Exposure to environmental pollutants: A mini-review on the application of wastewater-based epidemiology approach

Abstract

Wastewater-based epidemiology (WBE) is considered an innovative and promising tool for estimating community exposure to a wide range of chemical and biological compounds by analyzing wastewater. Despite scholars' interest in WBE studies, there are uncertainties and limitations associated with this approach. This current review focuses on the feasibility of the WBE approach in assessing environmental pollutants, including pesticides, heavy metals, phthalates, bisphenols, and personal care products (PCPs). Limitations and challenges of WBE studies are initially discussed, and then future perspectives, gaps, and recommendations are presented in this review. One of the key limitations of this approach is the selection and identification of appropriate biomarkers in studies. Selecting biomarkers considering the basic requirements of a human exposure biomarker is the most important criterion for validating this new approach. Assessing the stability of biomarkers in wastewater is crucial for reliable comparisons of substance consumption in the population. However, directly analyzing wastewater does not provide a clear picture of biomarker stability. This uncertainty affects the reliability of temporal and spatial comparisons. Various uncertainties also arise from different steps involved in WBE. These uncertainties include sewage sampling, exogenous sources, analytical measurements, back-calculation, and estimation of the population under investigation. Further research is necessary to ensure that measured pollutant levels accurately reflect human excretion. Utilizing data from WBE can support healthcare policy in assessing exposure to environmental pollutants in the general population. Moreover, WBE seems to be a valuable tool for biomarkers that indicate healthy conditions, lifestyle, disease identification, and exposure to pollutants. Although this approach has the potential to serve as a biomonitoring tool in large communities, it is necessary to monitor more metabolites from wastewater to enhance future studies.

Introduction

Humans are exposed to various environmental contaminants through inhalation, ingestion, and dermal routes [41]. Several approaches have been developed to detect contaminants in the environment or measure their metabolites in biological matrices to obtain valuable data on human exposure to chemicals [8]. Over the past years, there has been an increase in the number of studies that have focused on wastewater analysis to determine human exposure to a wide range of chemical and biological compounds [17, 18, 31, 48, 49]. The analysis of wastewater collected in a specific community can potentially monitor real-time data on geographic and temporal patterns of a wide range of compounds found in wastewater analysis. It enables scientists and researchers to get comprehensive data on the health status of the population or estimation of consumption of or exposure to a target compounds.

This relatively novel technique is called wastewater-based epidemiology (WBE) and was introduced in 2001 by Daughton to estimate illicit drug consumption [11]. Since then, WBE has received particular attention due to its wide application, ranging from the estimation of human drug consumption, and detection and monitoring of pathogens such as bacteria, protozoa, and viruses to assessing lifestyle and dietary habits, identifying communities at risk, and predicting future outbreaks [49]. Then, it was expanded to include other human biomarkers that provide qualitative and quantitative data about health, such as pharmaceuticals, illicit drugs, or new psychoactive substances. Additionally, it includes information about diseases, like the COVID-19 pandemic by SARS-CoV-2 detection and monitoring in wastewater.

Recently, the WBE approach has been developed to estimate exposure to chemicals such as pesticides, personal care products, and plasticizers [1]. The study conducted by Aghaei et al. showed that studies are moving towards environmental pollutants, and the trend of studies in some pollutntas such as pesticides is increasing [1].

Exposure to pollutants such as pesticides, heavy metals, and household products is being studied for its impact on human health [44, 45, 46]. The use of these resistant compounds raises concerns for both the environment and human health. Pesticides have a wide range of applications, and once they are released into the environment, they break down through various processes, including photolysis, hydrolysis, oxidation, reduction, microbial activity, and metabolism in plants or animals. However, both the parent compounds and their derivatives can be detrimental. [3, 19]. Heavy metals are of concern to human health due to their toxicity, carcinogenicity, and mutagenicity nature [2]. Heavy metals cannot be completely biodegraded but can only be converted to non-toxic forms, thereby interfering with the human body's metabolism.

