Tire-derived Crumb Rubber

Ground rubber is a derivative of tire scrap waste. It is also known as “crumb rubber.” It is the rubber component of tires, mechanically separated from other components, such as steel belts and fiber, prior to processing. Ground rubber is derived from two post-consumer waste streams: tire retread shops (buffings) and scrap dealers (whole tires). Crumb rubber is classified by mesh size. This refers to the size of the openings in the sieve that is used to separate smaller particles from larger particles, with smaller numbers corresponding to larger openings. It is primarily formed by two processes, ambient and cryogenic. In the ambient process tire scraps are ground into small pieces under ambient conditions using rotating blades and knives. In the cryogenic process tires are frozen in liquid nitrogen and shattered into small pieces with hammers. The resulting particles are smooth and more regular in shape with a lower surface area. Both processes contain additional steps designed to remove most of the steel and textile fibers from the tires, though the cryogenic method results in particles that may contain less fiber and steel content than particles formed in the ambient method. The data in the sources researched generally combined analyses of samples derived from both methods. Consequently, this CP is intended to cover products manufactured in either process. In addition, this CP does not cover crumb rubber that is coated with colorants, sealers, or biocides, which could all add additional chemical hazards to the product. 

Crumb rubber can be used in a number of different flooring products, playground and athletic surfaces, and in rubberized asphalt and other pavement. The advantage of incorporating crumb rubber into these products is that it has the potential to divert billions of pounds of tires from the waste stream into recycled products each year. This Common Product (CP) focuses on crumb rubber used as infill in athletic surfaces, with sizes generally ranging from 4 to 30 mesh. The use of crumb rubber in athletic surfaces is controversial as it contains chemicals known to be harmful to human health. Numerous risk assessment studies have concluded that athletes are not exposed to chemicals at levels that are harmful to human health for this applicaion, however, a study of tire-derived products conducted by the State of California did identify some chemicals emitted above the threshold of concern from new flooring products that contained crumb rubber. This report also emphasized that some chemicals that were identified in the products do not have established health thresholds. It is important to note that most chemicals do not have established health thresholds because they remain unassessed. In addition, testing only captures the chemicals that are being tested for and other hazardous volatile or non-volatile substances may be present that pose health concerns. Some substances found in crumb rubber, like lead, are known to have no safe threshold for exposure. In addition, exposures to many hazardous chemials from multiple sources do occur and are not well understood. Tire formulations are complex and residual concentrations of other chemicals are likely to be present.

Because recycled content can vary, this CP displays ranges for all chemicals identified in numerous academic studies. These ranges summarize testing data from samples tested prior to being used as infill in athletic turf, while field samples taken from athletic turf are excluded. Per the Common Product research methodology, chemicals present at less than 100 ppm in a product or material are included in the CP if they have an associated PBT, cancer, or respiratory hazard. Chemicals that were identified as present below 100 ppm that did not have one of these associated hazards at the time of the research, were excluded from the CP. Chemicals that were identified as present but not quantified, such as VOCs, can be found in the All Contents tab.