Species Sensitivity Distribution Approach Research Articles

A Chronic Aquatic Hazard Assessment for the Perfume Raw Material Octahydro-tetramethyl-naphthalenyl-ethanone.

Octahydro-tetramethyl-naphthalenyl-ethanone (OTNE) is a high-production volume fragrance material used in various down-the-drain consumer products. To assess aquatic risk, the Research Institute for Fragrance Materials (RIFM) uses a tiered data-driven framework to determine a risk characterization ratio, where the ratio of the predicted-environmental concentration to the predicted-no-effect concentration (PNEC) of <1 indicates an acceptable level of risk. Owing to its high production volume and the conservative nature of the RIFM framework, RIFM identified the need to utilize a species sensitivity distribution (SSD) approach to reduce the PNEC uncertainty for OTNE. Adding to the existing Daphnia magna, Danio rerio, and Desmodesmus subspicatus chronic studies, eight new chronic toxicity studies were conducted on the following species: Navicula pelliculosa, Chironomus riparius, Lemna gibba, Ceriodaphnia dubia, Hyalella azteca, Pimephales promelas, Anabaena flos-aquae, and Daphnia pulex. All toxicity data were summarized as chronic 10% effect concentration estimates using the most sensitive biological response. Daphnia magna was the most sensitive (0.032 mg/L), and D. subspicatus was the least sensitive (>2.6 mg/L, the OTNE solubility limit). The 5th percentile hazardous concentration (HC5) derived from the cumulative probability distribution of the chronic toxicity values for the 11 species was determined to be 0.0498 mg/L (95% confidence interval 0.0097-0.1159 mg/L). A series of "leave-one-out" and "add-one-in" simulations indicated the SSD was stable and robust. Add-one-in simulations determined that the probability of finding a species sensitive enough to lower the HC5 two- or threefold was 1/504 and 1/15,300, respectively. Given the high statistical confidence in this robust SSD, an additional application factor protection is likely not necessary. Nevertheless, to further ensure the protection of the environment, an application factor of 2 to the HC5, resulting in a PNEC of 0.0249 mg/L, is recommended. When combined with environmental exposure information, the overall hazard assessment is suitable for a probabilistic environmental risk assessment. Environ Toxicol Chem 2024;00:1-12. © 2024 SETAC.

Aquatic and sediment ecotoxicity data of difenoconazole and its potential environmental risks in ponds bordering rice paddies

Difenoconazole has a widespread agricultural use to control fungal diseases in crops, including rice. In edge-of-field surface waters the residues of this lipophilic fungicide may be toxic to both pelagic and benthic organisms. To allow an effect assessment we mined the regulatory and open literature for aquatic toxicity data. Since published sediment toxicity data were scarce we conducted 28 d sediment-spiked toxicity test with 8 species of benthic macroinvertebrates. Ecotoxicological threshold levels for effects were assessed by applying the species sensitivity distribution approach. Based on short-term L(E)C50’s for aquatic organisms from water-only tests an acute Hazardous Concentration to 5% of the species (HC5) of 100 µg difenoconazole/L was obtained, while the HC5 based on chronic NOEC values was a factor of 104 lower (0.96 µg difenoconazole/L). For benthic macroinvertebrates the chronic HC5, based on 28d-L(E)C10 values, was 0.82 mg difenoconazole/kg dry weight sediment. To allow a risk assessment for water- and sediment-dwelling organisms, exposure concentrations were predicted for the water and sediment compartment of an edge-of-field pond bordering rice paddies treated with difenoconazole using the Chinese Top-Rice modelling approach, the Chinese Nanchang exposure scenario and the Equilibrium Partitioning theory. It appeared that in the vast majority of the 20 climate years simulated, potential risks to aquatic and sediment organisms cannot be excluded. Although the HC5 values based on laboratory toxicity data provide one line of evidence only, our evaluation suggests population- and community-level effects on these organisms due to chronic risks in particular.

