Understanding the potential for exposure to styrene is key to evaluating human health risk. In 2019 SIRC completed a comprehensive assessment of styrene exposure to add to the body of knowledge of styrene.
Exposure to styrene occurs in workplaces that manufacture styrene or use styrene to produce important polymers and co-polymers of industrial, consumer, and medical importance, including: polystyrene, styrene butadiene rubber, unsaturated polyester resins, styrene butadiene latexes, acrylonitrile–butadiene–styrene, and styrene–acrylonitrile.
The general public may be exposed to styrene from environmental sources, such as from industrial emissions, engine exhaust, tobacco smoke, certain consumer, office, and household products, and styrenic building and furnishing materials, or from ingestion of styrene naturally present in foods and beverages or from migration from food packing materials.
The following sections review the current knowledge of styrene exposure to workers and the general public. Because these sections summarize the exposure information, specific references are not provided but are available upon request.
General Public Exposure
The highest exposures to styrene occur in workplaces that produce styrene or use styrene to make other materials.
The primary route of exposure of styrene to workers is by inhalation. Workers may also come into skin contact with styrene, however compared to the inhalation route where uptake through the lungs is high, only minimal amounts of styrene (< 5%) pass through the skin into the body.
Workplaces have the highest airborne concentrations of styrene, although there is significant variability among types of facilities and processes. The highest occupational styrene exposures occur during the production of fiber-reinforced polymer composites (FRP) by open-molding processes that involve manual application of materials. The central tendency and upper-bound time-weighted average styrene concentrations for workers engaged in open-mold processes is about 30 ppm and 60 ppm, respectively, assuming no respiratory protection, and around 4 ppm and 7 ppm, respectively, with protection. Closed-mold processes reportedly produce central tendency and upper-bound time-weighted average styrene concentrations of about 11 ppm and 18 ppm, respectively.
General Public Exposure
The primary routes of styrene exposure for the general public are by inhalation of air and ingestion of food containing amounts of styrene. Less frequent exposures may occur from toys and consumer products made of styrene-containing polymers.
The general public is exposed to very low levels of styrene from air. The highest inhaled styrene concentrations occur in smokers.
Styrene is often detected in ambient air as a result of releases from natural sources, industrial processes and products, and human activities. Annual average styrene air concentrations in the United States from 2012 to 2016 ranged from 0.03 to 0.05 ppb, with an overall five-year average of 0.05 ppb.
Indoor air concentrations of styrene are important as most individuals spend the majority of their time indoors and because concentrations of styrene are typically higher in buildings than in ambient air. Studies have shown that the home is the locus of greatest exposure, followed by work and school. Styrene present in residential indoor air can arise from multiple indoor and outdoor sources, such as automobile exhaust, tobacco smoke, certain consumer, office, and household products, and styrenic building and furnishing materials. Typical styrene air concentrations inside homes range from 0.2 to 0.9 ppb. Central tendency and upper bound estimates of personal styrene exposure concentrations range from about 0. 2 to 1.1 ppb, respectively, for non-smokers, and about 0.5 to 1.5 ppb, respectively for smokers.
Low levels of styrene occur in foods from natural sources or from transfer from styrene-based packaging material.
Styrene occurs naturally in many foods and beverages, although it is not clear in some cases whether it is formed endogenously or if its presence is due to partitioning from the environment or food packaging materials. Styrene is commonly found at low concentrations in both raw agricultural commodities and processed foods. High concentrations (as high as 40 ppm) occur in cinnamon because styrene is the product of oxidation of its major flavoring compound, cinnamaldehyde. Styrene in food packaging materials may volatilize and be inhaled by the consumer, or leach into foods and be ingested. Migration may be strongly influenced by the interaction of food components with the packaging material, as well as enhanced by microwaving. The average dietary exposures for children to adults reportedly range from 0.001 to 0.01 mg styrene per day.
The general population is not regularly exposed to measurable levels of styrene in drinking water.
Although localized contamination of drinking water sources can occur in connection with specific sources, the general population is not regularly exposed to measurable levels of styrene in drinking water.
Some consumer products may release styrene with use.
Children may be exposed to small amounts of styrene through mouthing of plastic toys made of styrene-containing polymers. The average and upper-bound estimates of styrene ingestion by a child by mouthing is 0.0001 mg to 0.0004 mg per day. These levels are significantly lower than ingestion from dietary exposures indicating that mouthing of styrenic toys is unlikely to be a significant source of children’s exposure to styrene.
Consumer use of liquid and paste resins containing styrene (used mostly for automobile and boat repair) may result in inhalation and skin exposures. Estimated exposures are high for these uses (10 ppm for inhalation exposure and 1.7 to 5.5 g per day for skin exposure) but the estimates are based on conservative assumptions and hence are highly uncertain.