The use of scrap tire chips for landscaping has become controversial, due to the leaching of metals and other contaminants from the tire pieces. Zinc is concentrated (up to 2% by weight) to levels high enough to be highly toxic to aquatic life and plants.[88] Of particular concern is evidence that some of the compounds that leach from tires into water, contain hormone disruptors and cause liver lesions.[89]
Tires are dirty, drive on dirty roads, and then leach out nasty stuff.
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Part of the toxic nature of rubber leachate is due to its mineral content: aluminum, cadmium, chromium,
copper, iron, magnesium, manganese, molybdenum, selenium, sulfur, and zinc have all been identified in
laboratory and field leachates. If rubber products have been exposed to contaminants during their useful
lifetime, such as lead or other heavy metals, they will adsorb these metals and release them as well. Of
these minerals, rubber contains very high levels of zinc – as much as 2% of the tire mass. A number of
plant species, including landscape materials, have been shown to accumulate abnormally high levels of
zinc sometimes to the point of death. One USDA researcher who has studied zinc and other metals in
soils and plant materials for decades strongly believes that ground rubber should not be used “in any
composting, or in any potting medium, or casually dispersed on agricultural or garden soils” because of
zinc toxicity. Acidic soils and aquatic systems are particularly sensitive, since heavy metals and other
positively charged elements are less tightly bound to the soil and more available to plant and animal
uptake.
Rubber leachates are complex solutions. They include not only the minerals and organic building blocks
of rubber, but also various plasticizers and accelerators used during the vulcanizing process. In high
enough concentrations, some of these rubber leachates are known to be harmful to human health; effects
of exposure range from skin and eye irritation to major organ damage and even death. Long term
exposure can lead to neurological damage, carcinogenesis, and mutagenesis.
Some of these materials break down quickly, while others are known to bioaccumulate. One of the more
common rubber leachates is 2-mercaptobenzothiazole, a common accelerator for rubber vulcanization. In
addition to its known human health concerns, it is highly persistent in the environment and very toxic to
aquatic organisms: its environmental persistence may cause long-term damage to aquatic environments
constantly exposed to rubber leachates. Another family of organic leachates under scrutiny are the
polyaromatic hydrocarbons (PAHs). These compounds, used as rubber softeners and fillers, have been
repeatedly demonstrated to be toxic to aquatic life. PAHs are released continually into solution, and after
two years in a laboratory test leachates were shown to be even more toxic than at the study’s inception.
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