Analyzing species sensitivity distribution of evidently edible microplastics for freshwater biota

Assessing the ecological risks of microplastics is difficult because of the limited availability of reliable ecotoxicity data. Although freshwater is a valuable sink for microplastics, the current framework for ecological risk assessment using traditional toxicity data is not applicable to freshwater ecosystems. Herein, species sensitivity distribution (SSD) curves were compared for edible and all microplastics exposed to aquatic organisms based on traditional endpoint-based and all-endpoint-based databases. Freshwater toxicity data for microplastics were screened after verifying microplastic presence in test species (56 toxicity datapoints for one microalga, three water fleas, one fish, and one crab; 0.02–100 µm-sized microplastics). SSD and curve parameters were compared with or without non-traditional toxicity endpoints. The HC50 in all endpoint databases was more sensitive than that in the traditional endpoint database and showed a good fit. SSD curves derived from the database for all microplastics were compared and analyzed with edible microplastics. HCx increased for edible microplastics (0.02–100 µm-sized) than for all microplastics (0.02–200 µm-sized), and the size of edible microplastics was lower than of all microplastics. Thus, using non-traditional toxicity data, the SSD approach compensates for the limited ecotoxicity data on microplastics while considering the internalization of microplastics in biota.

Determination of soil environment criteria for ecological safety of benzo[a]pyrene in soil based on the species sensitivity distribution approach

Benzo[a]pyrene (BaP) is a highly carcinogenic organic pollutant. Its distinct toxic effects have raised significant concerns among regulators and researchers. However, the absence of established soil environmental criteria for ecological safety of BaP has led to a lack of evidence in the current risk assessment for soil ecological security. To address this gap, we conducted a comprehensive study to investigate the toxicity of BaP using multiple soil organisms, including rice, wheat, lettuce, rape, cucumber, tomato, leonurus, and earthworm. The toxicity data obtained were applied to the species sensitivity distribution (SSD) approach, and the soil environment criteria for ecological safety of BaP under different land use types were derived. The soil environmental criteria for ecological safety of BaP were estimated to be 1.07 (Natural reserve and agricultural land), 1.63 (Parkland), 3.84 (Residential land) and 6.46 (Commercial service and industrial land) mg·kg−1, respectively. This study is the first to conduct toxicity testing and derive soil environmental criteria of BaP in terrestrial ecosystems. These findings are effective supplement for the current soil environmental quality standards and will function as the basis for ecological risk assessment of BaP.

Chronic toxicity testing including transcriptomics-based molecular profiling in Cloeon dipterum

The so-called EPT taxa have been shown to be highly sensitive to various environmental pollutants. However, there are only few published studies on toxicity testing with EPT representatives and there is a particular lack of protocols for chronic toxicity testing, e.g., for integration into species sensitivity distribution (SSD) approaches. To address this gap, we performed a long-term 38-day semi-static toxicity test with the European mayfly species Cloeon dipterum using the insecticide fipronil as model substance. The functionality of the test system was confirmed by the high emergence rate of 85% in the control condition. We found a high sensitivity with regard to larval development with an EC50 of 180 ng/L and a NOEC of 38.0 ng/L after 7 days exposure. After 38 days, an LC50 value of 185 ng/L and an EC50 value of 160 ng/L for emergence (both: NOEC = 38.0 ng/L) were calculated. In a short-term 7-day toxicity test, we found a similar effect on larval development. In addition to the physiological endpoints, we examined fipronil-induced gene expression changes at the transcriptome level in this test. Our results revealed a concentration-dependent increase in the number of differentially expressed genes, as well as observed effects on larval development. Notably, we identified marker gene candidates involved in nervous system development, mirroring the known mode-of-action of fipronil in C. dipterum. The affected genes primarily play crucial roles in neurological processes. Concluding, within this two-step approach we were able to identify fipronil effects on the sublethal physiological endpoint larval development and to complement these effects at the molecular level by gene expression changes in the transcriptome. Thus, this assay proved to be suitable to assess sublethal effects as well as the mode-of-action of substances in the non-standard organism C. dipterum already after a short-term exposure of 7 days. However, further testing is required to validate the procedure.

