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 in vivo studies as described in this report was to determine if there was evidence of systemic exposure to crumb rubber constituents following exposure to crumb rubber in experimental animals.
Crumb rubber was obtained from multiple commercial sources, combined into a single lot, and size fractionated for use in these studies: 37–170 µm for oral gavage and ≥170 µm for feed and bedding studies. NTP conducted 14‑day studies in female B6C3F1/N mice by oral gavage (0 or 1,250 mg/kg/day in corn oil), dosed feed (0 or 50,000 ppm), or by housing on crumb rubber mixed bedding (bedding only or 50:50 wt:wt). Plasma and urine were collected from all groups to determine internal exposure. Hematology, bone marrow cytology, and limited histopathology were evaluated as conventional approaches to assess systemic exposure through evidence of biological effect.
Untargeted chemical analysis of urine and plasma revealed no apparent difference in chemical profiles between control and treated animals using principal component analysis, for any route of exposure. While overall chemical profiles were not affected, some constituents potentially originating from crumb rubber were detected at higher levels in urine or plasma of crumb rubber-treated animals, compared to controls. No biological relevant effects were observed on survival, food consumption, body weight, or organ weights following crumb rubber exposure by any route tested. Small changes in hematology parameters were observed in crumb rubber-treated groups, but none of these changes was considered biologically meaningful. No histopathologic lesions were observed in animals exposed to crumb rubber via dosed feed or mixed bedding. Animals dosed with crumb rubber by oral gavage had higher occurrences of esophageal inflammation compared to the respective controls; the average severity of this lesion was similar between controls and the crumb rubber gavage group. In conjunction with chemical characterization and in vitro testing of the same material, this work will contribute to what is known about potential human exposure to crumb rubber constituents and biological effects resulting from contact with crumb rubber used in synthetic turf.
National Toxicology Program (NTP). 2019. NTP research report on synthetic turf/recycled tire crumb rubber: 14-day exposure characterization studies of crumb rubber in female mice housed on mixed bedding or dosed via feed or oral gavage. Research Triangle Park, NC: National Toxicology Program. Research Report 14. https://doi.org/10.22427/NTP-RR-14