When we--and others--have asked about the health concerns of cleanup workers in the Gulf, BP's response has been that extensive air sampling is being done, and that none of the samples have shown levels of chemical exposure that go beyond federal exposure limits set by the Occupational Safety and Health Administration.

That may be true. What the company doesn't mention, however, is that some of the samples [PDF] have come back with exposure levels that exceed different federal limits -- limits set by the National Institute for Occupational Safety and Health, an agency that's part of the Centers for Disease Control and Prevention.

Sampling results released by BP this week show that both benzene (a carcinogen) and 2-butoxyethanol ( a compound in Corexit, one of the dispersants being used) have been found above the recommended exposure limits set by NIOSH, while still within the permissible exposure Limits set by OSHA. For both these chemicals, OSHA's limit is greater than NIOSH's by a factor of 10. (We've called OSHA to get a comment for this story, but have not yet heard back.)

I asked some experts to explain how it works between the two agencies with these two different standards.

"NIOSH is an advisory board and their numbers don't have force of law. They can just follow the science," explained John Kissel, a professor of environmental and occupational health sciences at the University of Washington. OSHA, on the other hand, is more politically hamstrung. There's "a lot of discussion about how protective are OSHA standards, given that they don't get updated because Congress listens to the regulated community."

OSHA has in the past tried to adopt more stringent standards along the lines of NIOSH's recommendations, but its efforts to make these changes must go through proposed rulemaking stages and public comment periods, giving industry a chance to make arguments about the cost of stricter standards outweighing any proven benefits to worker health. And because epidemiology is a "blunt instrument," Kissel said it's often difficult to prove causation.

"You're often left with an ‘I don't know,'" said Kissel. And if you're on the side of ‘it-would-cost-me-money-if-standards-were-changed,' you say there's no problem."

According to Gina Solomon, a practicing physician and a senior scientist with the Natural Resources Defense Council, this means workers, including the ones in the Gulf, may end up being exposed "to levels that are perhaps perfectly legal, but not safe."

Even the more stringent, NIOSH exposure limits may not be sufficient, Solomon told me. That's because all of the NIOSH exposure limits are calculated based on a 40-hour work week and eight-hour work day.

"Really none of the workers on the spill are working an eight-hour day," she said. "Or even if they are, some are sleeping on boats out there, so they're still in same environment."

What we do know is that workers have reported experiencing many of the same symptoms of benzene and 2-butoxyethanol exposure, although some of these symptoms could also be caused by heat.

Franklin Mirer, a toxicologist and Hunter College professor, has been looking closely at BP's air sampling results. Mirer said that given the exposure levels, he's not surprised by worker symptoms, and that OSHA's permissible exposure limits "allow exposures where these symptoms are observed."

But faced with so many potential causes, it's hard to make an argument for more stringent chemical exposure standards, according to Kissel.

"Standards are not precise harm/no-harm boundaries. They're supposed to be protective and they might be, they might not be," he said. "It costs money to protect people to a greater extent."