO₂-eq/FU and 128.926 kgCO₂-eq/FU respectively, with a break-even point of environmental performance between them of 16.886, which implies that cotton masks will reverse the trend and become more environmentally friendly after 17 washes, emphasizing the importance of improving the washability of cotton masks. Additionally, the total carbon footprints of disposable surgical masks and KN95 respirators were 154.328 kg CO₂-eq/FU and 641.249 kg CO₂-eq/FU respectively, while disposable medical masks and disposable surgical masks were identified as alternatives with better environmental performance in terms of medium- and high-risk environments respectively. The whole-life-cycle oriented carbon footprint evaluation further indicated that the four masks have greater potential for carbon emission reduction in the raw material processing and production processes. The results obtained in this study can provide scientific guidance for manufacturers and consumers on the production and use of protective masks. Moreover, the proposed model can be applied to other personal protective equipment with similar properties, such as protective clothing, in the future.

Graphical abstract

1. Introduction

Our planet is currently facing unprecedented health and environmental threats, with COVID-19 pandemic and climate change being the two crises of greatest concern. Even though it has been more than two years since the COVID-19 pandemic was first detected (Xu and Li, 2020), people around the world are still suffering from the emerging SARS-CoV-2 virus on a massive scale. By the end of Sept. 2022, the cumulative number of confirmed cases worldwide had exceeded 613 million, and the death toll was more than 6 million (World Health Organization, 2022b). With regard to climate change, accidental disasters and events directly or indirectly caused by it have occurred with greater frequency in recent years, such as the vast Australian bushfires that burned over 45 million acres (Latkin et al., 2022), the massive rock and ice avalanche at Chamoli, Indian Himalaya that destroyed two hydropower projects and left more than 200 people dead or missing (Shugar et al., 2021). What's more, the ravages of COVID-19 are also thought to be inseparable from climate change (Latkin et al., 2022).

The adoption of personal protective equipment (PPE) (Ranney et al., 2020), such as face masks (including respirators), gloves and isolation gowns, is considered crucial in curbing the spread of the virus and protecting from droplet infections (Leung et al., 2020; Liu and Zhang, 2020). Of these, face masks have arguably played the most critical role in the COVID-19 pandemic (Eikenberry et al., 2020; Leung et al., 2020). There are many different types of masks on the market currently, offering varying degrees of protection to users (Long et al., 2020; Ramakrishnan, 2020). They may be reusable or disposable; the former refers to industrially used half- or full-face respirators with cartridge filters attached, and homemade or commercial cloth masks, while the latter includes loose-fitting medical and surgical masks and well-fitting respirators (e.g. N95, KN95, etc.) with filtration efficiencies characterized as FFP1 (80%), FFP2 (94%) and FFP3 (99%) in Europe, N95 (95%), N99 (99%) and N100 (99.97%) in the United States, and KN90 (90%), KN95 (95%) and KN100 (99.97%) in China (Czigány and Ronkay, 2020; Gan et al., 2020). Their ability to filter contaminants and pathogens, and hence the level of protection, depends on the materials used and the engineering design (Davies et al., 2013; Mueller et al., 2020). Since SARS-CoV-2 viruses are between 60 and 140 nm in size (Zhu et al., 2020), masks made of materials with larger pore sizes, such as cotton and synthetic fabrics, are often ineffective in filtering these viruses or the tiny virus-laden droplets. In order to enhance the filtration properties and function of traditional masks, especially reusable cloth masks, a range of means, such as coating with waterproof and antibacterial layers, increasing the number of layers, and using hybrid or novel alternative materials, have been adopted (Chua et al., 2020; Konda et al., 2020). Despite the fact that the protection offered is still limited and may not provide sufficient protection against viruses for healthcare workers who work for long periods of time and have frequent contact with infected patients, reusable cloth masks can serve as an emergency substitute for disposable surgical masks in the general public (Agency for Science Technology and Research, 2020; Eikenberry et al., 2020; Javid et al., 2020).

With the massive worldwide spread of coronaviruses, the global demand for commercial masks has surged, leading to an unprecedented increase in mask production and consumption. As estimated by the World Health Organization (WHO), 89 million medical masks are required worldwide every month to cope with COVID-19 pandemic (World Health Organization, 2020b), with global consumption of masks peaking at billions per day, equivalent to 12,000 masks being used every second. China, as the largest exporter of industrial textiles worldwide, has provided approximately 350 billion face masks and more than 4 billion protective suits to the international community since the outbreak of COVID-19 pandemic (Wang et al., 2021). This has resulted in the generation of large amounts of mask waste and resource consumption (Akarsu et al., 2021; Shen et al., 2021). More importantly, it also poses new challenges to climate change-related issues (Chua et al., 2020).

Excessive emissions of greenhouse gases (GHGs) are blamed for global climate change, and carbon footprint (CF) is therefore often applied to quantify the environmental impacts associated with climate change (Luo et al., 2021). Van Straten et al. (2021) assessed the CF of disposed and reprocessed face masks based on the life cycle assessment (LCA) method, and a 58% reduction in CF was observed when the disposable face masks were reprocessed and reused five times, compared to new masks that were used only once. Türkmen (2022) conducted a LCA study on disposable medical face masks. The results showed that the CF of a single mask was 21.5 g CO₂-eq, of which the main contributor was the raw material supply (40.5%), followed by packaging (30.0%) and production (15.5%). And a total of 1.1 Mt CO₂-eq was further estimated based on global mask use in 2020. Allison et al. (2020) estimated that if every person in the UK used a disposable mask every day for a year, the climate change impact would be 10 times greater than using reusable masks. The computational results of Lee et al. (2021) indicated that the use of each disposable surgical mask and embedded filtration layer (EFL) reusable face mask would contribute 18.7 g CO₂-eq and 0.338 kg CO₂-eq to climate change, respectively. Schmutz et al. (2020) compared the environmental impact of surgical masks and 2-layered cotton masks based on two different user behavior scenarios (i.e., strict and less strict scenarios). Cotton masks were found to perform better in terms of CF, and further sensitivity analysis revealed that the lifespan and weight of cotton masks were two important variables affecting the overall environmental performance. The environmental impacts of disposable (i.e., MD-Type I) and reusable (i.e., MD-Type IIR) face masks were compared by Morone et al. (2022) using the ReCiPe (H) method and found that the impacts of the former were mainly related to raw material consumption, energy requirements and waste disposal, with the latter's use phase and raw material consumption contributing the most.

