SUSTAINABILITY INDICATORS FOR END-OF-LIFE CHEMICAL RELEASES AND POTENTIAL EXPOSURE

Abstract

Understanding the chemical risk to environment and human health is an important issue when a waste management strategy and a control risk system is analyzed and selected. This is even more important at the end-of-life (recycling, recovery and disposal) scenario for a chemical due to the uncertainty in respect of the most susceptible receptors (e.g., workers), pathways (e.g., groundwater), routes (e.g., inhalation) and hazard (e.g., cancer) associated to a chemical exposure. Hence, selecting a group of sustainability performance indicators for estimating the chemical risk when evaluating end-of-life scenarios is a crucial task. Therefore, this manuscript focuses on a critical analysis of the sustainability indicators taxonomy which are used to assess chemical risk to the environment and human health during end-of-life scenarios. The insights from performing an extensive literature search in the largest database of peer-reviewed literature provide that chemical intake, hazard quotient, hazard index, and carcinogenic risk have been the most commonly used for human health chemical risk. In addition, previous research has been less focused on environment chemical risk, with ecological risk index being the most widely used indicator for. The most employed human health chemical risk sustainability indicators are part of a methodology suggested by U.S. Environmental Protection Agency for chemical risk assessment.

Graphical Abstract

1. Introduction

The interest for understanding the risk to both environment and human health which a chemical may have on its compound life-cycle (CLC) has been increasing over the years. For that reason, the amended Toxic Substance Control Act (TSCA) has given the U.S. Environmental Protection Agency (U.S. EPA) the authority for developing and implementing an evaluation procedure to determine whether a chemical at U.S. market exhibits an unreasonable risk to environment and human health through its manufacturing, processing, use (industrial, commercial and consumer), and disposal [1].

The end-of-life (EoL) scenarios that a chemical may have is one of the main concern areas when performing risk evaluation due to its high uncertainty level as well as the lack of studies and information for achieving some sustainable waste management goals (e.g., recycling, recovery, etc.) [2]. Currently, EoL studies consider risk and exposure during the disposal (e.g., landfilling, incineration) of chemical waste. However, an unforeseen effect is the proliferation of new chemical exposure pathways and scenarios when combining many EoL consumer products from different use (e.g., plastics) for performing industrial scale recycling and recovery and convert these as feedstocks for the manufacturing of new goods for new uses.

According to the Plan-Do-Check-Act (PDCA) cycle, “if you cannot measure it, you cannot manage it”. For that reason, it is recommended to identify of the most relevant sustainability indicators which help to quantify the performance of different EoL scenarios for chemical waste. Thus, the aim of this work is to provide a critical analysis of the sustainability indicators taxonomy for assessing the chemical risk to environment and human health in EoL scenarios, for supporting an appropriate selection of a sustainable chemical waste management and risk control strategy. For doing this, a literature search strategy is performed, using Scopus as the literature search manager, a search equation, which has a combination of search terms such as end-of-life, chemical risk, environment, human health, and waste management, and terms related to waste management strategies (e.g., landfill, recycling, etc.). Other search parameters employed as a filter, are the manuscripts published since 2008, which have the search terms in their title, keywords, and/or abstracts. After that, a skimming is used to select the most relevant papers according to the searching criteria parameters. Finally, an exhaustive and complete reading and evaluation is used as the last filter to select the most appropriate manuscripts (see Table 2).

Table 2.

EoL usage context of sustainability indicators in literature.

Paper	Context	Risk assessment indicator
		Environment								Human health
		1e	2e	3e	4e	5e	6e	7e	8e	1h	2h	3h	4h	5h	6h
[5]	Mechanical recycling, incineration for energy recovery and landfilling of waste plastic	x	x	x						x
[6]	Polybrominated Diphenyl Ethers (PBDEs) and Alternative Halogenated Flame Retardant (AHFR) in abandoned e-waste recycling sites										x
[7]	Heavy metals in e-waste processing sites: dismantling, burning, acid-leaching, and abandoned sites				x	x					x	x
[8]	PBDEs and heavy metals at informal e-waste recycling sites										x	x	x
[9]	Air emissions from landfill and composting facilities													x
[10]	Reuse and recycling of pavement systems										x		x
[11]	PBDEs at e-waste recycling workshops										x	x
[12]	Typical water reclamation plants										x	x		x	x
[13]	Metals present in waste dumps from an abandoned mine										x	x	x		x
[14]	Heavy metals in abandoned mines						x	x			x	x	x		x
[15]	Pyrolysis and combustion of non-metallic fraction of printed circuit boards					x
[16]	Chemical pyrolysis of non-metallic fraction of printed circuit boards					x
[17]	Chemicals in recycled rubber										x	x	x	x
[18]	Heavy metals in e-waste recycling sites										x	x		x
[19]	Polycyclic Aromatic Hydrocarbons (PAHs), Persistent Organic Pollutants (POPs), and Polychlorinated Biphenyls (PCBs) in landfills												x	x
[20]	Polychlorinated Dibenzo-p-Dioxins and Dibenzofurans (PCDD/Fs)										x		x
[21]	Analysis of landfill soils													x
[22]	Groundwater near a non-sanitary landfill facility										x			x
[23]	Solid waste management by means of landfills and thermal treatment										x	x	x	x	x
[24]	Heavy metals in informal e-waste recycling sites				x				x
[25]	Aromatic compounds in landfill										x	x		x	x
[26]	POPs in waste dumping sites										x			x
[27]	Holistic risk analysis approach in landfills										x	x	x	x	x
[28]	Heavy metals in e-waste recycling sites										x		x	x
[29]	Sediments and water polluted by heavy metals from a landfill					x
[30]	Mobile e-waste recycling plant										x	x	x	x
[31]	Air emission from a landfill										x	x	x	x
[32]	Volatile Organic Compounds (VOCs) from plastic solid wastes										x	x	x	x
[33]	Integrated waste management facility										x	x
[34]	Landfilling and recycling of cadmium telluride thin-film photovoltaic panels										x	x	x
[35]	Solid waste landfilling										x	x	x	x
[36]	Municipal waste organic fraction treatment plant										x	x	x	x	x
[37]	Municipal solid waste incineration plant										x		x	x
Number of times that the indicator was used		1	1	1	2	4	1	1	1	1	25	18	17	18	7