Deriving seawater quality criteria of tris(2-chloroethyl) phosphate for ecological risk assessment in China seas through species sensitivity distributions

Tris(2-chloroethyl) phosphate (TCEP), one of the widely used organophosphorus flame retardants (OPFRs), has been frequently detected in the marine environment in the seas off China. The existing freshwater biotoxicity data are not suited to derivation of the seawater quality criteria of TCEP and evaluating the associated ecological risks. This study aimed at deriving water quality criteria (WQC) of TCEP for marine organisms based on species sensitivity distribution (SSD) approach using the acute toxicity data generated from multispecies bioassays and chronic toxicity data by converting acute data with the acute-to-chronic ratios (ACRs); the derived WQC were then used to evaluate the ecological risk for TCEP in China Seas. According to median effective concentration (EC50) and median lethal concentration (LC50), TCEP had a moderate or low toxicity to eight marine species selected, among which mysid Neomysis awatschensis (96h-LC50 of 39.65 mg/L) and green alga Platymonas subcordiformis (96-h EC50 of 395.42 mg/L) were the most sensitive and the most tolerant, respectively. The acute and chronic hazardous concentrations of TCEP for 5% of marine species (HC5) were estimated to be 29.55 and 2.68 mg/L, respectively. The short-term and long-term WQC were derived to be 9.85 and 0.89 mg/L, respectively. The risk quotient (RQ) values indicated that TECP at current levels poses a negligible risk to marine ecosystems in China. These results will provide valuable reference for the government to establish a seawater quality standard for TCEP.

Integrative ecotoxicity evaluation of Cd, Cu, Zn and Ni in aquatic animals reveals high tolerance of Artemia franciscana

Heavy metals pose a significant threat to animals in aquatic environments due to the adverse effects they exert. Species of the genus Artemia have been described as heavy metal tolerant, but the sensitivity/tolerance range for these species has not been established. In the present study, the toxicity of Cd, Cu, Zn and Ni as reported in the ECOTOX and Web of Science databases was examined for Artemia franciscana and compared with other species and taxonomic groups using an integrative ecotoxicity evaluation. The hazard concentration for 5% of the species (HC5) of acute toxicity tests (24–96 h), obtained through a species sensitivity distribution (SSD) indicated that Cu (0.02 mg/L) and Cd (0.03 mg/L) were the metals with the highest toxicity to aquatic animals followed by Zn (0.15 mg/L) and Ni (0.23 mg/L). In addition to the higher hazard of Cu and Cd to aquatic animals, the comparison of acute LC50 values for A. franciscana indicates lower toxicity of Cd followed by Cu, Zn, and Ni (200.0, 14.5, 9.5, and 0.6 mg/L, respectively). Using the SSD and physiological sensitivity (S) approaches, it was demonstrated that A. franciscana is relatively tolerant to Cd (SSD= HC99; S = 2.21), Cu (SSD= HC97; S = 2.00), Zn (SSD= HC90; S = 1.29) and Ni (SSD= HC83; S = 0.96) compared with other species and taxonomic groups. It appears that tolerance to the metals Cd, Cu, Zn and Ni is unique to the family Artemiidae within the order Anostraca, as the families Streptocephalidae and Thamnocephalidae are not tolerant (have negative S values). Our study confirmed that as expected, A. franciscana presents higher tolerance to Cd, Cu, Zn, and Ni than other aquatic animals. Our findings confirm that A. franciscana can be used as a model organism to understand mechanisms involved in tolerance to heavy metals, mainly Cd and Cu, which are considered highly toxic to other animals.

Advantages of model averaging of species sensitivity distributions used for regulating produced water discharges.