Despite considerable attention being paid to the impact of face masks on climate change, a holistic view is missing for the comprehensive consideration of both functionality and ecological sustainability of face masks. While environmental protection is important, it may not be a priority at the most critical moment (Klemeš et al., 2020), as in the context of a raging epidemic, where individual lives need to be protected first. It is therefore necessary to learn how to save natural resources and address environmental problems caused by the massive use of masks on the premise of ensuring the safety of human life. Moreover, there are a variety of protective masks available on the market (NCS-TF Swiss National COVID-19 Science Task Force, 2020), coupled with different applicable scenarios (Burch and Walsh, 2021), but very little information was supplied concerning the environmental impact assessment in terms of specific scenarios. Cotton or cloth masks, for example, despite being thought to have some antiviral effects (Chughtai et al., 2020), are known to be ineffective substitutes in high-risk situations due to their varied filtration efficiencies (Prata et al., 2021; World Health Organization, 2022a). Therefore, it is somewhat meaningless to compare the environmental performance of different masks, such as cloth masks and filtering facepiece respirators (e.g., N95, KN95, etc.) (Do Thi et al., 2021), apart from the applicable scenarios of face masks.

In this context, three scenarios suitable for low-, medium- and high-risk settings were determined in this study, and four commonly used protective face masks including reusable cotton masks, disposable medical masks, disposable surgical masks and KN95 respirators were selected as the research objects for the above scenarios. Both the protective performance of masks and life-cycle environmental impacts associated with climate change were taken into account. The purpose of this study is to establish a more scientific and comprehensive evaluation method for the environmental impact assessment of anti-epidemic masks, thereby objectively analyzing the potential impact of products with different protective properties and scope of application on the environment, aiming to help medical staff and the public choose and use protective masks rationally, and also contribute to the energy saving and emission reduction of the manufacturing industry. Despite the focus on face masks, the evaluation method discussed in this study is generic and applicable to other PPE with similar properties. The novelties of this work consist mainly in: i) the extension of methodology by introducing the protective performance of face masks into the environmental impact assessment model; ii) the integration of practical applicable scenarios with respect to different exposure risks; iii) the detailed comparison of various COVID-19 protective masks from a full life cycle perspective and the resulting recommendations related to mask production and use.

2. Material and methods

2.1. Types of masks and applicable scenarios

With reference to the guidance provided by WHO on the use of masks (World Health Organization, 2020a) and the Technical Guidelines on the Selection and Use of Masks for the Prevention of COVID-19 Infection in Different Groups issued by China's National Bureau of Disease Control and Prevention (National Bureau of Disease Control and Prevention, 2020c), the applicable scenarios of common anti-epidemic masks on the market can be divided into the following five exposure risk levels depending on the target population, settings and activities.

• ●Low-risk level, including home activities, outdoor exercisers, workers and students in well-ventilated places, etc.;
• ●Slightly low-risk level, including people stranded in crowded places, indoor working environments where people are relatively gathered, the general public who go to medical institutions for medical treatment, and students who concentrate on study and activities, etc.;
• ●Medium-risk level, including staff in general outpatient clinics, wards and densely populated areas, and residents in home isolation, etc.;
• ●Slightly high-risk level, including emergency medical staff, investigators of close contacts, and testing personnel of epidemic-related samples, etc.;
• ●High-risk level, including fever clinics in epidemic areas, medical staff in isolation wards, high-risk medical workers such as intubation and incision, and on-site investigators for confirmed and suspected cases, etc.

For the consideration of model simplification, both low and slightly low risks were then classified as low-risk scenarios, while high and slightly high risks were considered as high risks. Accordingly, the three risk scenarios of low, medium and high were finally determined.

A variety of face masks are commercially available with different protective properties for different risk scenarios. A meta-analysis on betacoronavirus (including SARS-CoV-2) concluded that while masks are generally effective in preventing virus transmission, respirators such as N95 are more protective than disposable medical masks or reusable multilayer (12-16-layer) cotton masks (Chu et al., 2020). Similarly, Do Thi et al. (2021) noted that reusable/washable (cloth) masks and disposable masks (both surgical masks and respirators) did help control the spread of SARS-CoV-2, although the highest protection was provided by respirators, followed by surgical masks, and finally cloth masks. Ho et al. (2020) concluded, based on experiments with cotton versus medical masks worn by respiratory infected persons, that cotton masks could be a potential substitute for medical masks in air-conditioned micro environments and, given their washable and reusable nature, could be used by healthy individuals in community settings. Klemeš et al. (2020) summarized the available results regarding the filtration efficiency of masks made of different materials. In general, reusable masks are not as efficient in filtration as N95 respirators and surgical masks, but there are exceptions, not absolutes. Prata et al. (2021) stated that all masks contribute to the reduction of COVID-19 transmission, but higher protection is obtained with respirators, followed by surgical masks, and finally reusable or cloth masks. Furthermore, they argued that the use of disposable masks should be specifically enforced only in high-risk situations, such as shared indoor environments (e.g., hospitals, offices, stores, and markets), while maintaining reusable masks in low-risk situations (e.g., walking outside). The Centers for Disease Control and Prevention (CDC) recommends general use of cloth masks, preferably with multiple layers of high-thread-count fabrics, which can block 50–70% of fine droplets and particles (Centers for Disease Control and Prevention, 2021). Rowan and Moral (2021) agreed that the type and degree of non-pharmacological strategies employed are influenced by the perceived severity of relative risk; for example, in high-risk healthcare settings, such as ICU (Rowan and Laffey, 2021), full PPE should be worn. Conversely, in community settings where the relative risk is low, the wearing of improvised cloth face coverings, hand hygiene and social distancing are adopted.

Based on the above analysis, reusable cotton masks, disposable (ordinary) medical masks, disposable surgical masks and KN95 respirators without valves were selected as the research objects of this study, and the four types of masks were corresponding to different risk levels with reference to the guidelines for mask use, as illustrated in Fig. 1 .

Fig. 1.

Types of masks and the risk level of their application scenarios.

Of the four types of masks, reusable cotton masks are the earliest and relatively simple masks that intercept and filter larger particles or powders in the air through the multi-layer physical accumulation of cotton gauze or cotton cloth. Previous studies have confirmed that cotton masks can offer protection against certain bacteria or viruses (Chughtai et al., 2020; Ho et al., 2020), as well as reduce aerosol exposure (Davies et al., 2013). The filtration capacity of these masks varies from 50% to 90% (Do Thi et al., 2021), depending on the closeness of the gauze/cloth threads, the number of gauze/cloth layers, and the type of gauze/cloth (Chughtai et al., 2013). While most cotton masks on the market are still not as effective as disposable products (e.g., medical masks) in terms of filtration, they can be reused after washing and maintain the desired protective performance over a certain number of washes if designed properly and used correctly (Chughtai et al., 2020). Besides, some additional processing methods and finishing techniques can give these masks enhanced protective performance (Chua et al., 2020). The cotton masks studied in this paper consist of three layers of fabrics, as shown in Fig. 2 , of which the outermost and innermost are waterproof, antiviral and antibacterial treated cotton fabrics, and the middle is polypropylene (PP) melt-blown non-woven fabric, which can meet the protection needs of some low-risk scenarios, although their functions will eventually be weakened or even lost after multiple washes.

