The End of the Road

What happens to old tires when they die? Researchers examine the risks of using recycled rubber in playgrounds and gardens.

By Mollie Day

   Finding a home for the world’s mammoth mound of recycled scrap tires is a challenging task. Historically, millions of scrap tires in the US were stacked-up each year in landfills or provided breeding grounds for mosquitoes and rodents when stockpiled or illegally dumped. Markets now exist for 80.4% of these spent tires, according to the US EPA. Instead of going to waste, some recycled truck and car tires are being used in artificial playing grounds, as soil amendments and surface mulches. But what could be the perfect solution for one of America’s biggest waste problems is also a cause for concern.
   Crumb rubber, shredded rubber and ground rubber are among the materials that can be made from scrap tires. During the recycling process steel and fluff are removed from the tires as best as possible. The remaining rubber is then ground and formed into pellets or chips of various natural or decorative colors. There is no disguising the fact that heavy metals, including lead and cadmium, were once used to produce tires. Potentially toxic chemicals remain a part of the rubber manufacturing and recycling process. They are also found in recycled rubber products.
   Recycled tires are increasingly being incorporated into ground cover for playgrounds and gardens, and soil amendments. The recycled materials are “safe”, “long-lasting”, and “weed retarding,” as touted by the manufacturers. As children, pets, plants and wildlife are becoming increasingly exposed to the controversial materials – and their potentially harmful elements – some researchers and child-advocates are beginning to question the merits and potential dangers of recycled rubber.
   A significant amount of research demonstrates potentially serious risks associated with recycled rubber, causing consumers to reconsider their yards and public spaces as a safe grave for scrap tires. There is also a good bit of research, largely funded by the tire industry, demonstrating no adverse effects could result from using recycled rubber as suggested. Scientists continue to explore the potential effects components of recycled rubber may have on soil, air, water and living things, but for the consumer who can’t analyze lab results, conflicting conclusions can be confusing.
   Rufus Chaney of the USDA Agricultural Research Service, having analyzed twenty years of research, concludes that the majority of research demonstrates that for the zinc factor alone, ground or chipped rubber should never be used in gardens or composts. When asked if Zinc kills plants, he replies, “Of course it does!” sounding exasperated, and with good reason.
   Chaney is not opposed to the idea of using rubber or zinc in gardens; in fact, he is preparing to deliver a paper on the potential benefits of using very small doses of rubber as a zinc fertilizer in the appropriate soils. “Today, highly purified zinc oxide is used to make rubber,” says Chaney.
   He also says that those who believe today’s tires are made with heavy metals are wrong, calling into question many reports. “Rubber is, for commercial reasons, not made with lead, cadmium and other heavy metals. Its not contaminated,” says Chaney of horticultural-grade rubber products. “The lead in ground rubber is substantially lower than in soil,” he remarks.
   Chaney suggests that an old story, based on mishandled research about roadside contamination is still inflated today because of authors who aren’t doing their homework. “They’re quoting old literature,” says Chaney who often finds himself misquoted in articles.
   There are reasons not to like recycled rubber, whether it’s harmful or not. Producers of organic soil softeners and wood-based mulches will be glad to learn that the stink being raised about synthetic ground cover is widespread. Opponents of rubber dislike its foul odor and ability to increase the surface soil temperature, making the garden an inhospitable environment for gardeners and plants alike.
   Hydrocarbon, an element found in crude oil and also synthetic rubber, causes a light to intense rubber smell, depending on the conditions. The aroma of rubber may not be pleasant and it may cause an allergic reaction in some people, but it isn’t toxic. The offending rotten egg odor tires emit arises from sulfide and sulfur dioxide, heavy ends of the petroleum process used to make rubber. Hydrogen sulfide and sulfur dioxide are respiratory irritants.
   Synthetic, recycled rubber will also release volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs). These foul smelling gasses can be toxic and in heavy doses can cause irritation of nasal and respiratory passages, central nervous system damage, depression, nausea, dizziness, headaches and a many other ailments.
   But the smells of recycled rubber aren’t “the real problem,” says Wilma Subra, an environmental consultant, chemist and MacArthur Genius Award-winning scientist who reviews crumb rubber research and advocates against use of it on playgrounds. “The real problem is the heavy metals that are dispersed in the air and on the fine end particles as the rubber breaks down,” says Subra.
   “The issue is getting a lot of attention because of the children’s exposure on playgrounds,” Subra adds. Concerns about potentially harmful elements found in recycled tires are of particular concern to many researchers.
   The Connecticut Agricultural Experiment Station, a state agency, conducted several experiments with crumb rubber under laboratory conditions. The station’s studies showed that under relatively mild conditions of temperature and leaching solvent potential dangerous components of crumb rubber volatized into vapor and leached into water in contact with the crumbs. Those components include two volatile organic compounds (VOCs), carcinogens, as well as butylated hydroxyanisole (a carcinogen and toxicant), n-hexadecane (an irritant) and 4-(t-octyl)phenol (corrosive to mucus membranes and a suspected endocrine disruptor).
