If you grew up in America – or much of the rest of the world – in the last 30 years, there’s a good chance you played on artificial turf.
The small, spongy black balls used as infill material in most artificial turf fields are called rubber crumbs, which have long been touted as a major benefit for recycling. However, conflicting studies have alternately identified granulated rubber as either safe for people playing on it or dangerous to human health.
New research from Northeastern University has studied the decomposition cycle of crumb rubber, made from old tires. By simulating the conditions under which rubber disintegrates, such as strong sunlight, they discovered that crumb rubber is very reactive, generating hundreds of previously unknown chemicals as it disintegrates, some of which are dangerous to humans.
Swim upstream
Zhenyu Tian, assistant professor of chemistry and chemical biology, says researchers have long known that tire rubber produces harmful processing chemicals when it breaks down. A processing chemical is the product of a chemical reaction, the new chemical left behind. In the case of artificial grass, transformation results from factors such as sunlight, rain and natural degradation over time.
Tian, in previous research, has quantified some of the deleterious effects of these processing products on the environment. Tian identified a chemical used to make car tire treads more durable, called 6PPD, which interacts with ozone to produce a processing chemical called 6PPD-quinone.
They found that 6PPD-quinone is highly toxic to coho salmon, a fish that spends most of its life in the Pacific Ocean but migrates up freshwater rivers to spawn. Less than one microgram per liter of water can kill a juvenile coho salmon in less than an hour. By weight, it’s about 100,000 times lighter than a metal paper clip.
Rainwater runoff from roads introduced 6PPD-quinone into waterways where salmon return to spawn. In streams affected by 6PPD-quinone, up to 90% of coho salmon died before spawning, the New York Times previously reported.
Tian notes that while they don’t know how 6PPD-quinone interacts with human physiology, other chemicals identified in their crumb rubber research are known to be hazardous to human health.
Tian already knew there were potentially dangerous chemicals associated with tire aging, but how might the many chemicals involved in tire manufacturing interact as they degrade?
In a new experiment, Tian and his team of researchers exposed rubber crumbs to a photoreactor, which accelerates the solar conditions an artificial turf field might experience, and measured the many transformation chemicals that resulted. Madison McMinn, a Ph.D. candidate working in Tian’s lab and first author of the paper, said he identified at least 572 distinct transformation products.
Smaller molecules getting bigger
McMinn says the research is “the result of a multidisciplinary, collaborative effort combining the expertise of three different research groups,” including data scientists and Northeastern’s Plastics Center.
Tian says they were able to identify, in addition to 6PPD-quinone, two other chemicals known to be dangerous to humans. One of them, 4-HDPA, is an endocrine disruptor suspected of causing breast cancer. The other, 1,3-DMBA, mimics the stimulant effects of amphetamine.
A 2019 report from the Environmental Protection Agency suggested that human exposure to toxins in artificial turf fields was limited due to the way these fields are used. Additionally, the concentration of a chemical varies in the real world, because tires tend to come from multiple manufacturers and represent many different ages, Tian notes.
“The take-home message,” Tian says, is that while communities may view rubber crumbs in their fields as recycled or reused, “when you put them in the ground, they’re still very active. Things change, some things are generated. And, over a period of four or five months, they don’t disappear.”
Additionally, because of the high reactivity of the chemicals used in making tire rubber, scientists don’t just see the chemicals breaking down into smaller pieces, but also “smaller pieces turn into larger pieces,” he says.
It will likely take two to three years for the transformation process to stop, Tian says, but many artificial turf fields are being replaced at about the same rate.
For the majority of identified processing chemicals, they simply do not yet know what their effects are on the human body or the environment.
“God knows what it feels like,” Tian said.