Fig. 2.

Fabric composition of the four masks. (PP: polypropylene).

Medical masks, surgical masks and KN95 respirators are all single-use and need to be replaced regularly (Do Thi et al., 2021). Both medical and surgical masks are composed of three layers of PP non-woven fabrics, i.e., PP spun-bond, PP melt-blown and PP spun-bond non-woven. The effectiveness of both masks is defined by bacterial filtration efficiency, differential pressure (Pa/cm²), splash resistant pressure (kPa), and microbial cleaning (Rowan and Moral, 2021). In contrast, the former has relatively low filtration efficiency for bacteria and particles, and provides limited shielding against some pathogenic microorganisms. They therefore commonly serve as barriers or protection in ordinary environments, such as low-risk and some medium-risk scenarios. While the latter is capable of blocking particles larger than 4 μm in diameter with a filtration efficiency of about 90% (Chua et al., 2020), suitable for medium-risk and some high-risk medical settings. The inner layer absorbs moisture from the wearer; the middle filter prevents a specific range of particles, aerosols, and pathogens from penetrating in any direction (Do Thi et al., 2021); and the water-resistant outer layer provides better protection against splashes of biological fluids, such as blood during surgery or infectious droplets in the air during patient care (Aragaw, 2020; NCS-TF National COVID-19 Science Task Force, 2020).

KN95 respirators (which comply with Chinese standards and provide a level of protection comparable to N95 respirators that meet U.S. testing standards) are tight-fitting masks that possess a filter capacity of at least 95% for 0.3 μm airborne particles (Burch and Walsh, 2021). They are constructed of five layers of fabrics, including PP spun-bond non-woven (the innermost and outermost layers), PP melt-blown non-woven (the second and fourth layers) and cotton (the middle layer) (Fig. 2). Filtration is accomplished by a physical barrier of various complex PP microfibers as well as electrostatic rates (Das et al., 2021). When worn consistently and properly, the highest degree of particle protection can be provided, including filtering of the SARS-CoV-2 virus. In addition, they can block respiratory droplets and particles from being exposed to others (Centers for Disease Control and Prevention, 2022a). Therefore, they are applicable to high-risk scenarios.

2.2. Functional unit

The functional units (FUs) employed for analysis in this study are masks used by 100 individuals over a period of 1 month. Reusable cotton masks are discarded after 6 days of use; in other words, they can withstand up to 5 washes while maintaining their protective properties, as recommended by the surveyed factories. Despite the fact that there are many different brands of reusable masks on the market that may vary in the maximum numbers of washes (TESTEX Community Mask Label, 2022), with some reportedly able to withstand up to 30 washes (Forever Family, 2022), due to limited research conditions and survey funding, production data were only available for masks that could be washed up to 5 times. Given that the cost people pay for them (retail price: CNY 7 per mask) is quite low relative to disposable masks (retail price: CNY 0.35 per medical and surgical mask and CNY 2.5 per KN95), we developed an evaluation model based on the number of washes in Section 2.4.2 to reduce the uncertainty associated with low settings for the number of washes.

Regarding disposable masks, including medical masks, surgical masks and KN95 respirators, they are discarded after each use according to the recommendations given by WHO (World Health Organization, 2021). In this case, it is assumed that each person uses one disposable mask per day, as presented in Table 1 . It should be noted that wearing one mask per day may be considered optimistic, as the number of masks necessary is highly dependent on the individual's behavior, but differences in environmental impact outcomes will remain relative between scenarios. If more than one mask is used per day on average, a scaling factor can thus be applied to the final results to reflect the actual impact (Allison et al., 2020).

Table 1.

Number of masks required to support mask use for 1 month of 100 people.

Mask type	Mask use per person per day (piece)	Time boundary	Mask treatment	Quantity demanded (pieces)
Reusable cotton mask	1	1 month (30 days)	Disposed after 6 days of use (5 washes)	500
Disposable medical mask	1	1 month (30 days)	Disposed at the end of day	3000
Disposable surgical mask	1	1 month (30 days)	Disposed at the end of day	3000
KN95 respirator	1	1 month (30 days)	Disposed at the end of day	3000

2.3. System boundaries

For our comparison of various masks for epidemic prevention, a cradle-to-grave scale was chosen, which includes raw materials extraction, industrial manufacturing, transportation, consumer use and end-of-life disposal, as depicted in Fig. 3 . The raw materials used in mask manufacturing mainly include non-woven fabrics made of PP, cotton fabrics, aluminum strips and polyester/nylon blends. In the production of masks, non-woven fabrics and cotton fabrics are acted as the body of the mask, aluminum strips are inserted as the material for the nose wire, and polyester/nylon blended materials are applied to make ear loops. In addition, the cotton masks have also undergone a water-repellant treatment, which is distinct from the other three.

Fig. 3.

System boundaries for four types of face masks.

The finished masks produced are sterilized and then packaged with polyethylene (PE) films and cartons. In the process of mask use, the reusable cotton masks are disposed of after repeated washing for 5 times; while the three types of disposable masks are discarded after each use, as described in Table 1. Hence, washing, sterilization and drying (being exposed to the sun) are only included within the system boundaries of cotton masks. Finally, all the masks are collected into the waste disposal site for incineration. In terms of the transport process, long-distance transport including raw materials being delivered to mask manufacturing plants and finished masks being distributed to consumers is considered in this case, while the transport of waste is covered in the end-of-life disposal process.

Other processes and material flows, such as human labor, manufacturing and maintenance of mechanical equipment, livestock providing transport services, as well as infrastructure investment (e.g., roads and construction) are excluded from the boundaries, considering their irrelevance to the environmental profiles measured (Luo et al., 2022).