Phthalates, bisphenols, and other household and personal care products (PCPs) are frequently released into the environment, exposing people to these chemicals through ingestion, inhalation, and dermal routes [16, 39, 40]. These chemicals enter the environment primarily due to the inefficiency of wastewater treatment in removing all pollutants [39]. Consequently, they contaminate the soil and can be absorbed by plants and bioaccumulate in organisms' fatty tissues and other organs. Consequently, humans can be re-exposed to these chemicals through the consumption of aquatic products and plants [39].

According to studies, exposure to these environmental pollutants can result in various adverse health effects (e.g., reproductive and developmental effects). Hence, assessing human exposure is an issue of concern [19, 39]. Although many epidemiological studies have been designed to estimate exposure to pollutants, “wastewater surveillance” has been introduced as a new non-invasive, cheap, and fast exposure monitoring method to determine human exposure to these chemicals in a large population [48]. However, there are some limitations and issues that will be described in the next section.

Recently, research has been directed toward using WBE to estimate human exposure to environmental pollutants using wastewater analysis. The researchers declare that WBE could be potentially used as an early warning system to promptly identify communities with a higher risk of exposure to pollutants [52]. The current review focuses on the feasibility of the WBE approach to assessing some environmental pollutants including pesticides, heavy metals, personal care products, phthalates, and bisphenols, emphasizing the limitations and challenges of previous studies in this field.

Challenges and limitations of WBE studies estimating the exposure to environmental pollutants

Monitoring of exposure to environmental pollutants is important for public health. WBE can provide useful information on human exposure at the population level. Although WBE has been used in several studies, some limitations exist regarding estimating exposure to environmental pollutants. The lack of reliable evidence regarding the limitations provided below is one of the main problems in using the WBE approach for studies on environmental pollutants.

Appropriate biomarker selection

The selection of appropriate biomarkers is one of the key concerns in the wastewater-based epidemiology approach. The composition of wastewater and also the operational parameters of the sewer are other contributing factors in degradation processes [22]. It is necessary to evaluate biomarker stability to address its suitability for the WBE approach to avoid underestimating chemical exposure or substance consumption. Some biomarkers are not specific to human metabolism, and lack of specificity may decrease or increase measured exposure levels [48]. Studies have been conducted to evaluate the stability of various biomarkers, including illicit drugs, new psychoactive substances (NPS), and pharmaceuticals [34, 42]. Some studies have used wastewater-based epidemiology (WBE) to assess population exposure to environmental pollutants such as pesticides [44, 45, 46], flame retardants [4, 5], plasticizers [18], and UV filters [36]. Researchers have investigated the stability of these biomarkers and found that most pesticides show little degradation when stored in raw wastewater at room temperature or 4ºC for 24 h [47]. However, certain pesticide metabolites can form up to 24% in wastewater stored under these conditions. Most phthalate biomarkers remain stable in raw wastewater at a pH of 2 and room temperature for 12 h, except for mono-ethyl phthalate (MEPH), which degrades by 30% within 12 h [18]. Several compounds commonly found in raw wastewater, including MEPH, MnBP, 5OH-MEHP, MiBP, MECPTP, and MEHA, experienced significant loss (ranging from 35 to 65%) after being stored in wastewater at 22 °C for 24 h. This indicates that these compounds cannot be relied upon as biomarkers to assess plasticizer exposure. Further research is necessary to understand the stability of environmental pollutants in sewers under real-world conditions.

The studies reported that 3-PBA is a common urine metabolic product of roughly 20 pyrethroids that cannot be differentiated; hence, it cannot be called an exclusive biomarker of a specific pesticide [47]. No study was found on establishing or using heavy metal biomarkers in WBE studies. However, three types of markers for the detection of heavy metals have been identified and suggested in the Markosian study, including parental substance biomarkers (PSBs), metabolic substance biomarkers (MSBs), and non-substance biomarkers (NSBs) [33]. If heavy metal enters the body to be affected by metabolic changes or interactions with other relevant molecules, it can lead to the production of MSBs and NSBs, while if it is excreted unchanged, it forms the PSB biomarker. As a result, according to metabolism, biomarkers can be measured to assess the level of exposure [33].