Produced water (PW) generated by Australian offshore oil and gas activities is typically discharged to the ocean after treatment. These complex mixtures of organic and inorganic compounds can pose significant environmental risk to receiving waters, if not managed appropriately. Oil and gas operators in Australia are required to demonstrate that environmental impacts of their activity are managed to levels that are as low as reasonably practicable, for example, through risk assessments comparing predicted no-effect concentrations (PNECs) with predicted environmental concentrations of PW. Probabilistic species sensitivity distribution (SSD) approaches are increasingly being used to derive PW PNECs and subsequently calculating dilutions of PW (termed "safe" dilutions) required to protect a nominated percentage of species in the receiving environment (e.g., 95% and 99% or PC95 and PC99, respectively). Limitations associated with SSDs include fitting a single model to small (six to eight species) data sets, resulting in large uncertainty (very wide 95% confidence limits) in the region associated with PC99 and PC95 results. Recent advances in SSD methodology, in the form of model averaging, claim to overcome some of these limitations by applying the average model fit of multiple models to a data set. We assessed the advantages and limitations of four different SSD software packages for determining PNECs for five PWs from a gas and condensate platform off the North West Shelf of Australia. Model averaging reduced occurrences of extreme uncertainty around PC95 and PC99 values compared with single model fitting and was less prone to the derivation of overly conservative PC99 and PC95 values that resulted from lack of fit to single models. Our results support the use of model averaging for improved robustness in derived PNEC and subsequent "safe" dilution values for PW discharge management and risk assessment. In addition, we present and discuss the toxicity of PW considering the paucity of such information in peer-reviewed literature. Integr Environ Assess Manag 2024;20:498-517. © 2023 Commonwealth Scientific and Industrial Research Organisation. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Non-traditional species sensitivity distribution approaches to analyze hazardous concentrations of microplastics in marine water

Owing to their ubiquitous nature, microplastics are a major environmental concern. This study reviewed the toxicity data of microplastics in marine water, and analyzed their species sensitivity distribution (SSD) curves and hazardous concentrations (HCs). Toxicity database of no-observed effect concentration (NOEC), 50% effect concentration (EC50), and highest observed no-effect concentration (HONEC), and lethal, developing, reproductive, biochemical, and behavioral toxicity endpoints was used. Using 169 chronic NOEC databases, all non-traditional toxicity endpoint databases showed stronger HC values, better fit, and more variable toxicity sensitivity than those derived from traditional values. Moreover, using 426 chronic NOEC, EC50, and HONEC data points, HC values calculated from traditional plus HONEC toxicity values showed weaker HC values, slightly better fit, and more variable toxicity sensitivity than those derived from traditional toxicity values. The SSD approach using non-traditional toxicity and marine water toxicity data can expand the marine water toxicity database, including information on SSD curves and HCs of diverse microplastics.

Contaminant occurrence, water quality criteria and tiered ecological risk assessment in water: A case study of antifouling biocides in the Qiantang River and its estuary, Eastern China

Antifouling biocides may cause adverse effects on non-target species. This study aims to determine the distribution, sources, and ecological risks of antifouling biocides in the surface waters of the Qiantang River and its estuary in eastern China. The concentrations of total antifouling biocides were ranged from 12.9 to 215 ng/L for all water samples. Atrazine, diuron and tributyltin were the major compounds in the water bodies of the study area. The acute and chronic toxicity criteria for tributyltin, diuron and atrazine were derived for freshwater and saltwater, respectively, based on the species sensitivity distribution approach. The freshwater and saltwater criteria were slightly different, and the toxicity to aquatic organisms could be summarized as tributyltin > diuron > atrazine. The graded ecological risk rating showed that the long-term risk of TBT was significant in coastal waters. The pollution of TBT in the Qiantang River deserves further attention.

