Rubber mulch is a ground cover material created from shredded, recycled vehicle tires, often used in landscaping and playgrounds for its durability and shock-absorbing qualities. The toxicity of this material is a public concern because tires are complex products containing various chemical additives. This article explores the chemical makeup of recycled rubber mulch and details the potential health and environmental risks. The presence of known toxic substances drives the debate regarding its appropriate use in areas frequented by people, especially children.
The Composition of Rubber Mulch
Rubber mulch is manufactured primarily from end-of-life vehicle tires, which are shredded into small pieces. The composition of these tires introduces chemical compounds into the final mulch product that are necessary for tire function. These compounds include Polycyclic Aromatic Hydrocarbons (PAHs), which are potentially carcinogenic and are present in the aromatic oils used in tire manufacturing.
Heavy metals are also present, notably zinc, which is used as zinc oxide to facilitate the vulcanization process, making up to 2% of the tire mass. Other heavy metals, such as cadmium and lead, can be found in lower concentrations. Volatile Organic Compounds (VOCs) and semi-volatile organic compounds (SVOCs) are also detected, including substances like benzothiazole and isoprene, which can be released into the air.
Potential Health Risks to Humans
The health risks associated with rubber mulch are generally tied to three main exposure pathways: inhalation, dermal contact, and ingestion. Children are particularly vulnerable to these risks.
Inhalation risks arise from fine rubber dust particles and the off-gassing of VOCs and SVOCs, which become more pronounced in warm weather. Exposure to these airborne chemicals, such as benzothiazole, has been linked to respiratory and nasal passage irritation.
Dermal contact is another pathway, where chemicals like PAHs and zinc can be transferred from the rubber surface to the skin. Studies have detected these compounds in skin wipe samples following contact. This exposure can potentially lead to skin irritation or allergic reactions.
The greatest concern involves young children who frequently use playgrounds surfaced with this material. Their developmental behaviors, such as frequent hand-to-mouth activity, increase the risk of ingesting small rubber particles. The ingestion of heavy metals like lead and cadmium is a serious concern for the developing nervous system. High temperatures on playgrounds can significantly increase the rate of volatile chemical release, intensifying the potential for exposure.
Environmental Impact and Chemical Leaching
The environmental concerns surrounding rubber mulch center on the leaching of chemical additives into the surrounding soil and water systems. When the mulch is exposed to rain, water can pass through the material, carrying soluble chemical compounds. This leaching process introduces contaminants into the environment, potentially affecting the underlying soil and local waterways.
Zinc is a major leachable metal, as it is present in high concentrations in the original tire material, and its release is well-documented. High concentrations of zinc in runoff can be harmful to aquatic life, including plankton and marine organisms. PAHs and other organic compounds can also leach into the soil.
When rubber mulch is used directly in garden beds, the chemicals can exhibit phytotoxicity, or toxicity to plants. The elevated levels of leached zinc have been shown to accumulate in plant tissues, potentially inhibiting germination and affecting growth in certain species.
Scientific Consensus and Usage Guidelines
A multi-agency effort, including the U.S. Environmental Protection Agency (EPA) and the U.S. Consumer Product Safety Commission (CPSC), has been conducted to characterize the chemicals and potential exposures associated with recycled tire materials. These studies confirmed the presence of various chemicals, including metals and PAHs, but have not resulted in an outright ban on the material for playground use. The consensus is that while risks exist, especially given the increased exposure potential for children, the data collected so far does not establish a definitive health risk for the general population. Regulatory bodies recommend caution and offer practical guidelines to mitigate exposure.
Mitigation Guidelines
- Limit time spent on rubber-surfaced playgrounds on extremely hot days to minimize inhalation of off-gassed VOCs.
- Wash hands and exposed skin after playing to reduce dermal and ingestion exposure to surface chemicals.
- For landscaping use, placing a barrier, such as landscaping fabric, between the mulch and the underlying soil can reduce chemical leaching into the ground.
- Wash newly installed rubber mulch to remove any loose surface contaminants.
- Ensuring good ventilation in any enclosed area where the product is used helps to minimize the buildup of airborne volatile compounds.