Public health concern for playing on synthetic turf fields with crumb rubber infill has increased in recent years. Crumb rubber manufactured from recycled automobile tires contains potential carcinogenic and toxic substances, and, with over 12,000 synthetic turf fields in the United States, the potential for exposure is widespread. The National Toxicology Program (NTP) conducted research to improve the understanding of potential human exposure to crumb rubber and its biological activity. The objective of the studies as described in this report was to evaluate likely exposure scenarios in humans that could be translated into exposure routes for in vivo testing. Feasibility trials, without animals, were conducted to evaluate various crumb rubber formulations for bedding (incidental contact), feed (indirect ingestion), oral gavage (direct consumption), and dermal (direct contact) exposures.
Due to the physical characteristics of crumb rubber (irregularly sized particles of ground tires encompassing a large range in size and composition), particles were physically manipulated prior to testing. Milling was not feasible due to the characteristics of rubber (elasticity and thermal properties) and the additives employed during the grinding process; therefore, crumb rubber was sieved using standard mesh sizes into various particle sizes for evaluation in each exposure scenario. Formulations for testing of the various exposure scenarios were prepared at concentrations that allowed for maximum exposure to the crumb rubber material. Bedding and feed formulations, using sieved fraction sizes of crumb rubber greater than 420 µm, were prepared at 50:50 (wt:wt) crumb rubber:bedding and at 50,000 ppm crumb rubber in feed, respectively. Feed formulations were rotated on an orbital shaker at ambient temperature for 7 days to evaluate uniformity. Bedding formulations were rotated on an orbital shaker at ambient or elevated temperature for 4 days to evaluate uniformity and potential for vapor off-gassing. Uniformity of the feed formulations was maintained over 7 days of shaking. Larger crumb rubber particles (approximately 1,410 µm or greater) maintained uniformity with the bedding, but smaller particles (less than 1,410 µm) settled to the bottom of the cage. No vapor off-gassing was observed for the bedding.
Corn oil gavage formulations were prepared as homogenous suspensions at concentrations up to 200 mg/mL using crumb rubber with a particle size no greater than 170 µm. Particle sizes and concentrations greater than 170 µm resulted in blockage of the gavage needle during dispensing. Dermal administration of crumb rubber suspensions of any size fraction tested was determined not to be feasible due to clumping of crumb rubber in the vehicle, preventing homogeneous formulations. From this study, three potential human exposure scenarios (incidental exposure, indirect ingestion, and direct consumption) were identified as feasible exposure regimens for in vivo testing.
National Toxicology Program (NTP). 2019. NTP research report on synthetic turf/recycled tire crumb rubber: feasibility study in support of non-inhalation in vivo exposures of synthetic turf/recycled tire crumb rubber. Research Triangle Park, NC: National Toxicology Program. Research Report 13. https://doi.org/10.22427/NTP-RR-13