Playing It Cool

By Thomas P. Shay

Depending on whom you ask, the shredded Styrene-Butadiene Rubber (SBR) infill—or crumb rubber as it is more commonly known—typically used in synthetic-turf athletic fields, is perfectly safe or frighteningly hazardous. Both sides of the argument are currently supported by limited research and studies, making it nearly impossible for many owners and administrators to make heads or tails of the issue. Luckily, there are other options for synthetic-turf infill that the general public is decidedly more cool-headed about, making synthetic-turf field installation possible without stepping into tricky territory.

A form of recycling, crumb rubber is produced by shredding discarded tires made from natural or synthetic rubber. Because these materials contain a mixture of chemicals that are potentially harmful, school administrators, municipal officials, and athletic coaches have expressed concern over the risk of exposure to athletes who play regularly on this type of cushioning. Because many schools and municipalities continue to find their existing natural-grass fields cannot adequately support sports and recreation programs, crumb rubber alternatives are a mostly conflict-free solution.

Contradictory Cases Lead To Confusion
In the midst of conflicting perspectives, it is helpful to understand why crumb rubber is used in the first place; it is a cost-effective material that creates a playing surface with properties acceptable for modern sports competition. In addition, it removes tons of used tires from the waste stream. Plus, synthetic-turf fields can reduce maintenance costs and athletic injuries while increasing the life of a field. A typical multi-purpose synthetic-turf field using crumb rubber mixed with silica sand has approximately 6 pounds of infill per square foot with 50 percent being rubber. That equates to approximately 250,000 pounds of recycled tires for one 85,000-square-foot field. The vast majority of synthetic-turf fields has been and continues to be infilled with crumb rubber.

Those opposed to using crumb rubber assert that recycled rubber on athletic fields increases the risk of cancer and other illnesses. One of the most notable pieces of this debate was a college soccer coach’s assertion that goalies—who come into more contact with crumb rubber than other players on the field—have a higher incidence of non-Hodgkin’s lymphoma and other rare types of cancer. Some states have thus begun to take legislative action; for instance, California’s state senate recently heard legislation proposing a ban on synthetic-turf field installations that contain crumb rubber until a comprehensive health-risk assessment is completed.

However, other states have conducted their own research into these health hazards. A study conducted by the Connecticut Department of Public Health found that the potentially harmful chemicals in crumb rubber are not released into the air during active use at a high enough level to have detrimental health effects on athletes. Until a definitive answer is reached through scientific consensus in a majority of studies, field owners have the option to steer clear of this disagreement. This is where the alternatives come into play.

Looking To Synthetic Substitutes
Several different types of alternative infill products can serve as replacements for crumb rubber while the jury is still out on its safety. Some alternatives replace both the crumb rubber and the mix with silica sand, while others maintain a significant level of silica sand in the system. Everything else associated with the field generally remains the same.

So far, more than 28 million pairs of shoes have been used to make Nike Grind, another rubber-composite infill that some are turning to instead of SBR. The rubber outsoles of shoes are put through shredders and refined to make a cushioned surface for synthetic-turf fields that vendor reports claim does not exceed hazardous levels for formaldehyde, volatile organic compounds, or heavy metals, such as lead and mercury. Some municipalities and schools are mixing in this post-industrial, 100-percent recycled product with crushed SBR for the fields, and others are shifting exclusively to the use of the former sneakers.

Ethylene propylene diene monomer (EPDM) and thermoplastic elastomer (TPE) are other options for synthetic turf. EPDM is a type of synthetic rubber that has had varied success as an infill product. It is more resistant to oxidation and heat, and has a wide array of other applications, such as weather seals, roofing, and garden hoses. TPE, on the other hand, is an infill composed of plastic pellets with many of the characteristics of rubber combined with the recyclability of plastic. It is 100-percent non-toxic, free of heavy metals, and comes in a variety of colors, making it an attractive option for owners wanting a custom-designed field. A key aspect related to the durability of these types of alternative infills is the quality of the source materials and manufacturing processes.

Acrylic polymer-coated, round sand is being used in place of crumb rubber as well, with many noting its durability as an infill material. By coating the pure silica sand with nontoxic acrylic, this material is protected from bacteria, mold, and mildew—all of which can produce stains or odors and cause deterioration. The round sand provides a firm surface, mimicking a sand-based natural-turf field, and has the unique advantage of being able to be reclaimed for subsequent uses.

Possibly the most interesting alternative, organic infill, is commonly comprised of coconut fibers and other natural additives, such as cork or processed nutshells. Cork presents some advantages: it is sustainable and natural, and there is plenty of it. Its porous quality lends itself well to the purposes of athletic fields, providing adequate cushioning for most uses. Since cork is so lightweight, though, it can be washed away or become wind-blown fairly easily. Using a variation of organic infill provides a system more similar to natural grass, without the downtime.

Using Alternatives Wisely
Each of these alternative infill products performs slightly differently than a traditional crumb rubber and sand infilled system, so each athletic field will need to be engineered accordingly. The most important elements for owners and field designers to consider are the pile height of the turf, the thickness of the infill for proper shock absorption, vertical deformation and force reduction, drainage, and adequate maintenance of the infill. For example, most fields using organic infill require the use of a shock pad in order to achieve a safe level of performance, and these same fields require different maintenance methods than crumb-rubber fields would, such as more frequent topdressing and irrigation to keep the infill moist and in place. However, this type of field will better mimic a natural-grass playing surface without the down-time needed to appropriately maintain a natural-grass field at a safe level.

It is important to take a holistic view when considering the cost of using an alternative infill. The upfront capital cost is greater when including an alternative infill in the turf system; however, a cost-benefit analysis may show that a given alternative infill could be cheaper over multiple-turf life cycles, since some can be reused during turf replacement. These savings are in addition to the value of using more environmentally responsible materials.

Environmental responsibility is more important today than ever, and part of implementing sustainable practices for field administrators is being knowledgeable about what goes into each of these materials and where it will end up at the end of the field's service life. It is also necessary to consider other sustainable properties when evaluating infill options, such as the reusability of materials in the next field, repurposed, or recycled. As the situation currently stands, institutional administrators, city officials, and even designers are all waiting for more definitive answers when it comes to crumb rubber, especially about the significance of possible health hazards. For those institutions in need of synthetic-turf fields today, time is of the essence, and waiting for additional research is sometimes out of the question. Fortunately, alternatives that satisfy parties on both sides of the debate are available to suit a variety of field needs until studies confirm that crumb rubber is, indeed, safe. It is important to take the time to review all of the variables in order to choose which infill is right for you and your players.

Thomas P. Shay is a technical manager at Woodard & Curran, an integrated engineering, science, and operations firm. He is a licensed professional engineer specializing in athletic field and recreation-facility design, and a former college athlete. Reach him at