Back in the spring of 2002, as Benepe was extolling the benefits of Green Acres, Brigham Young University installed a synthetic-turf athletic field next to a natural- grass one, and football coaches immediately noticed the artificial turf was much hotter than the grass—one coach's feet blistered through his tennis shoes.

On a warm day that June, two BYU professors monitored the fields from 7 a.m. to 7 p.m. and discovered the synthetic turf 's rubber and plastic absorbed more of the sun's heat than the grass did. Synthetic turf reached extraordinary temperatures that were 37 degrees hotter than asphalt and 86.5 degrees hotter than natural turf. When the air temperature hovered around 81 degrees, the turf averaged 117 degrees. When the air temperature reached 98 degrees, the turf clocked in at a scorching 200 degrees. The researchers found the field would cool down when doused with water, but the effects were short-lived: Within 20 minutes, the shocking heat returned. Far from offering a surface that would combat obesity by being available at all times, synthetic turf got so hot, these findings suggested, that it could actually discourage play during the warmer months, when demand was sure to be at its highest.

That same year, doctors were cataloging injuries common to synthetic turf, including heat exhaustion. Nationwide, heat stress has been blamed in the deaths of about 30 high school football players during the last decade, though it's not clear how many were playing on turf. Athletes had long complained about the old carpet-style AstroTurf, with 87 percent of NFL players saying they preferred grass. Synthetic turf was promoted as a safer alternative, but doctors were seeing the same kinds of injuries, including ankle sprains, ligament tears and "turf burns," skin abrasions due to athletes sliding on the surface.

These burns, researchers believed, exposed the players to infection, which could be passed among teammates in a variety of ways, from sharing towels to using the same locker room facilities. One scientist at the University of Missouri called artificial turf a breeding ground for harmful bacteria, leading turf manufacturers to market pretreated antimicrobial fields as well as new chemical solutions and equipment to clean surfaces of sweat, spit and blood. "Regular maintenance of artificial turf is necessary to keep playing surfaces clean," warned the turf management company Synpro.

In 2003, the federal Centers for Disease Control and Prevention blamed turf burn for providing an entry point for the antibiotic-resistant staph infection known as MRSA. The CDC found eight cases of MRSA in five players on the St. Louis Rams, and skin scrapings from those infected pointed to turf burn as the cause. After a St. Louis game that season against San Francisco, some players on the 49ers were also diagnosed with MRSA.

Artificial turf ended up playing a prominent role in Mayor Bloomberg's PlaNYC program which, among other things, called for turning schoolyards into parks. The city's argument for turf has often stressed its environmental attributes. In 2007, Benepe attacked turf 's critics on WNYC's The Brian Lehrer Show.

"There's a phony environmentalism that says synthetic turf is somehow much worse than so-called real grass," Benepe said, claiming that natural-grass fields require "lots of chemicals" and "fossil-fuel-burning machines" to mow them. The Parks Department, in fact, has 843 mowers, of which just one is electric.

"These synthetic fields use recycled tires," Benepe bragged. A single soccer field recycled 27,000 automobile tires. "I think environmentalists need to walk the walk and talk the talk, and synthetic-turf fields really just make sense for inner- city sports."

The environmental claims were surprisingly common for a product drawn from scrap tires. FieldTurf Tarkett, the world's largest installer of artificial turf, told organizations that its reused-rubber content could help them earn the necessary points for construction projects to be certified as green by the U.S. Green Building Council.

The Parks Department had long claimed that artificial turf was mostly replacing asphalt lots. But of the Parks Department's total 111 artificial-turf fields as of early last year, 39 had gone down on asphalt, and 72 replaced grass. The loss of grass bothered Bill Crain, a developmental psychologist at the City University of New York, because he believed nature is essential for city dwellers. "Children need contact with plants and trees to develop a sense of calm," he explained, citing studies that showed attention-deficit disorders were reduced after kids spent time in nature.

He had once collected 600 signatures on an anti-turf petition, but he wasn't a firebrand. A soft-spoken professor and the author of two books on childhood development, Crain had a philosophical bent, quoting the maxims of Romantic writers like Jean-Jacques Rousseau. His opposition to turf was purely professional, he said, until a sense of urgency grabbed him when, during an April 2006 stroll in Riverside Park, he came upon four new athletic fields.

From a distance, the 4-acre swath looked lush and green, but a closer inspection of the soccer field at 107th Street revealed bright plastic strips, almost like Easter basket grass, poking out of a black sea of rubber crumbs.

"These granules are loose," Crain said, as he bent to pick up a handful of the rubber pellets. Leaning against a fence was Deborah Peretz, a mother of two who lived nearby. Her children liked the new soccer field, laughing as they ran on the springy surface, but Peretz had concerns about the acrid scent. "You can smell the rubber," she told Crain. The crumbs came home in her son's shoes and pockets. "I find them everywhere."

Crain was startled. Scrap tires are a dirty business, he knew, and their disposal is strictly regulated. If they catch on fire, the blaze emits toxins and is difficult to extinguish; a 1983 tire fire in Virginia burned for nine months, polluting the air and groundwater.

"I thought, my goodness," Crain recalled, "her son is carrying around these rubber crumbs, and these rubber crumbs are shredded from old tires, so children are probably playing on all kinds of toxic chemicals."

He decided to pay for a toxicology test, collecting a sample of the rubber crumbs in a juice glass and sending it to a lab at Rutgers University. The lab's chief was Junfeng Zhang, chair of the department of environmental health and associate dean at the University of Medicine and Dentistry of New Jersey. Zhang found the crumbs did indeed contain toxins, including metals like lead and chemicals linked to lung, stomach and skin cancer.