   Outdoors, toxic odors from recycled tires may, or may not, be of harm to children or pets, though this has not been scientifically proven. The greater concern however, is that a child might ingest the particles. Loose pellets from rubber used in artificial turf and playground cover can travel in an athlete or child’s shoe tread, from the playing ground to the home, where a child might ingest the smaller than pea-sized pieces. This could also happen on the playing ground.
   “We’ve found areas of concern especially for very young children,” says child advocate Dr. William Crain. Crain is the author of several articles that discuss the issue of children and recycled rubber.
   In “Hazardous Chemicals in Synthetic Turf,” an article co-written by Dr. Junfeng Zhang and Crain, the authors discuss their research and analysis of a sample for two possible sets of toxicants -- polycyclic aromatic hydrocarbons (PAHs) and toxic metals. Jungfeng is Chair of the Department of Environmental and Occupational Health at UMDNJ-School of Public Health. His studies focus on assessing contaminant exposures and resulting health effects in susceptible populations (e.g., asthmatic patients and children).
   There are three ways in which PAH’s can get into the body: ingestion, skin contact and inhalation. Zhang’s research, and others, shows that it is unlikely a child’s body would absorb the carcinogens if he ingested the rubber. According to Crain researchers need to look in “all directions” when it comes to PAHs in order to determine whether or not children are vulnerable to contamination by other means.
   “They [researchers] haven’t examined children’s blood levels, they haven’t looked at anything direct, evidence of potential hazards that should be researched further,” Crain reports.
   Lead poisoning is a strong concern for parents with young children. Although rubber does not contain high levels of the heavy metal, effects are cumulative. Simulated lab studies show that lead from rubber does dissolve into the digestive fluid. If a child were to swallow crumbs, researchers are confident the body would absorb the lead, but they’re not certain whether or not it would absorb into the blood. Too much lead in the blood can cause developmental difficulties and brain damage.
   In their article, Zhang and Crain acknowledge that their studies are preliminary, but report that the results are worrisome. Of the PAH’s found in their first samples, taken from a playground surface in Manhattan, six were above the concentration levels that the New York State Department of Environmental Conservation (DEC) considers sufficiently hazardous to public health to require their removal from contaminated soil sites. A second sample yielded similar findings.
   FieldTurf, a Montreal based company that manufactures and installs artificial playing grounds, ALIAPUR (a French consortium founded by tire makers), and the French Environment and Energy Management Agency released their own joint report concluding that there was no cause for concern to human health by inhalation of VOCs released by recycled tire crumb rubber surfaces. Several other tire and petroleum industry commissioned reports favor the use of crumb rubber.
   “They have more rubber than they know what to do with,” says Chaney.
   No official ruling has been made regarding the safety – or lack there of – of recycled rubber grounds. Crain argues that health agencies should have provided the money to make sure that recycled rubber playing fields were safe before the installations went in.
   “Now we’re playing catch-up—going about it backwards in terms of a health policy,” says Crain.
   Zhang and Crain’s research, conducted at the Environmental and Occupational Health Sciences Institute of Rutgers University, also revealed levels of zinc in both samples that exceed the DEC's tolerable levels. Most studies show recycled tires contain very high levels of zinc, which is another concern for small children – or pets – that might ingest the rubber pieces and particles. Zinc has also been shown to be particularly harmful to fish and other aquatic life, a red flag for researchers.
   Zinc, as Chaney suggested, will raise a gardener’s brow over rubber in soil amendments and mulch – as will the increase temperature that can accompany a black soil-cover. Both can be harmful to ornamental plants.
   Fine ground rubber used to soften soils has been tested and discovered to kill ornamental plants due to their zinc content. While rubber does have a slight nitrogen value, the toxicity from zinc might outweigh the chance of a benefit depending upon the pH of the soil, according to Chaney, who has done extensive research on the subject.
   If the soil’s pH is 6 or below, the zinc uptake from ground rubber can be too much for plants. Rubber also contains sulfide, which increases the acidity in soil and consequently also the opportunity for zinc toxicity. In acidic southeastern soils zinc can be harmful. “Not very much is needed to kill any number of plant species,” says Chaney.
   In soils with a pH of 7.5 and higher, zinc may not harm ornamental plants. In calcareous southwestern soils, zinc binds with limestone preventing toxicity. Chaney is experimenting with using small quantities of ground rubber as a zinc fertilizer in these types of soils. “It might be an alternative to purchasing zinc chemical fertilizers,” says Chaney.
   What about rubber mulch? Over time, microbes do biodegrade rubber mulch. As the zinc degrades rubber can get into the soil, according to Chaney. Or – rubber mulch can be raked off the garden each year and sent back to square one, the landfill.
   Research with recycled rubber is also addressing the effects of rubber-related heat in the garden. When a thin layer of black rubber is exposed to the sun it can increase soil temperature. In the spring when soil is cool warmer soils may be favorable for transplanting or starting seeds. In the summer an increase in soil temperature can harm plant roots.
   “The story is complex,” says Chaney in discussing the pros and cons of recycled rubber for garden use. Until more research is completed, and agreed upon, it seems the saga of recycled rubber environments will also go on.