Under the circumstances, keeping off-roaders out of the Clear Creek Management Area seemed like a no-brainer.

With its rugged scenery and miles of challenging trails, the federal tract in California’s San Benito and Fresno counties has long been a cherished destination for thousands of dirt bike and ATV riders. But the site straddles an immense deposit of chrysotile asbestos, and airborne dust from vehicle tires contains high levels of the cancer-causing fibers. After years of on-again, off-again study and debate, the federal Bureau of Land Management ordered an emergency closure in May 2008, based on a U.S. Environmental Protection Agency finding of long-term cancer risk.

There were strong emotions from the off-road community, but gratitude wasn’t among them.

At a public meeting to explain the decision, “we probably had 600 pretty ticked off OHV (off-highway vehicle) people,” recalled Rick Cooper, an area manager for the BLM. “There were some personal attacks, and there were some attempts at intimidation and threats. You’ve got people coming up to you and asking you, kind of offhandedly, ‘How many death threats did you get today?’”

Don Amador, western representative of the Blue Ribbon Coalition, which represents off-road groups, said the closure was “arbitrary and capricious,” because the EPA had failed to conclusively show “there was a significant risk factor.” So far, the group has voiced opposition in comments on a Clear Creek management plan, Amador said, and may take legal action unless the bureau lifts the closure.

He noted that off-road groups do not approve of hostility or threats, “and, in fact, we do our best to tamp that down.”

Cooper said he could understand the riders’ frustration. To be involved in the sport, “you’ve got to have a little bit of a daredevil in you,” he said. “The last thing that they’re even worried about is breathing asbestos fiber and what’s going to happen to them 30 years from now.”

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Related: ATV Industry Straddles Safety Debate as Off-Road Deaths Mount