Styrene, Polyphenyls, and Related Compounds

Abstract

The class of chemicals described in this section include aromatic hydrocarbons whose molecular structures contain single aromatic rings separated by single chemical bonds from other such rings, or from simple groups containing unsaturated carbon atoms such as vinyl (CHCH2), ethynyl (acetylenyl) (CCH), or allyl (CH2–CHCH2). Aromatic compounds containing multiple aromatic rings sharing one or more sides are not included. These aromatics differ vastly in chemical, physical, and biological properties from the aliphatic and alicyclic hydrocarbons, including increased toxicity to humans and other mammals. Of prime importance in this respect is the carcinogenicity of alkenylaromatic hydrocarbons such as styrene. Included in this chapter are (a) alkenylbenzenes such as styrene and allylbenzene, (b) alkynylbenzenes such as phenylacetylene, and (c) di- and terphenyls and triphenylene. These compounds are poorly to moderately reactive under ambient conditions but readily undergo photochemical degradation, for instance in the atmosphere. They generally occur as volatile liquids under normal conditions, but possess lower vapor pressures, volatility, absorbability, and solubility in aqueous media than aliphatic or alicyclic compounds with a similar number of carbon atoms. Higher molecular weight derivatives are volatile solids. These properties contribute to their biological activities. All are also characterized by high lipid solubility, and donor–acceptor and polar interactions. Because of their low surface tension and viscosity, low molecular weight analogs may be aspirated into the lungs during ingestion, where they can cause chemical pneumonitis. These hydrocarbons are widely used as chemical raw materials, intermediates, solvents, in oil and rosin extractions, as components of multipurpose additives, and extensively in the glue and veneer industries because of their rapid drying characteristics. Aromatics serve in the dry-cleaning industry, in the printing and metal processing industries, and for many other similar applications. They are important constituents of aviation and automotive gasolines and represent important raw materials in the preparation of pharmaceutical products. The polyphenyls are obtained as products or by-products in petroleum or coal refining, burning, or pyrolysis. In coke-oven operations, the aromatics are recovered from the gases and the coal tars. In crude oil distillation, they are produced by fractionated distillation, solvent extraction, naphthenic dehydrogenation, alkylation of benzene or alkenes, or from alkanes by catalytic cyclization or aromatizations. These aromatic compounds are primary skin irritants, and repeated or prolonged skin contact may cause dermatitis and corneal irritation and damage. Direct aerosol deposition or contact from ingestion and subsequent aspiration can cause severe pulmonary edema, pneumonitis, and hemorrhage. These hydrocarbons are absorbed rapidly and cause local irritation changes in endothelial cell permeability, and secondary effects have been observed in the liver, kidney, spleen, bladder, thymus, brain, and spinal cord in animals. Even a single dose exhibits a special affinity to nerve tissue and these hydrocarbons accumulate in marine animals to a greater extent and are retained longer than alkanes. Once absorbed, higher molecular weight hydrocarbons are released more slowly. These compounds are present in smoke from regular and flavored cigarettes. At lower molecular weights these hydrocarbons are mainly liquids that are soluble in fats, oils, and organic solvents. Their mutagenic or carcinogenic properties have been linked to physicochemical properties, such as electronegativity, electrophilic potency, dipole moment, intramolecular and subcellular binding, hydrophobicity, and others. However, these characteristics alone are inadequate for specific predictions. Metabolism occurs through epoxides and hydroxides, which are excreted as conjugates. Various hydroxide–epoxide or hydroxide–oxide combinations have been identified. Rat liver microsomes can also produce 3- or 6-hydroxymethyl metabolites. Enzyme systems, such as aryl hydrocarbon hydroxylase (AHH), are present in almost all human and animal cell tissues and are inducible by noncarcinogenic and potentially carcinogenic hydrocarbons. The stability of cytochrome P450 epoxidase may depend on immunologic competence, as does the epoxide hydrase. Among compounds of this class, styrene is of high human mutagenic and carcinogenic concern. Accumulation of aromatic hydrocarbons of these types in marine animals occurs to a greater extent and retention is longer compared to alkanes. In all species tested, the accumulation depends primarily on the octanol/water partition coefficient. Once absorbed, higher molecular weight hydrocarbons are released more slowly. From the standpoint of industrial hygiene, these aromatic hydrocarbons require close monitoring and evaluation, particularly styrene. Within the past several years, threshold limit values have been lowered incrementally in some cases because of the development of better sampling and analytic techniques and more extensive toxicity testing. Industrial monitoring programs should be continually evaluated. Where excursion values are found, biological monitoring should be carried out in addition to regular medical surveillance programs. Sampling techniques may be compound specific (see below) and also include the collection of air particles using an absorbent glass sampler, desorption with pentane, and quantification using spectral analysis. Collection on acrylonitrile-PVC filters is also recommended. Analytic quantification is also achieved by using gas chromatography high-resolution mass spectrometry or chemiluminescence. Methods for cleanup from waste water are also available. Keywords: phenylacetylene; polyphenyls; styrene