2.4. Theoretical model

2.4.1. Carbon footprint calculation and evaluation model

The CF calculation of mask products was carried out in accordance with the principles and basic methodology of ISO 14067: 2018 (International Organization for Standardization, 2018). With the help of Global Warming Potential (GWP), all kinds of GHGs such as CH₄ and N₂O are uniformly converted into CO₂ equivalents to reflect the contribution of various GHGs relative to CO₂ to climate change-related impacts. The CF of a single product can first be obtained by Formula (1):

$$C{F}_{prod}\left(x\right)=\sum _{i=1}^m\sum _{j=1}^{n}A{D}_{pro{d}_i}\left(x\right)\times E{F}_{i,j}\times GW{P}_j$$ (1)

where CF _prod (x) is the carbon footprint of a single product (kg CO₂-eq), and a single product here refers to a mask; x is the specific stage in the product life cycle, x = 1–5, representing raw material extraction, industrial manufacturing, distribution and transport, consumer use and end-of-life disposal respectively; $A{D}_{prodi}\left(x\right)$ is the activity data of GHG emission source i for the specific life cycle stage x (the unit varies with the emission source); EF _i,j is the emission factor of GHG j emitted by source i, i.e., the amount of GHG j emitted per unit of source i (the unit varies with the emission source); GWP _j is the global warming potential of GHG j (dimensionless); m is the number of GHG emission sources; n is the number of GHGs.

The total CF of the products can thus be calculated by multiplying the number of products and washes defined by the functional unit. The calculation methods of disposable and reusable products are described in (2), (3) respectively:

$$C{F}_{disp}=\sum _{x=1}^{5}C{F}_{prod\_disp}\left(x\right)\times M_{disp}$$ (2)

$$C{F}_{reus}=\left[\sum _{x=\mathrm{1,2,3,5}}C{F}_{prod\_reus}\left(x\right)+\sum _{x=4}C{F}_{prod\_reus}\left(x\right)\times N_{wash}\right]\times M_{reus}$$ (3)

where CF _disp is total carbon footprint of disposable products; CF _reus is total carbon footprint of reusable products; CF _{prod_disp}(x) and CF _{prod_reus}(x) are the carbon footprint of a single disposable product and reusable product at a specific life cycle stage x, respectively; M _disp and M _reus are the number of disposable and reusable products defined by the functional unit, respectively; N _wash is the number of washes per product defined by the functional unit.

2.4.2. Comparative model of disposable and reusable products

According to Formula (2), when the number of products is determined, the impact of different types of disposable products on the environment depends on the sum of the impacts of each life cycle process. And it can be inferred from Formula (3) that the CF of reusable products is not only affected by various life cycle processes, but also has a great relationship with the number of washes in the use stage. The maximum number of washes allowed is inherently limited by product performance, as multiple washing activities will loosen the fabric structure (changes in pore size and shape) and damage the fibers, thereby weakening the barrier against viruses and bacteria (Neupane et al., 2019). In the study by Allison et al. (2020), a total of 183 washes was assumed for each washable mask, which was considered unrealistic, as no mask has yet been able to withstand such extensive washing, leading to controversial conclusions. While in the case study of this paper, as previously mentioned, the number of washes is considered low at only 5 times. Therefore, in order to obtain more realistic results, this paper further proposed a general model for CF evaluation of different products, and built a comparison model of disposable and reusable products on this basis, thereby reflecting the trade-offs between product performance and environmental impact of protective products.

First, we assumed the same usage patterns for disposable and reusable products, i.e., the same number of products used per person per day. Accordingly, given the number of users and duration of use, the total number of uses for the two products is the same. It can thus be expressed as Formula (4):

$$M_{disp}=\left(N_{wash}+1\right)\times M_{reus}$$ (4)

As recommended by MacIntyre et al. (2020), at least two masks are used in rotation to allow for adequate cleaning and drying prior to use, but the number of reusable masks that can be rotated per person depends on personal preference and economic feasibility. As the number grows, they can be washed centrally, especially if multiple washable masks are worn throughout the day or if there are multiple members of a household. Hence, an adjustable parameter n is introduced here, which represents the number of products per wash and can be modified in accordance with the actual situation. The total number of washes of washable products (N _{wash_Total}) can be obtained from Formula (5):

$$N_{wash\_Total}=\frac{M_{reus}}{n}\times N_{wash}$$ (5)

Consequently, the CF calculation models of the two types of masks can be transformed into (6), (7), respectively:

$$C{F}_{disp}=\sum _{x=1}^{5}C{F}_{prod\_disp}\left(x\right)\times \left(N_{wash}+1\right)\times M_{reus}$$ (6)

$$C{F}_{reus}=\left[\sum _{x=\mathrm{1,2,3,5}}C{F}_{prod\_reus}\left(x\right)+\sum _{x=4}C{F}_{prod\_reus}\left(x\right)\times \frac{N_{wash}}{n}\right]\times M_{reus}$$ (7)

Therefore,

If $\sum _{x=1}^{5}C{F}_{prod\_disp}\left(x\right)\times \left(N_{wash}+1\right)=\sum _{x=\mathrm{1,2,3,5}}C{F}_{prod\_reus}\left(x\right)+\sum _{x=4}C{F}_{prod\_reus}\left(x\right)\times \frac{N_{wash}}{n}$, disposable and reusable products are considered to have equal environmental performance;

If $\sum _{x=1}^{5}C{F}_{prod\_disp}\left(x\right)\times \left(N_{wash}+1\right)<\sum _{x=\mathrm{1,2,3,5}}C{F}_{prod\_reus}\left(x\right)+\sum _{x=4}C{F}_{prod\_reus}\left(x\right)\times \frac{N_{wash}}{n}$, disposable products appear better environmental performance (with smaller CF) than reusable products;

If $\sum _{x=1}^{5}C{F}_{prod\_disp}\left(x\right)\times \left(N_{wash}+1\right)>\sum _{x=\mathrm{1,2,3,5}}C{F}_{prod\_reus}\left(x\right)+\sum _{x=4}C{F}_{prod\_reus}\left(x\right)\times \frac{N_{wash}}{n}$, disposable products exhibit worse environmental performance (with larger CF) than reusable products.

2.5. Data collection

The data involved in the case study were mainly derived from the primary data surveys of three manufacturing enterprises in Zhejiang, China. Missing data from the surveys were supplemented by the literature (Allison et al., 2020; Ariel, 2020). Detailed descriptions of the life cycle process, along with the material and energy inputs within the system boundaries, are given separately for the four types of masks, as presented in Table 2 . Background data (e.g., production and supply of various materials, chemicals, energy, etc.) were collected from the China Life Cycle Database (CLCD version 0.7), China Products Carbon Footprint Factors Database (2022), and some other relevant references. For more information, please see Appendix A in Supplementary Materials.

Table 2.

Life cycle inventory of four masks for different life cycle processes.

