Recycled rubber, primarily derived from shredding scrap tires into small granules known as crumb rubber, is a widespread material used in consumer and infrastructure products. This material repurposes millions of tires that would otherwise fill landfills. As the material is integrated into areas with frequent human contact, public concern has grown regarding the possibility of chemical toxins leaching or off-gassing. The central question is whether the benefits of recycling outweigh the potential health risks associated with the complex chemical makeup of tires.
Potential Contaminants in Recycled Rubber
Recycled rubber contains chemical additives incorporated during tire manufacturing. Among the substances of concern are Polycyclic Aromatic Hydrocarbons (PAHs), organic compounds formed during incomplete combustion. PAHs, including known carcinogens like benzo[a]pyrene, have been detected in crumb rubber, sometimes exceeding regulatory limits.
The material also contains heavy metals, notably zinc, used as a vulcanization accelerator and often found in high concentrations. While zinc is an essential element, its concentration has been observed to exceed environmental limits. Other metals present include lead, cadmium, and copper, which are residues from the tire manufacturing process.
Volatile Organic Compounds (VOCs) and Semi-Volatile Organic Compounds (SVOCs) are also released from the granules. These include solvents, plasticizers, and stabilizers like benzothiazoles and phthalates. These contaminants result directly from the original tire formulation, which relies on hundreds of chemical components for durability and performance.
How Exposure Occurs in Common Applications
Crumb rubber is used as infill in synthetic turf fields, surfacing beneath playground equipment, and rubber mulch in landscaping, creating three primary routes for human exposure. Inhalation is a concern due to the release of VOCs and SVOCs, which can off-gas, particularly under high temperatures or in indoor environments. Air sampling on turf fields has detected these organic compounds, though often at levels comparable to background air.
Direct contact with the granules leads to dermal exposure, where chemicals can be absorbed through the skin, especially if the skin is abraded or sweaty. Athletes and children frequently experience this exposure when playing or falling onto synthetic turf or playground mats. The transfer of fine rubber dust and chemical residues to hands and clothing is a common mechanism.
Ingestion is a pathway of concern for young children on playgrounds, who may accidentally swallow small amounts of granules or dust. Studies suggest the bioaccessibility of metals from the material is low, meaning only a small fraction is released into the body’s digestive fluids. The potential for chronic, low-level exposure remains, especially since the smallest particles contain higher concentrations of toxic compounds.
Regulatory Guidance and Scientific Consensus
Major government and health organizations have conducted extensive research to assess the health risks of recycled rubber, primarily through the Federal Research Action Plan (FRAP) in the United States. This effort, involving the Environmental Protection Agency (EPA), the Centers for Disease Control and Prevention (CDC), and the Consumer Product Safety Commission (CPSC), sought to characterize the materials and estimate human exposure. The consensus from most human health risk assessments is that the level of exposure associated with typical use is below established health benchmarks.
Specific findings indicate that the concentrations of organic chemicals released into the air are low, with many showing air concentrations not significantly different from background samples at outdoor fields. A pilot biomonitoring study supported this, finding that the levels of metals measured in the blood of participants were similar to those in the general population. No difference was found in PAH metabolites in the urine of those using synthetic turf compared to those using grass fields.
Although chemicals are present in the recycled rubber material, studies suggest that actual human exposure is limited. While the CPSC continues to evaluate the data for specific application to young children on playgrounds, the overall scientific evidence indicates that the general risk from exposure to recycled rubber infill on playing fields is not substantial.