Some of the chemicals with federal standards have exemptions big enough to drive a tanker through. OSHA’s limit for benzene, which can cause leukemia, is 1 part per million. But for some workers, including those involved in oil and gas drilling, the limit is 10 times higher. Frank Parker, an industrial hygienist who runs a consulting firm in Texas, says high benzene exposures are endemic in the shale fields there and in neighboring states. Shale oil contains a bigger proportion of benzene than typical crude oil, Parker says — up to 5 percent, in his experience. Image from the Noun Project/ Roban Gupta.