Life cycle process	Category	Inventory	Unit	Amount
				Reusable cotton mask	Disposable medical mask	Disposable surgical mask	KN95 respirator
Raw material processing	Material	Polypropylene spun-bonded non-woven fabric	g/mask	/	1.700	1.624	2.438
		Polypropylene melt-blown non-woven fabric	g/mask	0.840	1.020	0.725	1.625
		Cotton fabric	g/mask	13.960	/	/	1.625
		Aluminum strip	g/mask	0.330	0.320	0.430	0.400
		Polyester/nylon blends	g/mask	1.600	0.440	0.460	1.600
	Chemical	Electret masterbatch	g/mask	0.013	0.041	0.035	0.061
Mask production	Chemical	Ethylene oxide	g/mask	0.902	/	0.176	0.417
	Energy	Electricity	kW·h/mask	0.010	0.004	0.008	0.009
	Water resource	Water	m3/mask	0.797	0.035	0.035	0.288
Mask packaging	Material	Polyethylene film	g/mask	5.420	0.062	3.350	4.540
		Ivory board	g/mask	/	0.988	0.974	6.070
		Corrugating medium	g/mask	2.000	0.480	0.554	2.000
Transport	Energy	Gasoline	km	1007	1007	1017	1007
Mask use	Chemical	Detergent	g/(mask·wash)	6.240	/	/	/
	Water resource	Water	L/(mask·wash)	12.000	/	/	/
End-of-life disposal	Waste	Waste	g/mask	24.163	5.051	8.152	20.358

The gasoline consumption during transport was determined by the distance traveled and the corresponding cargo capacity. As mentioned above, the transport phase included the delivery of raw materials, depending on the distance between suppliers and manufacturers, which was estimated based on the addresses provided by the surveyed enterprises; and the distribution of finished masks, assuming an average distance of 1000 km from a terminal in China. Table 2 gives the sum of the two.

Regarding the washing process for the use of reusable cotton masks, Ariel's hand-washing guidelines (Ariel, 2020) and the UCL research (Allison et al., 2020) were referenced in this study, in which used masks were washed manually in a mixture of 3 L of lukewarm water and 1 teaspoon of liquid detergent (approximately 6 ml, weighing 6.24 g). When the cleaning was complete, rinse it three times in the tub of detergent-free water. After that, leave to dry naturally in the sun.

3. Results

The CFs of four mask products were calculated based on (1), (2), (3), and then analyzed and discussed respectively according to different risk levels.

3.1. Results of two masks in low-risk scenarios

In scenarios with low exposure to the novel coronavirus, reusable cotton masks and disposable medical masks are considered sufficient to provide safety protection for the population. The CF results of both are shown in Fig. 4 . It can be found that the total CF of reusable cotton masks is 285.484 kg CO₂-eq/FU, while that of disposable medical masks is 128.926 kg CO₂-eq/FU. Obviously, the impact of the former is greater than that of the latter.

Fig. 4.

Carbon footprint results of two masks in low-risk scenarios.

The discrepancy between the two is mainly reflected in the production and use of masks, although the CF of disposable medical masks in other life cycle processes is higher than that of cotton masks. In terms of mask production, the reusable cotton masks produce a total CF of 185.901 kg CO₂-eq/FU, more than three times the carbon emissions of disposable medical masks. Since the use of disposables does not involve washing, medical masks have a CF of 0 during use, while cotton masks, which consume a lot of water and detergent due to repeated cleaning, generate a CF of 41.580 kg CO₂-eq/FU. This is mainly attributed to the carbon emissions from the production and supply processes of water and detergent, as well as the treatment of laundry wastewater.

Given that carbon emissions in the life cycle of masks are mainly derived from the various materials, chemicals and energy used in each process (i.e., carbon emissions from the production and supply of materials, chemicals and energy), calculating the CF contributions of different GHG emission sources above relative to the total impact allows the identification of key factors for those processes in the life cycle of masks where major environmental impacts occur, and thus potential opportunities for environmental optimization. Detailed results on carbon emission sources for the four masks can be found in Appendix B (see Supplementary Materials). It reveals that the carbon emissions in the production process of reusable cotton masks mainly come from water, which is 179.237 kg CO₂-eq/FU, accounting for 62.78% of the total impact. The main contributor to the use process is detergent with a CF of 28.080 kg CO₂-eq/FU, which is nearly 10% of the total. In contrast, the water consumption in the production stage of disposable medical masks appears a smaller impact (47.500 kg CO₂-eq/FU), despite the fact that it still answers for the largest CF contributions, at 36.84% of the total. In addition, the polyester/nylon blends and aluminum strip as raw materials of disposable medical masks also contribute significantly to the whole life cycle impact of the product, accounting for 13.92% and 11.76%, respectively.

The comparative model of disposable and reusable protective products indicates that the relative environmental performance between the two is dependent on the impact of each life cycle process and the number of washes. Combined with the above results, the impact of life cycle process is mainly reflected in raw material processing, mask production and cotton mask washing processes. Since the production chain of mask products is quite short and simple, with a high degree of industrialization, the variations in environmental impact caused by the upgrading of manufacturing technology are generally not much. On the other hand, the maximum number of washings (5 times) for cotton masks in this study was provided by the manufacturers surveyed, which is far below our expectations. When the number of washes increases, the corresponding costs and resource consumption will be reduced, as more new products can be substituted for production and use. To this end, a further analysis was carried out based on the evaluation model established in Section 2.4.2.

It is first assumed, based on the results obtained in Fig. 4, that the impact of each life cycle process is constant for both mask products, that is, the CF of a single disposable medical mask in its whole life cycle is 0.043 kg CO₂-eq/mask, while the life cycle CF of a single reusable cotton mask excluding the use stage is 0.488 kg CO₂-eq/mask, coupled with 0.017 kg CO₂-eq/mask of carbon emissions generated by each wash. As suggested by MacIntyre et al. (2020), two cotton masks are assumed to be used alternately (thus the number of reusable masks is 2) and washed separately after one use; whereas disposable masks are discarded after one use and replaced with new ones. Based on (6), (7) proposed above, the CF functions of the two masks relative to the number of washes can thus be obtained. The number of washes is then assigned from 1 to 25, from which the CF trend of the two masks can be observed, as shown in Fig. 5 .

Fig. 5.

Carbon footprints of reusable cotton masks and disposable medical masks with increasing number of washes.

It can be found from Fig. 5 that there is a break-even point between the two, at which the environmental advantage is switching from one type of masks to the other one. When the maximum number of washes for reusable cotton masks is below the break-even point, disposable masks are more advantageous than cotton masks; instead, reusable cotton masks are a better choice. After calculation, the break-even point is displayed as 16.886, which means that the CF of cotton masks washed 17 times or more will be less than the carbon emissions generated by the use of disposable medical masks. And in this case, the price of 18 disposable medical masks (CNY 6.3) is close to that of one reusable cotton mask (CNY 7).

