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Styrene is a polycyclic aromatic hydrocarbon (PAH), formed during incomplete combustion of organic compounds (as in cigarette smoke[1]) 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[2]. Phenylalanine heated together with 1-hydroxyacetone or methylglyoxal yielded only 0.03 mol% styrene.[3]

Health effects

Styrene is metabolized / activated in the human body to phenylglyoxylic acid (alters dopamine levels)[4] or genotoxic styrene-7,8-oxide (SO).[5] SO is neurotoxic (synergistically with acrylamide).[6] Styrene is not carcinogenic[7][8], but hepatotoxicity (when metabolized to SO) in glutathione-depleted mice.[9] Environmentally, commercially manufactured polystyrene nanoparticles are taken up by algae and accumulate in fish, resulting in weight loss and altered cholesterol distribution[10], though may be eliminated within bile.[11] In rats, prenatal styrene exposure decreases postnatal serotonine and catecholamine levels in the brain.[12] Chronic exposure causes remodelling of the intestinal villi[13] and structural changes in apolipoproteins.[14] The testis may be the major target for styrene toxicity.[15] In rats, prenatal low level exposure to estrogenic styrene trimers obstructed genital organ development, and disrupted the endocrine systems of male rat offspring.[16]

Migration from disposables

Similar to acrylonitrile[17], benzene[18], bisphenol A and phthalates, plastic drink containers[19]] and plastic liners in cans and other polystyrene foam packages[20] are a source of styrene exposure from food/drinks. Except for polystyrene egg cartons, which leak no styrene into eggs.[21] 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.[22] Migration of styrene from disposable cups (styrofoam and PS, not paper cups) into drinks highly depends on fat content and temperature of drinks.[23] 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.[24] Butter[25], yoghurt[26][27] may also contain styrene, as the migration of styrene from packaging material very much depends on fat-contents of the liquid/food (and temperature).[28] 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.[29]

Naturally produced

Similar to toluene and ethylbenzene, styrene is also released during food decaying processes[30] (as in spoiled salmon[31]). olives and olive oil may also contain styrene[32], due to the uptake of aromatics, metabolized into styrene.[33] Barley storage molds[34] and Blue-cheese fungi (eg Gorgonzola, camembert[35]) also produce styrene, as well as the plastics used for packaging.[36] Due to gram-negative bacteria in dairy, all raw milk cheeses also contain styrene (and o-dichlorobenzene; a derivative of benzene).[37] Cinnamon constituents naturally contain the styrene structure (incl. cinnamic acid[38]), which may get released due to the activity of fungal species present on cinnamon.[39]

Human exposure

Total daily styrene exposure is estimated at maximally 0.17 mcg/kg bw[40] and human lifetime risk for tumors is estimated to be very low.[41] 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).[42] 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.[43]