Ecological risk assessment for xylenes and propylbenzenes in aquatic environment using a species sensitivity distribution approach

Xylenes and propylbenzenes (PBZs) are volatile aromatic hydrocarbons with high aquatic toxicity. Xylenes can be present in three isomers: o-xylene (OX), m-xylene (MX), and p-xylene (PX), while PBZs include two isomers: n-propylbenzene (n-PBZ) and isopropylbenzene (i-PBZ). Their accidental spills and improper discharges from petrochemical industries can cause severe contamination in water bodies posing potential ecological risks. In this study, the published acute toxicity data of these chemicals for aquatic species were collected to calculate hazardous concentrations protecting 95% species (HC5) using a species sensitivity distribution (SSD) approach. The acute HC5 values for OX, MX, PX, n-PBZ, and i-PBZ were estimated to be 1.73, 3.05, 1.23, 1.22, and 1.46 mg/L, respectively. The risk quotient (RQ) values calculated based on HC5 indicated their high risk (RQ: 1.23 ∼ 21.89) in groundwater, but low risk (RQ < 0.1) in natural seawater, river water, and lake water. When xylenes or PBZs leaked into the sea, they were expected to pose a high risk (RQ > 1) at the start and then a low risk (RQ < 0.1) after 10 days due to natural attenuation. These results may help to derive more reliable protection thresholds for xylenes and PBZs in aquatic environment and provide a basis for evaluating their ecological risks.

Marine ecological risk assessment for the herbicide sulfometuron-methyl based on species sensitivity distribution approach

Marine ecological risk assessment for the herbicide sulfometuron-methyl based on species sensitivity distribution approach

The applicability of species sensitivity distributions to the development of generic doses for phytosanitary irradiation

Ionizing radiation is used as a phytosanitary treatment to prevent the introduction of pests through trade. Generic doses are a valuable means to increase the number of pest-commodity combinations that can be treated using phytosanitary irradiation. Generic doses allow for the treatment of the entire taxa for which the dose has been approved, allowing for the treatment of untested species. As such, the approval of a generic dose requires substantial supporting data and careful consideration of the risks involved. We adopt the Species Sensitivity Distribution (SSD) framework, already in widespread use in the field of ecotoxicology and environmental risk assessment, to evaluate generic doses for phytosanitary irradiation treatments. Parametric SSDs for Curculionidae and Tephritidae were developed using existing data on efficacious phytosanitary irradiation treatments. The resulting SSDs provided estimates of the taxa coverage expected by the generic dose, along with the margin of uncertainty. The SSD analysis lends support to the existing 150 Gy generic dose for Tephritidae and a proposed 175 Gy generic dose for Curculionidae. The quantitative estimates of risk produced by the SSD approach can be a valuable tool for phytosanitary rule making, improving the process for generic dose development and approval.

Advancing Fifth Percentile Hazard Concentration Estimation Using Toxicity-Normalized Species Sensitivity Distributions.

The species sensitivity distribution (SSD) is an internationally accepted approach to hazard estimation using the probability distribution of toxicity values that is representative of the sensitivity of a group of species to a chemical. Application of SSDs in ecological risk assessment has been limited by insufficient taxonomic diversity of species to estimate a statistically robust fifth percentile hazard concentration (HC5). We used the toxicity-normalized SSD (SSDn) approach, (Lambert, F. N.; Raimondo, S.; Barron, M. G. Environ. Sci. Technol.2022,56, 8278-8289), modified to include all possible normalizing species, to estimate HC5 values for acute toxicity data for groups of carbamate and organophosphorous insecticides. We computed mean and variance of single chemical HC5 values for each chemical using leave-one-out (LOO) variance estimation and compared them to SSDn and conventionally estimated HC5 values. SSDn-estimated HC5 values showed low uncertainty and high accuracy compared to single-chemical SSDs when including all possible combinations of normalizing species within the chemical-taxa grouping (carbamate-all species, carbamate-fish, organophosphate-fish, and organophosphate-invertebrate). The SSDn approach is recommended for estimating HC5 values for compounds with insufficient species diversity for HC5 computation or high uncertainty in estimated single-chemical HC5 values. Furthermore, the LOO variance approach provides SSD practitioners with a simple computational method to estimate confidence intervals around an HC5 estimate that is nearly identical to the conventionally estimated HC5.