3.2. Results of two masks in medium-risk scenarios

In the case of medium risk, two types of disposable masks, namely disposable medical masks and disposable surgical masks, can provide sufficient safety protection for the masses. Therefore, the two products are selected for this scenario, and their CF results are presented in Fig. 6 . It can be seen that the total life cycle CF of disposable medical masks is 128.926 kg CO₂-eq/FU, while that of disposable surgical masks is 154.328 kg CO₂-eq/FU, which is slightly larger than the former.

Fig. 6.

Carbon footprint results of two masks in medium-risk scenarios.

According to Fig. 6, the key CF contributors found throughout the life cycle of both masks are raw material processing and mask production. In terms of disposable medical masks, the combined impact of the two processes accounts for 87.60% of the total, while concerning disposable surgical masks, the contribution rates of the two processes are 37.94% and 44.61%, respectively. Nevertheless, due to the similarity in raw materials consumed, there is no significant difference between the two mask products in the raw material processing stage. Rather, the CF values vary more between mask production and packaging processes.

By contrast, the CF value of disposable surgical masks during production is 11.943 kg CO₂-eq/FU greater than that of medical masks, mainly due to higher power consumption and the use of ethylene oxide. As for the mask packaging process, disposable surgical masks exhibit 75.23% more carbon emissions, which is mainly attributed to the increased use of packaging materials (PE film).

3.3. Results of two masks in high-risk scenarios

Regarding high-risk environments, the two masks that can protect the public from exposure to the COVID-19 virus are disposable surgical masks and KN95 respirators, both of which are designed and regulated as single-use. Fig. 7 displays the life cycle CF results for the two mask products. The total CF of the disposable surgical masks is 154.328 kg CO₂-eq/FU, while that of KN95 is 641.249 kg CO₂-eq/FU. It is clear that KN95 respirators possess a far greater impact than disposable surgical masks.

Fig. 7.

Carbon footprint results of two masks in high-risk scenarios.

Despite the fact that the CF of KN95 in each life cycle process is larger than that of disposable surgical masks, the discrepancy between the two is more significant in the raw material processing, mask production and packaging processes. According to the comparative study, the carbon emissions of KN95 respirators in the process of raw material processing, mask production and packaging are 2.4 times, 6.1 times and 3.6 times that of disposable surgical masks respectively. This is mainly due to the higher filtration requirements of KN95, which increases the number of fabric layers, resulting in more raw materials and resource consumption. In particular, the impact of KN95 increases by 341.300 kg CO₂-eq/FU compared to the carbon emissions from water used in the production of disposable surgical masks, which is identified as the main driver of the difference in CF between the two masks. This increase is especially significant when scaled by the face mask consumption of China's population.

4. Discussion

4.1. Mask phenomenon in the context of the COVID-19 pandemic

Pandemics typically go through different stages, including emergence, peak, decline, and stabilization, which are often plotted as curves (number of cases versus time), as was the case of the COVID-19 pandemic. At different stages, guidance from medical experts and epidemic prevention authorities, government policies on epidemic management, and actual actions exhibited by the public will vary in terms of the wearing of masks (e.g., whether healthy individuals need to wear masks in public and what grade of masks to use, etc.). China, as the first country affected by SARS-CoV-2 virus, the mask phenomenon presented from the beginning of the outbreak to the normalization stage is considered to be highly typical and representative, as shown in Fig. 8 .

Fig. 8.

New confirmed cases of COVID-19 in China from Dec. 27, 2019 to May 31, 2020. Note: the figure is plotted based on data from the National Health Commission of the People's Republic of China; 15,152 new confirmed cases were reported on Feb. 12, of which 13,332 cumulative clinically diagnosed cases in Hubei Province were counted as new confirmed cases on that day at one time.

Five stages are covered in Fig. 8.

• (i)Emergence stage (Dec. 27, 2019 to Jan. 19, 2020): on Dec. 27, 2019, cases of pneumonia of unknown origin were monitored and detected in Wuhan, Hubei, China, with a few cases appearing before Jan. 19, 2020. On Dec. 31, 2019, the Wuhan Municipal Health Commission released a notice on the pneumonia outbreak in the city, prompting the public to wear masks outside (Wuhan Municipal Health Commission, 2019). However, it did not attract widespread attention.
• (ii)Peak stage (Jan. 20 to Feb. 20, 2020): the epidemic was in full swing with a rapid increase in the number of cases. On Jan. 20, 2020, China classified COVID-19 as a Class B infectious disease but subject to the preventive and control measures for a Class A infectious disease. The Chinese government and departments have issued a series of notices on epidemic prevention and control (Chinese Center for Disease Control and Prevention, 2020; National Bureau of Disease Control and Prevention, 2020a, 2020c; Wuhan Municipal Health Commission, 2020), requiring all people to wear masks in public places and instructing different groups to choose and use masks in a scientific and rational manner. The vast majority of the Chinese public actively cooperated in wearing masks, while the few who did not would be criticized by those around them.
• (iii)Decline stage (Feb. 21 to Mar. 17, 2020): the epidemic was effectively contained through scientific prevention and control policies, and the number of new cases showed a continuous downward trend. For the sake of protection, the public continued to wear masks voluntarily.
• (iv)Stabilization stage (Mar. 18 to May 7, 2020): the epidemic was basically controlled and stabilized in China. On Mar. 18, the National Bureau of Disease Control and Prevention classified the general public, people in specific places, key personnel and people with occupational exposure, and made scientific recommendations for wearing masks in different scenarios (National Bureau of Disease Control and Prevention, 2020b).
• (v)Normalization stage (May 8 to May 31, 2020): the epidemic was sporadic in general, with some areas experiencing clusters of epidemics caused by sporadic cases, and the national epidemic prevention and control efforts entering normalization. On May 8, the authorities issued a guideline for normalization of epidemic prevention and control (Joint Prevention and Control Mechanism of the State Council on COVID-19, 2020), requiring the public to wear masks in crowded and enclosed places, as well as when in contact with others at a distance of less than 1 m.

As can be seen, the Chinese government has always taken a positive attitude towards the protective effects of masks, which have proven to be effective in reducing the spread of the virus (in combination, of course, with measures such as maintaining social distance, hand hygiene and isolation). Now the world, including China, is ready to return to normalcy as much as possible while still being cautious about COVID-19 disease (Renardo, 2023). The fact that masks remain a necessity for public travel, leading to continued demand and consumption of masks, means that the environmental burden caused by face masks will be a significant issue currently and for a long time to come. During the initial and peak periods of the pandemic, although experts and officials advised the public to choose masks wisely according to different scenarios in order to achieve a rational allocation of resources, the public would use masks with the highest protection level possible out of fear of the SARS-CoV-2 virus and concern for personal safety to provide maximum protection, regardless of the economic cost and environmental impact. However, with increased awareness of the virus, people are likely to shift to the rational selection and use of masks based on different applicable scenarios to reduce waste of resources and costs. In this regard, the research in this paper is of high practical significance and guidance value.