As mentioned earlier, these biomarkers have a selective function. The presence of a specific biomarker in urine or faeces indicates exposure to a specific metal; for example, the presence of MSB indicates the presence of a specific metal. While the presence of NSBs does not indicate specific metals, precautions should be taken when measuring this biomarker; for example, an increase in urinary δ-Aminolevulinic acid (δ-ALA) indicated overall metal exposure such as Pb, As, Mn, and possibly Cd. The presence of MT in urine or faeces can indirectly indicate many metals, including As, Cd, Cu, Hg, Pb, and Zn. This difference in WBE is a useful factor for measuring biomarkers [33].

It should be noted that MSBs and NSBs produced from metal metabolism in the human body have sufficient stability for sampling in the wastewater treatment plant (WWTP). Table 1 represents proposed biomarkers as indicators of WBE for heavy metals based on the review conducted by Markosian and Mirzoyan in 2019. The selection of biomarkers based on analogous toxicity endpoints can be a useful tool for predicting the risk of toxicity [47].

Table 1.

Suggested biomarkers of heavy metals and mixtures of these elements in WBE [33]

Metals	Media	(PSBs)*	(MSBs)*	(NSBs)*
Chromium	Urine	Total Cr(III)	Cr(III)	-
Copper	Urine	Total Cu	-	MT
Manganese	Urine	Total Mn	-	δ-ALA
Selenium	Urine	Total Se	MMSe, TMSe, SeCys, SeMet, SeUr, Se-methyl-N-acetylgalactosamine, Se-methyl-N-acetylglucosamine	-
Zinc	Urine	Total Zn	-	MT
Nickel	Urine	Total Ni	-	-
Arsenic (As)	Urine	Total As, arsenite, arsenate, MMA (V), DMA(V)	MMA(V), DMA(V), MMA(III), DMA(III), AsB	δ-ALA, MT, ZPP
Cadmium (Cd)	Urine	Total Cd	-	δ-ALA, MT, ZPP
Lead (Pb)	Urine	Total Pb	Diethyl Pb, ethyl Pb, Pb	δ-ALA, MT, ZPP
Mercury (Hg)		Total Hg, inorganic Hg, methyl Hg	Inorganic Hg	MT

MMA(V), monomethylarsonic acid; DMA(V), dimethylarsinic acid; MMA(III), monomethylarsonous acid; DMA(III), dimethylarsinous acid; AsB, arsenobetaine; δ-ALA, δaminolevulinic acid; MT, metallothionein; ZPP, Zn protoporphyrin; MMSe, monomethylselenium; TMSe, trimethylselenium; SeCys, selenocystine; SeMet, selenomethionine; SeUr, selenourea

^* [Parental substance biomarkers (PSBs), metabolic substance biomarkers (MSBs), and non-substance biomarkers (NSBs.]

Some studies on phthalates identified biomarkers that are used or proposed for the WBE approach to assess human exposure to these chemicals [29, 48]. These studies can help the researcher select appropriate biomarkers for phthalates; however, two points should be considered for the selection of phthalate biomarkers: 1) parent compounds are not considered appropriate biomarkers because of analytical challenges in laboratory determination, and 2) monoesters of long-chain phthalates have a low excretion rate and underestimation may occur; therefore, secondary metabolites are suggested to be considered due to their longer half-lives [24].

Bisphenol A is the most frequently used of the bisphenols, which are quickly conjugated and excreted mainly as BPA-Glu (glucuronide), according to biomonitoring studies[51]. However, BPA-Glu is not a good biomarker due to its fair stability and rapid hydrolysis by enzymes produced from fecal bacteria [18]. Although data on the metabolisms of other bisphenols is rare, BPA-Sulfate (BPA-SO₄) has been recognized as a suitable biomarker for BPA, according to studies [31]. Further studies must be designed to complete data on the human metabolism of bisphenol.

There are limited studies and knowledge on PCP biomarkers and their stability in sewers. The concern about metabolites is the excretion rate, which might be very low or insufficient amounts in urine [23, 26, 37]. Although quantitative excretion data are not available for most of the metabolites, other challenges in PCP metabolites are related to determining parent compounds only [21], so more pharmacokinetic studies are required considering exposure routes.

