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The toxicity of plastic additives is quite variable given the diversity of their chemical classes. Determining the dose that kills 50% of a test animal (LD₅₀), such as Daphnia magna in 48‐hour exposures, is a basic standard aquatic toxicity test that allows for simple comparisons of toxicity across compounds. The range of LD₅₀ values for plastic additives exemplifies this diversity (Table 2.5). Acutely toxic additives (e.g., LD₅₀ values <1 mg/L) include lower brominated PBDEs, APs, some phthalates, such as DEHP, Basic Red 51 azo dye, cadmium, copper, and zinc. In contrast, other additives, considered not harmful because their LD₅₀ values are >10 mg/L, include 2,2‐bis(bromomethyl)‐1,3‐propanediol (BBMP), TCEP, and DEP. The antioxidant, Irganox 1010 has a high LD₅₀ for D. magna (86 mg/L, Table 2.5) and other animals; therefore, it is allowed in food‐contact plastic packaging (USFDA 2019). Lower toxicity provides some justification for replacing conventional additives with newer replacements. Even so, many replacements still exhibit some level of toxicity, some even at similar concentrations than the original additive, and should be more thoroughly studied (Behl et al. 2016; Luo et al. 2021).

The toxicity of many plastic additives has been reviewed previously (Table 2.6). Endocrine disruption is a prominent toxicological mechanism noted in the literature for many plastic additives. Endocrine‐disrupting chemicals (EDCs) are so‐called because they disturb any step in the complex feedback systems of hormones that regulate reproduction, growth, metabolism, and many other biological functions. Mechanisms of toxic action include interfering in the synthesis, activity, or elimination of hormones or their receptors. Phthalates, FRs, antioxidants, monomers, and metal‐based additives are known as EDCs, disrupting a range of different hormonal systems (Table 2.7). The main concern over EDCs is that they act at concentrations much lower than lethal concentrations and results in sublethal effects, such as reduced reproduction or slower growth, both of which could significantly harm populations or communities of marine organisms. For example, thyroid disruption and neurobehavioral effects were observed in crucian carp (Carassius auratus) at concentrations of HBCDs 10–100 times higher than environmental concentrations, leaving only a small margin of safety for wild fish populations (Dong et al. 2018).

Additives can also exert neurological, carcinogenic, developmental, immunotoxic, and organ toxicities. DEHP, for instance, is an animal and human carcinogen (Campanale et al. 2020). Toxicity tests of organophosphorous flame retardants (OPFRs) with mammals, birds, and fish resulted in neurotoxicity, oxidative stress, altered metabolic processes, developmental toxicity, and effects on the liver, kidney, and other organs (Du et al. 2019). Hindered phenolic antioxidants, including 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT) and butylated hydroxyanisole (BHA) and their metabolites, exhibit diverse toxicities, including endocrine disruption, kidney and liver effects, genotoxicity, tumor promotion or enhancement, reproductive effects, and lipid disruption (Liu and Maybury 2020). The diverse toxic effects of priority metals, such as As, Cd, Cr, Pb, and Hg, to multiple organs are well known (Liu et al. 2008).

Table 2.6 Examples of review articles discussing the toxicities of plastic additives.

Reference	Chemical class	Toxicological effect	Organismal focus for toxicology
Hermabessiere et al. (2017)	Multiple	Multiple	Marine
Liu et al. (2020)	Multiple	Ecotox proteomics	Aquatic
Pérez‐Albaladejo et al. (2020)	Multiple	Oxidative stress	Human and Aquatic
Oehlmann et al. (2009)	Phthalates, bisphenol A	Multiple	Aquatic and terrestrial
Staples et al. (1997)	Phthalates	Acute and Chronic	Aquatic
Bradlee and Thomas (2003)	Phthalates	Multiple	Aquatic
Yost et al. (2019)	Diisobutyl phthalate	Multiple	Human and mammals
Weaver et al. (2020)	Diethyl phthalate	Multiple	Human and mammals
Caldwell (2012)	De(ethylhexyl) phthalate	Genotoxicity	Human and Rodent
Brehm and Flaws (2019)	Phthalates, BPA	Transgenerational	Human
Luo et al. (2021)	Phthalate replacements	Multiple	Multiple
de Wit (2002)	BFRs (PBDEs, HBCD, TBBPA)	Multiple	Environment
Yu et al. (2015)	PBDEs	Thyroid, reproduction	Fish
Akortia et al. (2016)	PBDEs	Multiple	Environment
Covaci et al. (2006)	HBCD	Multiple	Mammal
Koch et al. (2015)	HBCD	Multiple	Mammal, bird, fish
Du et al. (2019)	OPFR	Multiple	Mammal, Bird, Fish
Liu and Mabury (2020)	Phenolic antioxidants	Multiple	Mammal and aquatic
Servos (1999)	Alkylphenols	Multiple (endocrine)	Aquatic
Tchounwou et al. (2012)	Metals	Multiple	Environment
Canesi and Fabbri (2015)	Bisphenol A	Multiple	Aquatic
Bhandari et al. (2015a)	Bisphenol A	Multiple (endocrine)	Aquatic Vertebrates and humans
Liu et al. (2021)	Bisphenol A	Multiple	Aquatic
Sharma (2009)	Titanium oxide nanoparticles	Multiple	Aquatic
Turan et al. (2019)	Engineered nanoparticles	Multiple	Aquatic

