Styrene is a polycyclic aromatic hydrocarbon (PAH), formed during incomplete combustion of organic compounds (as in cigarette smoke) and a (one of many) Maillard reaction product. Styrene polymeres are industrially produced for use in various products, including dyes and disposables.
Formation in cooked foods
Styrene is considered the phenylalanine-glucose counterpart of acrylamide (instead of asparagine-glucose). In the presence of sugars, phenylalanine, similarly to asparagine and lysine, can undergo carbonyl-assisted decarboxylative deamination reaction to generate styrene. Phenylalanine heated together with 1-hydroxyacetone or methylglyoxal yielded only 0.03 mol% styrene.
Styrene is metabolized / activated in the human body to phenylglyoxylic acid (alters dopamine levels) or genotoxic styrene-7,8-oxide (SO). SO is neurotoxic (synergistically with acrylamide). Styrene is not carcinogenic, but hepatotoxicity (when metabolized to SO) in glutathione-depleted mice. Environmentally, commercially manufactured polystyrene nanoparticles are taken up by algae and accumulate in fish, resulting in weight loss and altered cholesterol distribution, though may be eliminated within bile. In rats, prenatal styrene exposure decreases postnatal serotonine and catecholamine levels in the brain. Chronic exposure causes remodelling of the intestinal villi and structural changes in apolipoproteins. The testis may be the major target for styrene toxicity. In rats, prenatal low level exposure to estrogenic styrene trimers obstructed genital organ development, and disrupted the endocrine systems of male rat offspring.
Migration from disposables
Similar to acrylonitrile, benzene, bisphenol A and phthalates, plastic drink containers] and plastic liners in cans and other polystyrene foam packages are a source of styrene exposure from food/drinks. Except for polystyrene egg cartons, which leak no styrene into eggs. Bottled drinking water may contain styrene (up to 29.5 mcg/L; increased to 69.53 mcg/L after 1 yr storage) leached from the polystyrene (PS) bottle. Migration of styrene from disposable cups (styrofoam and PS, not paper cups) into drinks highly depends on fat content and temperature of drinks. Up to 150 mg styrene /kg was transferred from polystyrene disposables into sunflower oil and up to 90 mg/kg into individual serving milk products. Butter, yoghurt may also contain styrene, as the migration of styrene from packaging material very much depends on fat-contents of the liquid/food (and temperature). In the US, the exposure to styrene from polystyrene food-contact articles is estimated at 9 mcg per day, compared to 1 to 4 mcg /day for UK residents.
Similar to toluene and ethylbenzene, styrene is also released during food decaying processes (as in spoiled salmon). Olives and olive oil may also contain styrene, due to the uptake of aromatics, metabolized into styrene. Other vegetable oils contain polycyclic aromatic hydrocarbons as well. Barley storage molds and Blue-cheese fungi (eg Gorgonzola, camembert) also produce styrene, as well as the plastics used for packaging. Due to gram-negative bacteria in dairy, all raw milk cheeses also contain styrene (and o-dichlorobenzene; a derivative of benzene). Cinnamon constituents naturally contain the styrene structure (incl. cinnamic acid), which may get released due to the activity of fungal species present on cinnamon.
Total daily styrene exposure is estimated at maximally 0.17 mcg/kg bw and human lifetime risk for tumors is estimated to be very low. In another study the daily styrene exposure is estimated to range from 18.2 to 55.2 mcg per person (roughly 0.2 to 0.8 mcg/kg bw) with the greatest proportion coming from inhaled styrene (exhaust fumes, cigarette smoke, indoor heating). In Canada daily intakes of styrene for the general population were estimated at 0.004 to 0.17 mcg/kg bw from ambient air, 0.07 to 0.10 mcg/kg bw from indoor air and up to 0.58 mcg/kg from food. The estimated intakes from drinking-water and soil were negligible. Potential exposure from cigarette smoke (20 cigarettes / day) was estimated to be 2.86 mcg/kg bw per day.