SECTION III. Regulation and Remediation of Chiral Pollutants

Regulatory Perspectives and Challenges in Risk Assessment of Chiral Pollutants

INTRODUCTION

Chemicals have been recognized in the past centuries as essential for sustaining life. As humans abandoned the hunter-gatherer lifestyle thousands of years ago, chemicals played a significant role in paving the way for modern civilization (Johnson et al„ 2020). Isolation of natural chemicals from plants for use in medicine, agriculture, and later the textile industry was critical in improving the quality of life. The risk posed by chemicals was known even then with Paracelsus postulating in the 15th century that it was the dose that makes a poison. In other words, Paracelsus posited that any chemical was potentially a poison at the right dose. Natural chemicals were also used for their adverse effects to kill or maim people. For example, Greek philosophers such as Theramenes and Socrates were killed by poisoning using hemlock extracts.

The chemical industry rapidly grew in the past century due to rapid population growth, improvements in the standard of life, and an increased global food demand (Sanganyado et al., 2018). It is estimated that at least 95% of all commercial and consumer products are heavily dependent on the chemical industry (Wang et ah, 2020). A recent study found that there are more than 350 000 chemicals and mixtures registered for commercial production globally (Wang et ah, 2020). In the United States, it is estimated that an additional 1000 chemicals are introduced into the market each year (Wang et ah, 2020). Unfortunately, synthetic chemicals are released continuously into the environment or food products during production, distribution, use, and disposal of industrial or consumer

Examples of chemicals that have biological effects in humans following exposure

FIGURE 12.1 Examples of chemicals that have biological effects in humans following exposure.

products. Chemicals discharged from industries, hospitals, households, landfills, and wastewater treatment plants can enter the environment where they can bioaccumulate in organisms.

Since Rachel Carson’s Silent Spring, chemical pollution has been recognized as a major threat to biodiversity, water quality, food security, and human health (Figure I2.l). For example, the use of neonicotinoids as an insecticide has been linked to population collapse of honeybees (Chen et al.. 2019; Wang et al., 2013). Discharge of industrial effluent containing poly- and perfluoroalkyl substances (PFAS) in the environment has been linked to increased cancer risk, adverse effects on the immune system, and endocrine disruption (Buck et al.. 2011; Gallen et al., 2018; Krafft and Riess, 2015). It was found that more than 1.9 million people in Michigan, USA were exposed to

PFAS through drinking water. In the 1950s, consumption of food contaminated with mercury in Minamata, Japan was shown to cause severe neurological disease (Sakamoto et al., 2010). Hence, synthetic chemicals may cause adverse human and environmental health effects.

12.1.1 Implications of Chirality on Risk Assessment

More than 50% and 30% of pharmaceuticals and agrochemicals are chiral compounds that exist as two or more enantiomers (Sanganyado et al., 2020). A chiral compound is a compound that can exist as two nonsuperimposable mirror-images (enantiomers) since it possesses an asymmetric center (Sanganyado et al., 2017). The asymmetric center can be a chiral center or axis. For example, pharmaceuticals such as fluoxetine, atenolol, venlafaxine, and chloroquine contain one or more chiral centers which are carbon atoms (Kasprzyk-Hordern, 2010; Ribeiro et al., 2013; Zhang et al., 2018a). Flame retardants such as polychlorinated biphenyls and polybrominated biphenyls, on the other hand, contain a biphenyl bond that acts as an axis of chirality (Ruan et al., 2019, 2018; von der Recke et al., 2005). Interestingly, molecules with helical structures such as deoxyribonucleic acid and some metal complexes (e.g., cobalt(III) metallocryptand) are inherently chiral even though they do not contain a chiral center (Rickhaus et al., 2016). However, planar and helical chirality are not common in chemical technology. Enantiomers of a chiral compound often have different properties in biological systems (Connors et al., 2013; Gonqalves et al., 2002; Khan et al., 2014; Sanganyado, 2019). For example, enantiomers of a chiral pharmaceutical often have different therapeutic and toxicological characteristics (Brocks, 2006; Shen et al., 2013; Smith, 2009). Several studies have shown that enantiomers of chiral pesticides also have different efficacies and toxicities to non-target organisms (Gamiz et al., 2018; Mueller and Buser, 1995; Zhang et al., 2018b). As a result, the risk posed by chiral contaminants to human and environmental health may be enantiomer specific.

12.1.2 Scope of Chapter

Research interest on the implications of chirality on the efficacy, toxicity, distribution, and fate of chiral chemicals grew significantly in the past three decades. This is probably because several studies showed that enantiomers of chiral compounds have different properties in biological systems. However, enantiomer-specific differences in the occurrence, transport, fate, and toxicity of chiral contaminants in food and the environment are generally overlooked by regulatory agencies across the globe. Unfortunately, current approaches on assessing risk ignore the implications of chirality on the biological effects of chemical contaminants, and this undermines the accuracy of the risk assessments (Figure 12.2). This chapter provides general backgrounds on chemical regulation and risk assessment. Current regulations, and the lack thereof, on chiral contaminants in food and the environment are also discussed.

 
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