Data on toxicity of the newer nanoscale inorganic fillers are well under way. Titanium oxide nanoparticles, smaller than 100 nm in diameter, may be toxic to aquatic organisms because of their bioavailability (Sharma 2009). Suspensions of CB nanoparticles causes oxidative stress and activates lysosomal biomarkers in the digestive gland of mussels (Canesi et al. 2010), but the extent to which nanoparticles leach out of polymer nanocomposites is unknown. Estrogenicity of BPA and NP is well documented, but more recently, immunotoxicity their has also been observed in fish (Canesi and Fabbri 2015; Rastgar et al. 2019; Servos 1999). Benzotriazole UV stabilizers changed many immune response genes in zebrafish brain, liver, and embryos, as revealed by transcriptomics (Li et al. 2020). As toxicological tests become more sophisticated, such as rapidly advancing omics research (Liu et al. 2020), and our understanding of chronic, chemical mixtures, and multigenerational effects grows, and toxicological effects may be observed at even lower concentrations.

Table 2.7 Plastic additives that are endocrine‐disrupting compounds.

Additive class	Chemical	Endocrine‐disrupting action
Plasticizers	Phthalates	Anti‐androgenic
Flame retardants	PBDEs	Thyroid disruption
Flame retardants	HBCDs	Thyroid disruption
Antioxidants	Nonylphenol	Estrogenic
Monomers	Bisphenol A	Estrogenic
Monomers	Styrene	Inconclusive
Multiple	Cd, Pb, Zn	Multiple
UV stabilizer	Benzotriazoles	Thyroid disruption

For ideal risk assessments, the doses, route of exposure, and species used in toxicity tests must be relevant to environmental exposures. Many studies use doses far higher than those found in the environment (Brehm and Flaws 2019). These tests may miss sublethal, chronic effects or U‐shaped dose responses. Toxicology studies on marine species are rare in the literature. Studies that use rats and mice are common and important for assessing mammalian toxicology, but are not relevant to most marine species. The use of freshwater model species, such as D. magna and zebrafish, is more relevant but may not always be the best surrogate for marine organisms (Duran and Beiras 2017). For example, toxicity thresholds of BPA and NP spanned two to three orders of magnitude across saltwater species alone. Current regulatory standards for admissible concentrations in water are often based on freshwater organisms and may not adequately protect marine organisms (Duran and Beiras 2017). More testing is needed on model and nonmodel marine species, like the studies of Duran and Beiras (2017) and Delorenzo et al. (2008).

A common method for testing the toxicity of mixtures of plastic additives is to expose cells or organisms to leachate from plastic products. Sometimes, but not always, the chemicals are identified in the leachate to understand which could be causing the toxicity. For example, the leachate from three polymers (PVC, PET, and polybutylene adipate co‐terephtalate) in seawater was tested for in vitro estrogenic activity (Kedzierski et al. 2018). Microplastics collected from the North Pacific gyre leached chemicals that were estrogenic in in vitro bioassays, but upon analysis of the leachate they detected estradiol, a natural hormone found in pharmaceuticals but not a plastic additive, indicating that plastics are perhaps absorbing environmental contaminants that may interfere with studies that intend to focus only on plastic additives (Chen et al. 2019).