The next concern regarding biomarkers is their stability from transporting in sewer pipes to sampling, handling, and analysis in laboratories [30]. It should be pointed out that all metabolite stability studies are conducted only in laboratories with controlled conditions (mostly in defined time and pH at room temperature), and some others are limited to modeling studies; therefore, the main limitation is lacking enough field in-sewer studies under different sewer conditions for all discussed pollutants. However, some studies mimic real conditions in sewers.

In brief, as unspecific metabolites may be related to other possible exogenous sources and may not reflect human exposure, more studies are needed to identify and propose reliable and appropriate biomarkers of exposure.

Based on criteria proposed by experts in the WBE field, a suitable biomarker should have the following characteristics:

1. Stability in wastewater during sampling, transporting in the sewer, and analysis,

2. Detectability in wastewater,

3. Consistent excretion in urine,

4. Human metabolism and excretion are special sources of biomarkers and do not come from environmental sources [, 38,44, 47].

Additional sources and metabolites formation: Overestimation due to exogenous sources

Another challenging aspect of WBE is how to prohibit the presence of additional sources of parent substances and metabolites. Some metabolites can be formed spontaneously as metabolic processes or can be produced by other chemical classes. The wastewater is a complex matrix, and the most notable challenge is whether human excretion is the major source of the studied metabolites in the wastewater or whether exogenous sources are contributing. Although most studies have considered the assumption that human excretion is the primary source, other sources may be identified for some metabolites when studying environmental pollutants. The results of studies regarding investigated environmental pollutants are provided below.

Exogenous sources such as the mixing of wastewater with contaminated soil and water, leaching of metals from sewage system pipelines, and industrial effluents may cause overestimation of metal in wastewater analysis. However, no previous works related to heavy metals have focused on this issue.

Pollution by heavy metals in various environmental systems, such as the atmosphere, soil, water, etc., is a widespread issue. Accurately estimating the amount of heavy metals released and understanding their fate is crucial for health risk assessment and policy making [7].

Wastewater, which contains contaminants from domestic and industrial sources, has become a significant source of pollution in aquatic environments.

Previous studies have focused on discharging pollutants like heavy metals into water bodies [14, 15]. For instance, research in China revealed the release of 160,000 kg of total mercury and 280 kg of methylmercury from municipal wastewater into different environmental systems. The study mentioned in the text provided insights into releasing mercury (Hg) from municipal wastewater in China. Still, it did not include information about variations over time or the release of other heavy metals. It is recommended that the effluent of industries be controlled and analyzed and the amount of the pollutants from the industries be determined. In this way, the contribution of industries to the entry of pollutants can be considered in the calculations.

Studies on phthalate esters (PAEs) show that an individual's urine is unlikely to be the major source of these chemicals in wastewater. In a study, monoethyl phthalate (MEP) concentration was determined using both WBE and HBM approaches. The results showed that the estimated concentration by the WBE method (mean: 520 ng/ml) is higher than the concentration measured in HBM (mean: 69 ng/ml) in adults [17]. In this study, values measured in children's urine were higher than in WBE approaches because of the high exposure of children to phthalates. In another study, it was reported that the contribution of human excretion for short-chained phthalates biomarkers was less than 25% (MMP (0.33%), MiBP (5.4%), MBP (9.7%), MBzP (20%), and MEP (23%), indicating that the individual's urine is not the major source of these chemicals in wastewater [50]. As the mentioned profile may not be kept in the case of other phthalate esters (PAEs), it is recommended to conduct more studies considering all PAE metabolites separately. Further studies are also needed to compare the results of WBE and HBM to validate data.

Some reports indicated that most of the bisphenols, especially BPA, in wastewater are not from human excretion and are derived from hospital wastewater and landfill leachates mixed with municipal wastewater. While diet is known to be a primary source of BPA exposure, the non-dietary contribution influences daily intake significantly. [31]. For PCPs, it is hard to recognize the original sources and the extent of personal use and exposure to the substance due to the variety of sources of products.