4.2. Comparison of risk-oriented face masks and improvement opportunities

Due to the particularity of PPE, it is impractical and pointless to discuss the environmental impact issues in isolation from their protective performance. Unfortunately, they are rarely considered in existing publications. While our research model is built based on different applicable scenarios provided by official documents, that is, the environmental impact issues are considered in the context of meeting the protective requirements of masks, which will be more instructive for the general public.

One of the phenomena that can be observed in the COVID-19 pandemic is that the surge in demand for masks has led to a global shortage of supplies for both physical products and raw materials (Carias et al., 2015), one important reason being the short life cycle and rapid replacement of disposable products. Reusable masks have been proposed as safe and eco-friendly alternatives to disposable ones due to their washable and reusable characteristics (Ho et al., 2020; Makki et al., 2021). It should be noted, however, that the quality of reusable masks currently on the market varies and most are not yet ready for use in high-risk level scenarios, and in some cases the environmental performance of reusable masks is even worse than that of disposable masks. The results of this study suggest that the use of reusable masks can reduce the amount of GHGs released into the atmosphere only if they can withstand more than 16 washes. Conversely, if reusable cotton masks cannot be washed 17 times or more, that is, the required safety protective performance cannot be guaranteed after washing, then disposable medical masks are considered more beneficial to the environment and more economical for the public.

While cotton masks require continuous consumption of water and detergent during repeated washing, which increases carbon emissions to some extent, reuse can extend the service life of cotton masks, thus reducing the use of new products and avoiding the environmental impact of the production and transportation of new masks. In contrast, the CF of individual disposable medical masks is significantly smaller than that of cotton masks, because more resources and energy are required for the production of cotton masks; however, the service life of disposable products is fairly short, and the CF of cotton masks during use is less than that of disposable products throughout their life cycle. In other words, the CF variation of cotton masks is smaller than that of disposable medical masks. Therefore, as the number of washes increases, the gap between the CF values of the two masks will continue to narrow, and then gradually widen after reaching the break-even point (Fig. 5). Ultimately, disposable surgical masks exhibit a far greater environmental impact than cotton masks. In this regard, it is critical to improve the washing durability of cotton masks. More efforts are therefore needed to improve the washability and reusability of reusable masks.

For medium-risk environments, the CF of disposable medical masks was found to be smaller than that of disposable surgical masks over their whole life cycle. The reason is that surgical masks have an additional sterilization procedure with ethylene oxide in the production stage, resulting in higher energy and ethylene oxide consumption and a corresponding increase in CF. In addition, during the mask packaging process, disposable surgical masks are packaged individually, i.e., one mask is packaged in a bag made of PE film, whereas disposable medical masks are packaged in 50 pieces. As a result, more PE films are consumed by disposable surgical masks, resulting in more carbon emissions. Thus, for the sake of environmental protection, disposable medical masks rather than surgical masks should be used in cases of medium-risk level.

In terms of high-risk settings, the total carbon emission of disposable surgical masks is significantly lower than that of KN95 respirators, primarily due to the greater material requirements of KN95 respirators during each life cycle process. Not only a layer of cotton fabric, but also a layer of PP melt-blown non-woven fabric is additionally included in the KN95 respirators, resulting in a larger CF during raw material processing. In addition, due to the more complex product structure and production process of KN95, more materials are consumed in the production and packaging processes, including water, ethylene oxide and packing materials (e.g., PE film, ivory board and corrugating medium).

Based on the analysis results, some feasible environmental optimization strategies for future mask production and consumption can thus be proposed. With regard to the production of masks, it was found that all four types of face masks have large CFs in the raw material processing and production processes. Accordingly, the low-carbon strategies can give priority to these two aspects. In the case of disposable medical and surgical masks, the raw materials are predominantly fossil-based polymers, which, in addition to exhibiting a high dependence on non-renewable resources and energy, also pose other potential threats to the ecosystem due to their inherent non-degradability (Shruti et al., 2020; Dharmaraj et al., 2021). Therefore, it is recommended to use some renewable and/or biodegradable resources to develop mask materials (Choi et al., 2021; Garcia et al., 2021), such as polylactic acid (PLA) (Soo et al., 2022). For reusable cotton masks and KN95 respirators, the cotton fabric layers involved are processed from conventional cotton, with a contribution to the overall life cycle impact of 11.64% and 3.62% respectively. In this regard, organic cotton (reducing or avoiding the use of pesticides and fossil fertilizers) (Baydar et al., 2015) and recycled cotton (avoiding the impact of cotton cultivation process) (Esteve-Turrillas and de la Guardia, 2017) can be considered as potential alternatives.

On the consumption side, the public should select and use masks in accordance with the principles of reasonable resource conservation and pollution reduction under the premise of ensuring personal safety protection. In some low-risk scenarios such as home activities and well-ventilated places, reusable cotton masks are preferred for epidemic prevention if they can maintain the protective performance after being washed for 17 times and more. However, attention needs to be paid to the decontamination and maintenance of reusable masks, since incorrect cleaning not only fails to inactivate the virus and increases the risk of infection (Chua et al., 2020), but also accelerates the destruction of the mask structure and thus affects its protective capacity (Neupane et al., 2019). Disposable medical masks appear to be more environmentally beneficial and economical when cotton masks fail to meet the required performance after 17 washes. In the context of medium risk, such as medical visits and some intensive activities, disposable medical masks are considered a more environmentally friendly alternative. Disposable surgical masks are more recommended for use in high-risk settings, such as workers in densely populated areas and medical staff in isolation areas. In addition, some other decontamination methods, including physical methods (steam, heat (dry and humid), ultraviolet germicidal irradiation (UVGI)) and chemical methods (using ethylene oxide, activated chlorine, ozone, vaporous hydrogen peroxide, and other tested chemical reagents) (Liao et al., 2020; Ma et al., 2020; Tsogtbayar and Yoon, 2020) can be considered to increase the reusability of masks (including disposable masks), especially in the event of supply shortages, and to greatly reduce environmental stress from overuse and discarded masks.

