So my twitter feed exploded again. This time it was about the risks of synthetic turf fields to the kids in our soccer organization. The basis for this furor was a series of reports in The Province, CBC etc…. apparently all derived from an ESPN short documentary “The Turf War” by Julie Foudy. The documentary details an investigation into the risks of turf fields (and more specifically the little rubber tire crumbs used on the fields) to young athletes. The documentary was presented from the perspective of Amy Griffin a woman’s soccer coach at the University of Washington who believes that she has identified a particular risk to soccer goalies from the fields. In the last three days friends at the park and colleagues at work have approached me to help them understand the risk associated with playing on these fields.

Now I am going to break one of my cardinal rules of blogging and give you a fast and dirty answer to the question rather than forcing you to read to the end. To summarize, the toxicological research says that rubber tire crumbs do not pose a significant risk to players. That being said, the epidemiological evidence for Ms. Griffin’s hypothesis has not been compiled so we can’t tell whether the cancers she has seen are isolated cases or some form of cancer cluster. If the latter is true, then more research will be required to try and figure out the cause. As I described in my post Risk Assessment Epilogue: Have a bad case of Anecdotes? Better call an Epidemiologist that is not a trivial exercise and the result might be something we have not even thought of yet. However, as a chemist, my assessment is that the material will be shown to be safe. I base this conclusion solely on its chemical composition (which I will talk about below). I hope my simple assertion helps to assuage a bunch of you, but for the rest, I will provide more details below.

This is not the first time this topic has risen on my radar. You see I am not only a chemist with experience assessing the risks of chemical exposure to human health; I am also the father of young children who play soccer on artificial turf fields and I have been coaching soccer on such fields since my kids joined soccer. Like every other parent in our soccer organization, my house and garage are dusted with those little black crumbs and the pile in my garage is almost big enough to make a spare tire for my minivan.

The last couple times this topic came on my radar I dismissed it by citing the US EPA. The EPA carried out a scoping exercise in 2009. In that exercise they looked at the data and then conducted air and metals leachate sampling from a number of fields using synthetic turf. Their conclusion was that air samples were not generally worse than background samples and that “concentrations of components monitored in this study were below levels of concern”. This time I need to do a bit more work because an action group called Public Employees for Environmental Responsibility (PEER) challenged the EPA and successfully got the EPA to modify their reporting on the topic. The report was changed to indicate that the results only reflected the samples studied and curiously included a long list of compounds often identified in recycled tire materials. The report suggested that more work needed to be done on the topic and that role was taken on by the research community and public health authorities. These results have been pretty much unanimous. The crumbs have been shown to be relatively benign and not a likely threat.

In a previous post (Risk Assessment Methodologies Part 1: Understanding de minimis risk) I introduced readers to the concept of de minimis risk. De minimis risk is a risk that is essentially negligible and too small to be of societal concern. The post explained that while all things have risk some risks are sufficiently small that risk assessors and toxicologists agree to essentially ignore them. The reason I point out this term is that you will see it again and again when reading the literature on the topic of shredded tire rubber.

Here is a brief overview of what the toxicological and occupation and public health literature has to say on the risk of the tire crumbs used in soccer fields:

• Third generation artificial turf is not expected to result in exposure to toxic substances at levels that pose a significant risk to human health provided it is properly installed and maintained and users follow good hygienic practices. Toronto Public Health 2015.
• The health risks for players who use artificial turf are not significant and that it is completely safe to engage in sports activities on this type of outdoor field. Bulletin d’information en santé environnementale (Public Health Branch, Montreal 2008).
• Overall the metals, PAHs and semi-volatile compounds found all classes of materials to be at very low concentrations. Thus, for the metals and compounds identified there would be de minimis exposures and risk among anyone using fields with the exception of lead in a single new turf material. Lioy and Weisel, 2011 Crumb Infill and Turf Characterization for Trace Elements and Organic Materials.
• Outdoor and indoor synthetic turf fields are not associated with elevated adverse health risks…cancer and non-cancer risk levels were at or below de minimis levels of concern Ginsberg et al, 2011: Human Health Risk Assessment of Synthetic Turf Fields Based Upon Investigation of Five Fields in Connecticut.

As for the air sampling, the results were the same:

• A limited number of studies have shown that the concentrations of volatile and semi-volatile organic compounds in the air above artificial turf fields were typically not higher than the local background, while the concentrations of heavy metals and organic contaminants in the field drainages were generally below the respective regulatory limits. Health risk assessment studies suggested that users of artificial turf fields, even professional athletes, were not exposed to elevated risks Cheng, Hu and Reinhard, 2014 Environmental and health impacts of artificial turf: a review.