Further studies are needed regarding the excretion rates of parabens, synthetic musks, and UV filters, and their sewer longevity. Benzophenone-3 (UV filter), has an estimated excretion rate of 1% [43].

The main challenge in connecting PCP concentrations in the influent to product usage and human health is the abundance of sources, difficulty in determining the original source, and the level of personal intake and exposure to these products. Another constraint is the lack of accurate excretion data for certain substances, making assessing human exposure to pollutants difficult. Some pesticides may originate from other sources, such as plasticizers or flame retardants that have been hydrolyzed, as well as other industrial compounds, and are thus not limited to human exposure [47]. For example, environmental transformation products are the other potential sources of atrazine, terbuthylazine, simazine, and propazine, whose biomarker is atrazine desisopropyl. Also, residential dust is considered an additional source of 20 pyrethroids, and 3-phenoxybenzoic acid (3-PBA) was the biomarker of exposure in the Rousis study [46].

In a study by Rousis et al. [45], wastewater-based epidemiology (WBE) was used to measure European pesticide exposure. This method was applied for the first time in eight cities across Europe. Urinary metabolites of three class of pesticides (triazines, organophosphates, and pyrethroids) were analyzed. The study found that spatial differences in exposure to insecticides in the cities aligned with national statistics on pesticide exposure. WBE proved to be a valuable tool in providing new information about the average exposure of the population to pesticides [6, 45].

In a study conducted by Devault et al. [13], the researchers investigated the exposure of an urban population to pesticides on a Caribbean Island using wastewater-based epidemiology. The study's main objective was to analyze and measure the patterns of human exposure and compare them with data from other countries. The results showed that the population had lower exposure to triazines and organophosphates but higher exposure to pyrethroids than certain European cities. It is important to note that the mass loads of pesticides detected may indicate human exposure. Still, the selected biomarkers used in the study may not be entirely specific to human metabolism and could be influenced by other sources [12, 13].

Other general limitations

The challenges provided below are not specifically for environmental pollutants and are considered general limitations in WBE studies, and very few studies have been designed to address these limitations.

Environmental conditions

Environmental conditions can boost the limitations of WBE. Environmental conditions such as temperature and pH can affect the stability of biomarkers, pesticides, or other chemicals degradation and metabolization [13]. At room temperature, the concentration of 2-isopropyl-6-methyl-4-pyrimidinol increased by 23.7%, whereas at 4 °C, it decreased (14.6%) [47].

As high temperatures can accelerate the degradation of the compounds or their metabolites, and affect the fate of metabolites in sewage in tropical conditions, metabolization in sewer should be considered and studied for pollutants. Therefore, it is strongly recommended to design more studies on this issue to get useful information on the degradation of different chemicals in different situations.

Urine as an inappropriate matrix for monitoring or estimating exposure to some chemicals

For some chemicals, including heavy metals, urine might not be an appropriate medium for biomonitoring in research. Despite the difficulty of sampling and being invasive, blood analysis can usually give reliable data in biomonitoring for metals, as the blood is in balance with the organs and tissues in which chemicals are stored in the body [27]. For example, lead (Pb) has a half-life in the blood of approximately 30 days, so the concentration of Pb in the blood indicates a history of exposure in recent months. Although urinary Pb as a substitute for blood lead levels has been investigated in studies, caution is needed for the following reasons: Urinary excretion of Pb reflects recent exposure, and these levels are close to the detection limit of the analytical methods for low-level exposures. Also, there is complexity in determining lead in urine by altering renal function.

On the contrary, arsenic (As) in blood exhibits recent exposures or exposures to high As levels, and the metalloid As is rapidly cleared from the blood. So urination is the main route of excretion of arsenic, and urinary measurements have been considered more reliable than blood. Total urinary levels of As are used as exposure biomarkers to determine the extent of past cumulative exposure to the metalloid [20]. WBE assumes that after exposure to metals, a portion is excreted unchanged or as a metabolite in the urine and faeces, so WBE has the potential to be used to assess population exposure to metals. However, selecting the appropriate matrix for detecting and biomonitoring chemicals regarding their half-lives and other influencing factors should be considered.