Derivation of predicted no-effect concentrations for thirty-five pharmaceuticals and personal care products in freshwater ecosystem

Pharmaceuticals and personal care products (PPCPs) are contaminants of emerging concern. PPCPs have been detected in various environmental matrices, posing potential threat to human health and environment quality. Thus far, there are no water quality guidelines (WQGs) established for PPCPs for the protection of freshwater organisms and ecosystems. In this study, we used the species sensitivity distribution (SSD) and assessment factor (AF) approaches to derive the 5% hazardous concentrations (HC5) of 35 selected PPCPs using acute and chronic toxicity data. The predicted no-effect concentrations (PNECs) and the acute-to-chronic ratios (ACRs) of chemicals were inherently computed to support the derivation of WQGs and for ecological risk assessment. Among these, endocrine-disrupting chemicals and antipsychotics were shown to pose a greater threat to the freshwater environment and organisms. The highest PNEC was recorded for chloramphenicol (3,620 μg/L) and the lowest for fluoxetine (0.0000291 μg/L), which could have significant ecological risks. In addition, the commonly used default ACRs do not seem to provide adequate support for the prediction of chronic toxicity thresholds and WQGs, as the highest ACRs of these drugs reached 39,100 (e.g., chloramphenicol). The findings of this study provide critical scientific information regarding the development of WQGs for environmental management and the risk control of PPCPs.

Modeling Time-Dependent Aquatic Toxicity of Hydrocarbons: Role of Organism Weight, Temperature, and Substance Hydrophobicity.

Oil spill exposures are highly dynamic and are not comparable to laboratory exposures used in standard toxicity tests. Toxicokinetic-toxicodynamic (TKTD) models allow translation of effects observed in the laboratory to the field. To improve TKTD model calibration, new and previously published data from 148 tests were analyzed to estimate rates characterizing the time course of toxicity for 10 fish and 42 invertebrate species across 37 hydrocarbons. A key parameter in the TKTD model is the first-order rate that incorporates passive elimination, biotransformation, and damage repair processes. The results indicated that temperature (4-26 °C), organism size (0.0001-10 g), and substance log octanol-water partition coefficient (2-6) had limited influence on this parameter, which exhibited a 5th to 95th percentile range of 0.2-2.5 day-1 (median 0.7 day-1 ). A species sensitivity distribution approach is proposed to quantify the variability of this parameter across taxa, with further studies needed for aliphatic hydrocarbons and plant species. Study findings allow existing oil spill models to be refined to improve effect predictions. Environ Toxicol Chem 2022;41:3070-3083. © 2022 ExxonMobil Biomedical Science Inc. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Is the impact of atmospheric microplastics on human health underestimated? Uncertainty in risk assessment: A case study of urban atmosphere in Xi'an, Northwest China

Microplastic (MP) exposure in the environment has been commonly demonstrated to have adverse effects on human health. The majority of studies on MP were related to the aquatic and terrestrial systems, its potential risk for ecosystem and human health when exposed to the atmosphere is not well-understood. The presented study, taking Xi'an, a megacity in Northwest China, as an example, first estimated the possibility of local residents bearing MPs pollution. The results figured out an average abundance of MPs in TSP, PM10, and PM2.5 was 12.5, 3.5 and 0.8 particles/L, respectively. A total of 15 polymer types of MPs were identified in the atmosphere. Although a species sensitivity distribution (SSD) approach is acknowledged to be useful to estimate the potential risk of pollutants, the result of SSD when used to evaluate the risk of MPs is debatable. In this study, SSD-based risk assessment showed that the atmospheric MP pollution in Xi'an had not yet reached the level of threatening human. However, unlike chemicals, it is unreliable to assess risk using the relationship of dose-response for MPs because toxic effects of MPs can be influenced by not only the abundance but also the characteristics, e.g., morphological size, shape and oxidative potential. Since insufficient mechanistic understanding regarding the relative relationship between MP characteristics and their toxic effects and limitation of the quality and relevance of toxicity data, the uncertainty of risk assessment of the atmospheric MPs is inevitable and the risk of the atmospheric MPs was tended to be underestimated. This poses a challenge to manufacturers and public health authorities, as well as researchers alike, however, we are already being exposed to the atmospheric MPs.