2. Risk evaluation elements

The risk evaluation process is composed by different elements which are important to understand before selecting the indicators. These elements can be identified by TSCA and defined in a conceptual model when the problem formulation for risk evaluation is made. These elements are summarized in Figure 1.

Figure 1.

A generic conceptual model with example of possible elements and linkages for risk evaluation (adopted from U.S. EPA [3]).

In Figure 1 is represented the four elements considered in the search and identification of the sustainability indicators taxonomy for chemical risk: (a) the first one is the stressor which is defined as a physical, chemical, biological, or other entity that can cause an adverse response in a human or other organism or ecosystem [3], namely, those chemical stressors which cause some hazard risk under some conditions. (b) receptors which are the agents exposed to a chemical stressor, for instance, workers and occupational non-users at the waste management facilities, as well as general population, and aquatic and terrestrial species on the outside of the above plants. (c) exposure pathway and route which are elements establishing a relationship between the chemical stressor and the final receptor. An exposure pathway is the physical course (fate and transport) that a chemical or pollutant takes from its source to the receptor, while an exposure route is the way a chemical or pollutant enters an organism after contact, e.g., ingestion, inhalation, or dermal contact [4]. (d) the final element to be considered is the hazard also known as endpoint. As it is in Figure 1, the hazard is the effect of a chemical stressor has in a susceptible receptor such as cancer.

3. Methodology

The literature search strategy used here was performed on Scopus as the literature search manager. The search equation was (End-of-Life .OR. Waste Management .OR. Other More Specific Search Term) .AND. Chemical Risk .AND. (Environment .OR. Human Health). A more specific term in the above equation refers to a waste management alternative such as landfill, recycling, recovery, etc. Each paper found by Scopus had to have the terms in the search equation in its title, keywords, and/or abstract, as well as published from 2008, which is the year after the U.S. EPA Science Advisory Board (SAB) provided advice on updating U.S. EPA’s exposure guidelines and enhancing risk assessment practices².

In addition, using Scopus the manuscripts were sorted according to their relevance in descending order. After obtaining the papers, skimming these was used as a filter to determine if they may be relevant for the scope of this work. Therefore, based on the searching parameters, the selected articles were passed through the last filter, which was an exhaustive and complete reading of each manuscript to select the most appropriate studies based on the fact that the objective of these would have been to assess the chemical risk during an EoL scenario.

4. EoL chemical risk sustainability indicators

Table 1 shows the sustainability indicators found in the literature to estimate the chemical risk in EoL scenarios. A group of 14 indicators were employed in previous works, where 8 of these were used to conduct environmental chemical risk assessment (from 1e to 8e) while only 6 are employed for human health chemical risk assessment (from 1h to 6h).

Table 1.

Sustainability indicators using in the literature to assess chemical risk at EoL scenarios.