Low concentration of some biomarkers

The concentration of some biomarkers in urine is very low, and when it enters the wastewater, it will certainly decrease due to the dilution in a high volume of wastewater. Therefore, the diluted sample of the urine of a community can affect the analysis in the laboratories, and low amounts of some biomarkers in wastewater samples might hamper reliable quantitative determination. This challenge may be related to all biomarkers because their excretion rates are very low.

Analytical problems

Wastewater matrix is one of the most common analytical difficulties, followed by the importance of advanced analytical methods [35]. Analytical problems can arise in wastewater-based epidemiology (WBE) due to low quantities of biomarkers in urine, making detection in sewage difficult. Only one study has examined the relevance of biomarkers in suspended solids [31]. Furthermore, while the stability of some biomarkers is typically examined during method development, more comprehensive in-sewer stability experiments are needed to represent real sewage conditions accurately. These experiments should be conducted in the future to improve the accuracy of WBE [5, 28].

Exposure back-calculation

One of the most significant current challenges in WBE for most of the substances is back-calculation. The daily sewer loads of target residues are determined by multiplying the concentrations of the detected target residues by the daily sewage flow rates, which is how the back-calculation of consumption or exposure to a specified substance is carried out. Using a particular correction factor, considering the molecular mass ratio of the parent substances to its metabolite as well as the average excretion rate of a target residue, the total consumption is determined. To make comparisons between cities easier, daily values are divided by the total population that the treatment facility serves. Daily amounts (or daily doses) per thousand people is one way to express this number. The reliability of extrapolating from a small number of wastewater treatment plants to the entire nation is hard to estimate. It is advised to (i) cover a large portion of the population, (ii) rely on a good geographic distribution of places, and (iii) rely on a variety of town sizes and types. In order to estimate a national average, thresholds of at least 10% of the population and/or a minimum of five wastewater treatment plants have been applied recently.

A lack of understanding of most chemicals' fate and metabolism patterns makes it difficult to back-calculate intake. In addition, widely used chemicals such as phthalates in industrial or commercial sectors, which are unrelated to human usage, may cause some uncertainties in the obtained data in back-calculation and, consequently, in the WBE approach. For a limited number of compounds and nations, a relationship between consumption and the catchment area (population) of wastewater treatment plants was discovered.

Exposure estimation only at a community level

In contrast to human biomonitoring (HBM), the WBE does not provide data at the individual level (i.e., mg/l/person), but rather at the community level (mg/l/1000 persons). This makes it difficult to compare data in different age categories, children and adults or men and women. The results of WBE studies may also be confounded by the fact that they are one step away from the source (human), which can result in overestimations due to additional sources or lower estimations because of the instability of some biomarkers [44].

All the above-mentioned limitations and challenges may hamper obtaining accurate and reliable results in the WBE studies. In summary, all important issues related to WBE studies for estimating human exposure to environmental pollutants are provided in Fig. 1.

Fig. 1.

Summary of challenges of the WBE approach for estimation of human exposure to environmental pollutants

Although human biological monitoring (HBM) is the most common method to assess environmental and occupational exposure to chemicals, its invasiveness, high cost and time, ethical issues, and sampling bias make it less popular [25]. There are also problems in extrapolating individual results from HBM to the target population. The WBE method has the potential to overcome these problems. It can overcome the main limitation of HBM studies, which cover a small percentage of the population compared to the WBE.

Future perspective, gaps, and recommendation

The WBE method has the potential to be known as an early warning system for exposure in a large population by providing data on spatial and temporal trends at a community-wide level. It is also noteworthy that WBE, as an inexpensive tool, provides an opportunity to achieve public health information through faster analysis of fewer samples compared to the human biomonitoring approach. Therefore, in the future, WBE can be considered a relatively novel approach for achieving epidemiological information regarding exposure to pollutants and public health. Consequently, it can help researchers identify areas where the population is most at risk.

