The use of rubber mulch, derived from shredded, recycled vehicle tires, has grown in popularity for both landscaping and playground surfacing due to its durability and shock-absorbing properties. However, its manufactured origin has led to public concern regarding the potential for chemicals to leach out and cause health problems, particularly cancer. Since this material is frequently used where children play, understanding the associated risks requires examining the scientific evidence. This article examines the chemical content of rubber mulch and synthesizes major government-led health studies to clarify the current scientific consensus on its safety.
Chemical Composition of Recycled Rubber
The primary reason for health concerns stems from the complex chemical makeup of vehicle tires, which is directly transferred to the recycled rubber mulch. Tire manufacturing introduces numerous compounds, including polycyclic aromatic hydrocarbons (PAHs), which are known carcinogens at certain exposure levels. These PAHs are often contained within the aromatic extender oils used to soften the rubber material.
Tire rubber also contains a variety of heavy metals, with zinc being the most abundant. Other metals detected include lead, cadmium, and chromium, though concentrations of these are much lower. Additionally, the material can release volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) into the air, especially when heated. These substances, along with additives like benzothiazole and phthalates, are present as a result of the vulcanization and preservation processes required to make a durable tire.
Current Scientific Findings on Cancer Risk
Major government and public health studies have sought to quantify the risk posed by the chemicals found in rubber mulch, particularly the potential for cancer. The general consensus from large-scale risk assessments is that the risk of cancer from typical exposure levels is considered minimal or negligible. The U.S. Environmental Protection Agency (EPA), along with the CDC and CPSC, conducted a comprehensive multi-agency study on recycled tire crumb.
This extensive research found that people using fields with recycled rubber did not show differences in PAH metabolite concentrations in their urine compared to those using natural grass fields. Furthermore, concentrations of many chemicals measured during active play were not significantly different from background air samples. The EPA’s research concluded that there were no adverse health effects found from the use of recycled rubber on playing surfaces.
The low-risk assessment is supported by data on chemical bioaccessibility, which is the amount that can actually be absorbed by the human body. Studies show that only a small percentage (one to three percent) of bioaccessible metals in the material can be absorbed. State-level health departments, including those in California and Washington, have also concluded that the use of recycled tire rubber in outdoor applications does not suggest a significant public health risk.
Pathways of Chemical Exposure
Chemicals within rubber mulch can potentially transfer to the human body through three main routes: inhalation, dermal contact, and ingestion.
Inhalation
Inhalation exposure occurs when volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), such as PAHs, off-gas or evaporate from the rubber. This release is significantly accelerated when the mulch heats up on a hot, sunny day, creating a higher concentration of chemical vapors above the surface. Children can also inhale fine rubber dust particles generated as the mulch breaks down over time.
Dermal Contact
Dermal absorption involves the transfer of chemicals from the rubber directly onto the skin through physical contact. Since many chemical additives are oil-based, they can adhere to the skin, especially after prolonged exposure like sitting or lying on the surface. While the skin acts as a barrier, some compounds may be absorbed, leading to a potential internal dose.
Ingestion
The third main pathway is accidental ingestion, which is often the most concerning for young children. This occurs when small pieces of rubber are swallowed or when children touch the mulch and then put their hands in their mouths. This hand-to-mouth transfer is the mechanism by which heavy metals and other non-volatile compounds become bioaccessible within the gastrointestinal system.
Practical Safety Measures and Precautions
Despite the current scientific consensus that the health risk is low, individuals can take simple steps to minimize potential exposure to the chemicals in rubber mulch. The most effective action is to ensure children wash their hands immediately after playing and before eating or drinking. This measure significantly reduces the likelihood of accidental ingestion of rubber particles and surface-adhered chemicals.
Because chemical off-gassing increases with temperature, it is beneficial to limit play during the hottest part of the day. Cooler surface temperatures, such as those in early mornings or evenings, naturally reduce the release of volatile compounds like VOCs. For installation or landscaping applications, using gloves and long sleeves can reduce direct dermal contact. If the mulch is used in semi-enclosed areas, ensuring adequate ventilation will help disperse any accumulating vapors.