So the obvious question that must be answered is: if the tires contain all those potential compounds identified by the EPA then why is there so little risk? The answer comes down to the chemistry of the materials. You see when all the analytical chemistry was being done on the tire crumbs the analytical chemists were not asked to mimic natural conditions in their extractions, they were told to figure out what was in the tires. To do so they conducted what the EPA describes as “aggressive” analytical techniques. So what does aggressive mean in this case? To answer that question I have to discuss another useful chemistry topic: polar versus non-polar compounds and solvents:

In a previous post about oil spills (a non-specialist’s guide to how spilled hydrocarbons react in water) I introduced readers to the concept of polar and non-polar compounds and the idea of “like dissolving like”. I explained that polar compounds will dissolve in polar solvents and non-polar compounds will dissolve in non-polar solvents. Water is very polar. The rubber in tires are derived from petroleum hydrocarbons which are highly non-polar. Following the chemical rule of “like dissolves like” a non-polar compound will not dissolve in a polar solution. In their natural state oil products are “hydrophobic” which literally means “afraid of water”, so they do not readily dissolve in water.

Now I’m pretty sure your eyes glazed over, so let’s think of this from a kitchen chemistry perspective. If you’ve been crushing garlic, all the water in the world will not clean the garlic oil off your hands. Instead, we are all taught to rub cooking oil on our hands and then clean the mixture off with soapy water. The garlic oil will not dissolve in water but will dissolve in the cooking oil and only then can you wash that oil off with soapy water. Now consider the report by Marsili et al 2014 Release of Polycyclic Aromatic Hydrocarbons and Heavy Metals from Rubber Crumb in Synthetic Turf Fields: Preliminary Hazard Assessment for Athletes this is one of the studies that describes an evaluation of the compounds in those tire shreds. In their research they reported heavy metals (Zn, Cd, Pb, Cu, Cr, Ni, Fe) in nine samples using “microwave mineralization” while they identified the levels of the 14 US EPA priority polycyclic aromatic hydrocarbons (PAHs) using “Soxhlet extraction”.

You might ask what is “microwave mineralization”? Well the tire rubber was dissolved in a combination of concentrated nitric acid and hydrogen peroxide and then cooked in a microwave. As for the Soxhlet extraction? Well in that case the tire crumbs were dissolved in a solution of potassium hydroxide (caustic potash, a very strong base) and methanol cooked in a Soxhlet extractor where it underwent essentially continuous extraction using boiling fluids for four hours. The solvent was then extracted using a non-polar solvent (cyclohexane).

To call the above “aggressive” is a bit of an understatement. Soxhlet extraction is used when materials will not dissolve in water which is really the only solvent available on an outdoor soccer field. Even the human gut doesn’t come close to the conditions used in this extraction. What this means from a chemistry/toxicological perspective is that the contaminants components are stuck in the tire crumbs and are not “bioaccessible”. That means that while the crumbs may indeed contain all those compounds they are not available to make people sick. In fact, toxicologists have tested the materials using synthetic bodily fluids to see how accessible the components were and the results have also been virtually unanimous:

• PAHs were routinely below the limit of detection across all three biofluids precluding completion of a meaningful risk assessment. No SVOCs [semi-volotile organic compounds] were identified at quantifiable levels in any extracts based on a match of their mass spectrum to compounds that are regulated in soil. The metals were measurable but at concentrations for which human health risk was estimated to be low. The study demonstrated that for the products and fields we tested, exposure to infill and artificial turf was generally considered de minimus, with the possible exception of lead for some fields and materials. Pavilonis et al, 2013 Bio-accessibility and Risk of Exposure to Metals and SVOCs in Artificial Turf Field Fill Materials and Fibers
• PAHs contained in rubber granules had zero or near-zero bioaccessibility in the synthetic digestive fluids Zhang et al, 2008 Hazardous chemicals in synthetic turf materials and their bioaccessibility in digestive fluids.

So when asked whether we should be afraid of our kids’ soccer fields my answer is simple. The academic and public health research is essentially unanimous: rubber tire crumbs do not pose a significant risk to players. Put in the toxicological jargon: the contaminants of concern do not appear to exceed a de minimis exposure concentration. Had anyone from the journalistic community spoken to an environmental chemist they might have heard that the mode of action is chemically implausible.