WBE could be used to evaluate the current programs as well as the effects of the programs implemented by the relevant organization to reduce population exposure to pollutants. Studies in wastewater-based epidemiology areas to assess pesticide exposure showed that WBE has the potential to be a useful complementary biomonitoring tool. Tracing substance resistance could become a significant role of WBE in exposure estimation and public health, although this role is still a prospect [32, 47]. Trying to overcome the challenges associated with pesticides and their metabolite detection in wastewater strengthens the hypothesis that WBE can be a helpful complementary biomonitoring tool. Also, a comparison of the results of biomonitoring studies with WBE results can strengthen the evidence for WBE as a biomonitoring tool that can cover a wider population than traditional biomonitoring tools [12, 35].

In cases such as overestimation that cannot be representative of metabolic products with particular metal exposure, the concentration of metal-measured biomarkers should be considered cautiously [33]. By recognizing input source data for heavy metals, WBE will be improved depending on advances in fields such as metabolomics to provide meaningful information on general population health.

As well as other environmental pollutants, wastewater samples usually contain metals at concentrations above background levels and can be used for accessing heavy metal biomarkers. However, due to the wide presence and numerous potential sources of metals in the environment, it is necessary to consider the sampling location. For this problem, more acceptable results will be obtained if the samples are collected at more points along the treatment plant route or if the samples are combined.

The wastewater-based epidemiology studies to date have focused only on the estimation of community exposure to some chemicals, especially phthalates, parabens, and bisphenols, and the pharmacokinetic studies are still insufficient. They should be considered for different routes of exposure and in representative populations. More pharmacokinetic studies are needed to design for plasticizers and other chemicals. These studies can provide data on the rate of excretion, which is a key parameter for monitoring environmental pollution exposure in a community.

Few researchers have addressed the instability of potential biomarkers; only in-sample experiments have been conducted in laboratories under controlled conditions, and in-sewer stability studies considering complex conditions in real sewers are missing. Also, far too little attention has been paid to the possible contribution of interesting biomarkers from other sources. Therefore, further studies are needed to be designed to recognize additional contributing sources of phthalates, BPA, and other contaminants as biomarkers in wastewater other than human excretion [31]. In WBE as an interdisciplinary activity, expertise from epidemiology, environmental science, medicine, public health, and analytical chemists,are involved. Therefore, interdisciplinary collaboration in addressing the challenges of WBE is important to discuss enhancing the robustness of WBE studies.

Standardization and harmonization in WBE studies are also important to enhance comparability across different studies and regions.

Wastewater-based epidemiology would also be applied for direct measures of general population health by measuring biomarkers of health, food, and diet in the future. Although there are associations between the social, demographic, and economic characteristics of the various populations and the chemicals found in wastewater by prediction and modeling studies [9, 10], more research is needed to determine whether WBE measurements and socioeconomic or environmental agents can be used as effective tools to reach insights into public health.

Conclusion

Wastewater-based epidemiology (WBE) is a useful tool for assessing the health profiles of different populations. It involves monitoring biomarkers in wastewater to detect contamination and identify new substances and metabolites. WBE can be particularly beneficial in large communities where other methods like biomonitoring are not cost-effective or feasible. Furthermore, WBE is a useful tool for biomarkers that reflect healthy conditions, lifestyle, disease identification, and pollutant exposure. However, there are challenges associated with WBE, including selecting appropriate biomarkers, potential overestimation due to external sources, low concentration of certain biomarkers, exposure back-calculation, environmental conditions, and dealing with analytical issues related to the wastewater matrix. There is a substantial lack of information regarding these substances' excretion rates, sewer longevity, and sources. These limitations can affect calculations for assessing human exposure to pollutants. Although this approach can potentially be used as a biomonitoring tool in large communities, more metabolites must be investigated in wastewater to improve future studies.

Scientists, researchers, and policymakers are recommended to allocate more funds to WBE studies in order to acknowledge the necessity of sustained efforts to improve the approach and maximize its impact.

Authors contributions

M.A: Conceptualization, searching, drafting the manuscript, N.K. H. J.: searching, drafting the manuscript, M.H.D. R.R.K.: Conceptualization and design of study, Supervision, review & editing.

Funding

No funding was received for conducting this study.

Data availability

Not applicable.

Declarations

Competing interests

The authors of this review declare that they have no conflict of interest.