However, it is worth noting that the life-cycle environmental impact of face masks may vary considerably, depending on where the raw material (e.g., cotton) is produced, how it is produced (e.g. manual or machine-automated packaging), and where the mask is manufactured and with what technology. The carbon footprint obtained in this study regarding disposable surgical masks (0.051 kg CO₂-eq/piece) is similar to the results obtained by Allison et al. (2020) (0.059 kg CO₂-eq/piece) because both are produced in China. However, it is much larger than the result (0.019 kg CO₂-eq/piece) obtained in the study by Lee et al. (2021), which is mainly attributed to the different production sites and production technologies of the two. The latter occurs in Singapore, where electricity is mainly derived from natural gas. In China, however, most of the electricity still relies on coal. In addition, there are some differences between the two masks in terms of fabric weight. These variations also mean that the conclusions drawn in this paper may not apply to masks produced in other countries (with different carbon intensity of energy grids) and with different production technologies. In this regard, an update of the data inventory is necessary.

4.3. Sensitivity analysis

The contribution analysis has shown that, aluminum strips and polyester/nylon blends used in raw material processing, and water consumption in mask production for disposable masks, as well as cotton fabric used in raw material processing and detergent in the use phase for reusable cotton masks, were identified as the key factors contributing most to CF among the four types of face masks. However, the above parameters are of high variability and uncertainty due to monitoring errors, the diversity of processing techniques and the inconsistency of historical data series. Sensitivity analyses were therefore performed to fill these limitations by assessing the effects of changing the base case of these parameters. Each parameter was varied independently of all other parameters by ±10% from its base value, so that the extent of their impact on the base case could be assessed. Fig. 9 illustrates the results of the sensitivity analysis.

Fig. 9.

Sensitivity analysis of parameters affecting the carbon footprint of four masks. (P1: cotton fabric in raw material processing; P2: aluminum strip in raw material processing; P3: polyester/nylon blends in raw material processing; P4: water in mask production; P5: detergent in mask use).

As observed in Fig. 9, the CFs of all four types of face masks are most sensitive to the water consumed during mask production. When a 10% reduction in water consumption was assumed, the total CFs of reusable cotton masks, disposable medical masks, disposable surgical masks and KN95 respirators were reduced by approximately 6.28%, 3.68%, 3.08% and 6.06% respectively. The sensitivity analysis suggested that carbon emissions associated with the water required in the mask manufacturing process had a greater impact on the total CFs of face masks compared to other energy and materials involved in the product life cycle. In this regard, besides the possibility of reducing water dependency by improving production technologies and upgrading equipment, other options are worth considering, such as increasing water reuse to reduce environmental stress on water supplies and adopting sustainable technologies such as nitrifier-enriched activated sludge (NAS) for wastewater treatment (Sepehri and Sarrafzadeh, 2018; Sepehri et al., 2020), which can increase carbon capture while enhancing nutrient removal.

4.4. Limitations

With regard to the lifecycle inventory, although great efforts have been made to gather as complete and reliable foreground and background data as possible, there are still some deficiencies. For example, due to the uncertainty of mask distribution, a reasonable assumption was made that the average distance of a terminal in China is 1000 km, excluding exports. Moreover, the washing process of cotton masks was modeled with reference to available data on common usage patterns, without considering machine washing (Centers for Disease Control and Prevention, 2022b), sterilization with hot water (60 °C) (NCS-TF National COVID-19 Science Task Force, 2020; World Health Organization, 2020a) and dryer drying, which would result in higher power consumption and thus be underestimated.

On the other hand, considering the huge demand of mask products in the context of epidemic prevention and their relatively simple structure and ease of modeling, only mask products are taken as a case of this study. But it should be realized that the research framework and evaluation model proposed in this paper are somewhat generic and applicable to other similar PPE, such as protective clothing. In addition, not all masks currently on the market were covered in this study. Although the four most common types of masks in China were selected as far as possible to improve representativeness, given the diversification of the market and the upgrading of technology, there is an increasing variety of mask products, especially reusable masks. For example, reusable masks reusable masks embedded with reinforced filtration layers can maintain a bacterial filtration efficiency of 95% after 30 washes (Forever Family, 2022). Reusable masks with enhanced electrostatic and physical filtering through specific fabric combinations can achieve protection efficiencies similar to disposable masks at 90% (Konda et al., 2020). Due to the variations in materials and production processes, the conclusions drawn in this study may not be applicable to these masks; therefore, further exploration of other objects is necessary in the future, and the contribution of this study is to provide them with ideas and methods for analysis and evaluation.

5. Conclusions

Considering the variations in the risk levels of different applicable scenarios and the protective performance of various anti-epidemic mask products in the context of COVID-19, reusable cotton masks, disposable medical masks, disposable surgical masks and KN95 respirators were determined as the research objects in this study. The evaluation model of personal protective products based on the existing carbon footprint assessment methodology was proposed by integrating the protective performance of products, and the life cycle carbon emissions of face masks suitable for different risk levels were then calculated and evaluated.

The comparison of the two masks used in low-risk scenarios showed that the break-even point of environmental performance between reusable cotton masks and disposable medical masks was 16.886. In other words, reusable cotton masks are a better choice for the environment if they can maintain sufficient technical performance after more than 16 consecutive washes; otherwise, disposable medical masks are more favorable. Considering both environmental impact and protective performance can make the conclusion more realistic. Regarding medium- and high-risk settings, the disposable medical and surgical masks were identified as preferable alternatives respectively, due to their better environmental performance throughout the product life cycle. The results obtained can thus serve as a guide for the public to select and use mask products.

Further contribution analysis also provides technical guidance for mask manufactures to move towards more sustainable and cleaner production patterns. On the one hand, renewable and/or biodegradable materials could be considered as alternatives to fossil-based raw materials, as well as organic or recycled cotton to replace conventional cotton that relies on artificial irrigation, thus reducing the environmental burden of the four face masks during raw material processing. On the other hand, more attention should be paid to the protective performance and washability of reusable masks to improve their effectiveness in preventing infection. In addition, the sensitivity analysis revealed that variations in water consumption during production of the four masks had the most significant impact on the carbon footprint contributions. Hence, reducing carbon emissions associated with industrial water use will also lead to significant environmental improvements for these four masks, which can be achieved with the help of increased water reuse and sustainable technologies such as nitrifier-enriched activated sludge (NAS).

Credit authorship contribution statement

Yan Luo: Methodology, Visualization, Writing – original draft, Writing – review & editing. Mengfan Yu: Investigation, Writing – original draft. Xiongying Wu: Conceptualization, Supervision, Writing – review & editing. Xuemei Ding: Conceptualization, Supervision, Writing – review & editing. Laili Wang: Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Handling Editor: Yutao Wang

Appendix A. Supplementary data

The following is the Supplementary data to this article.

Data availability

I have shared the data in Supplementary File at the Attach File step.