N°	Indicator	Receptor	Exposure pathway*	Exposure routes*	Type of hazard*
1e	Marine ecotoxicity potential	Aquatic organisms	Surface water	By interaction	Acute, chronic, subchronic toxicity
2e	Freshwater ecotoxicity potential	Aquatic organisms	Surface water	By interaction	Acute, chronic, subchronic toxicity
3e	Terrestrial ecotoxicity potential	Terrestrial organisms	Outdoor air	By interaction	Acute, chronic, subchronic toxicity
4e	Potential ecological risk index	Terrestrial organisms	Soil	By interaction	Acute, chronic, subchronic toxicity
5e	Ecological risk index	Terrestrial organisms	Soil	By interaction	Acute, chronic, subchronic toxicity
6e	Risk to groundwater due to the contaminant in superficial soil	Aquatic organisms	Ground water	By interaction	Acute, chronic, subchronic toxicity
7e	Risk to groundwater due to the contaminant in deep soil	Aquatic organisms	Ground water	By interaction	Acute, chronic, subchronic toxicity
8e	Risk assessment code	Terrestrial organisms	Soil	By interaction	Acute, chronic, subchronic toxicity
1h	Human toxicity potential	General population	Outdoor air	Inhalation	Cancer, noncancer, genotoxicity
2h	Hazard quotient	General population, workers	Food, soil, air, dust	Ingestion, inhalation, dermal contact	Noncancer, genotoxicity
3h	Hazard index	General population, workers	Food, soil, air, dust	Ingestion, inhalation, dermal contact	Noncancer, genotoxicity
4h	carcinogenic risk	General population, workers	Dust, soil, food, air, surface water	Ingestion, inhalation, dermal contact	Cancer
5h	Chemical intake	General population, workers	Air, products, soil, food, water	Ingestion, inhalation, dermal contact	Cancer, noncancer, genotoxicity
6h	Risk index	General population, workers	Surface water. soil	Ingestion, inhalation, dermal contact	Cancer

^*The exposure pathways and routes, as well as the hazards and susceptible receptors in the table are retrieved from the reviewed literature.

Although there are qualitative and quantitative indicators for sustainability evaluation, but from the literature review, it is possible to notice that all of the indicators used for risk assessment are quantitative, which is expected due to the high needs of objectivity associated with risk evaluation. Additionally, the indicators to assess the environmental chemical risk consider some aspects like acute, chronic, and subchronic toxicity for both terrestrial and aquatic organisms, where the exposure pathways which were analyzed included ground and surface water, soil, and outdoor air.

As presented in Table 1, from the reviewed literature, the exposure pathways cancer, noncancer and genotoxicity effect of the chemical stressors in the human health chemical risk assessment, outdoor and indoor air, dust, deep and surface soil, ground and surface water, food were contemplated during chemical risk evaluation in EoL scenarios. Moreover, inhalation, ingestion and dermal absorption were taken in consideration.

The Table 2 presents the most remarkable manuscripts which were retrieved from the literature and the sustainability indicators to estimate environmental and/or human health chemical risk employed by each work. As it is possible to see at the bottom of Table 2, the main objective in these studies was the effect of a chemical stressor in human health according to the number of times of the human health risk assessment indicators were used.

In addition, the indicator 2h (hazard quotient, HQ) was the most widely used indicator for quantifying the human health chemical risk. It is important to highlight that this indicator is collected from the U.S. EPA’s Risk Assessment Guidelines for Superfund (RAGS), where HQ represents the ratio of a single substance exposure level over a specified time period (e.g., subchronic) to a reference dose for that substance derived from a similar exposure period [38]. In that way, HQ is a suitable indicator to evaluate chemical risk in different kind of exposure scenarios such as EoL chemical waste management.

From Table 2, it is also possible to grasp that indicators 3h (Hazard index, HI), 4h (carcinogenic risk), and 5h (Chemical intake) are also widely used. These are part of the U.S. EPA’s RAGS as well. HI is the sum of more than one HQ for multiple substances and/or multiple exposure pathways. The HI is calculated separately for chronic, subchronic, and shorter-duration exposures [38]. The indicator 5h is used many times to estimate the exposure level to a chemical substance over a specified period (numerator of HQ).

The indicator 4h, which is used 17 times, is suggested to assess carcinogenic risk to human health. This indicator is based on the Slope Factor, which is a plausible upper-bound estimate of the probability of a response per unit intake of a chemical over a lifetime [38], and the chemical intake by means of whatever exposure pathway and route is normally estimated using the indicator 5h.

As is described in Table 2, the abovementioned indicators have a wide range of application, from estimating the exposure to a chemical in a consumer product made from recycled materials to the chemical wastes generated and released during the different EoL scenarios including the remnants of a disposed chemical substance.

5. Conclusions

According to the performed exhaustive technical literature review and analysis, previous studies employed for human health chemical risk assessment, show that HQ, HI, chemical intake, and carcinogenic risk have been widely used to assess the chemical risk during EoL scenarios no matter the exposure pathway and route, human receptor, and end-point effect. The application, use, and validation of those indicators by the scientific community show the precedent and usefulness of the U.S. EPA’s RAGS for TSCA chemical risk evaluation under different scenarios along its CLC where the human health may be affected. However, by performing an extensive literature search in the largest database of peer-reviewed literature, it suggests that for future advances some performance indicators capable of assessing environmental and human health risk at EoL due to releases to air, soil, and water should be included. These indicators will have to deal with the uncertainty associated with new chemical exposure pathways and scenarios when combining many EoL consumer products from different use (e.g., plastics) for performing industrial scale recycling and recovery (and their releases) and convert these as feedstocks for the manufacturing of new goods for new uses. Moreover, HI would allow decision-makers perform some aggregation of risk from multiple chemical stressors, cumulative risk, which is especially important during EoL due to the high uncertainty in the composition of chemical waste from different sources for further management.