Ecological risk assessment for perfluorohexanesulfonic acid (PFHxS) in soil using species sensitivity distribution (SSD) approach

Perfluorohexanesulfonic acid (PFHxS) is one of the persistent organic pollutants that has been recommended to be listed in Annex A of the Stockholm Convention. It has gained increasing attention in recent years due to its toxic effects. The guideline values of PFHxS are commonly associated with PFOS in various countries and regulatory agencies. In this study, multispecies bioassays were conducted to determine the ecological toxic effects of PFHxS, including plants, soil invertebrates, and soil microorganisms, which indicated the EC10/NOEC values ranged from 2.9 to 250 mg/kg. Where possible, logistic models were used to calculate the EC30 values for various endpoints. The species sensitivity distributions were employed to estimate the ecological investigation levels for PFHxS contamination in soils using toxicity results from literature and this study. The calculation using EC10/NOEC values from both literature and this study indicated a most conservative HC5 as 1.0 mg/kg (hazardous concentration for 5 % of the species being impacted). However, utilisation of EC30 values derived from this study resulted in a much higher HC5 for PFHxS in contaminated soils (13.0 mg/kg) which is at the higher end of the existing guideline values for PFOS for protecting ecological systems. The results obtained in this study can be useful in risk assessment processes to minimize any uncertainty using combined values with PFOS.

Risk-based management framework for microplastics in aquatic ecosystems

Microplastic particles (MPs) are ubiquitous across a wide range of aquatic habitats but determining an appropriate level of risk management is hindered by a poor understanding of environmental risk. Here, we introduce a risk management framework for aquatic ecosystems that identifies four critical management thresholds, ranging from low regulatory concern to the highest level of concern where pollution control measures could be introduced to mitigate environmental emissions. The four thresholds were derived using a species sensitivity distribution (SSD) approach and the best available data from the peer-reviewed literature. This included a total of 290 data points extracted from 21 peer-reviewed microplastic toxicity studies meeting a minimal set of pre-defined quality criteria. The meta-analysis resulted in the development of critical thresholds for two effects mechanisms: food dilution with thresholds ranging from ~ 0.5 to 35 particles/L, and tissue translocation with thresholds ranging from ~ 60 to 4100 particles/L. This project was completed within an expert working group, which assigned high confidence to the management framework and associated analytical approach for developing thresholds, and very low to high confidence in the numerical thresholds. Consequently, several research recommendations are presented, which would strengthen confidence in quantifying threshold values for use in risk assessment and management. These recommendations include a need for high quality toxicity tests, and for an improved understanding of the mechanisms of action to better establish links to ecologically relevant adverse effects.

Assessment of a New Approach Method for Grouped Chemical Hazard Estimation: The Toxicity-Normalized Species Sensitivity Distribution (SSDn).

New approach methods are being developed to address the challenges of reducing animal testing and assessing risks to the diversity of species in aquatic environments for the multitude of chemicals with minimal toxicity data. The toxicity-normalized species sensitivity distribution (SSDn) approach is a novel method for developing compound-specific hazard concentrations using data for toxicologically similar chemicals. This approach first develops an SSDn composed of acute toxicity values for multiple related chemicals that have been normalized by the sensitivity of a common species tested with each compound. A toxicity-normalized hazard concentration (HC5n) is then computed from the fifth percentile of the SSDn. Chemical-specific HC5 values are determined by back-calculating the HC5n using the chemical-specific sensitivity of the normalization species. A comparison of the SSDn approach with the single-chemical SSD method was conducted by using data for nine transition metals to generate and compare HC5 values between the two methods. We identified several guiding principles for this method that, when applied, resulted in accurate HC5 values based on comparisons with results from single-metal SSDs. The SSDn approach shows promise for developing statistically robust hazard concentrations when adequate taxonomic representation is not available for a single chemical.