"Administrator Jackson said — and I'm using her words — has said that it's a 'myth' that the EPA is proposing to regulate farm dust,'" he said on the Senate floor. "But then her agency says 'No, we can't distinguish farm dust and other dust subject to regulation.'"

EPA has consistently rejected the idea that it is looking to regulate farm dust, and an EPA spokesman has said the EPA has no plans to put standards in place that would affect farmer. Still, Johanns and Grassley sent a letter to Jackson on Thursday that invited her to publicly support the Senate Republican bill.


"Given your previous statements, which appear to be completely consistent with this legislation, we ask you to publicly support it," they wrote. "Please submit a letter of support for this legislation within the next three weeks to provide much-needed certainty to farmers, ranchers, and other rural stakeholders across the country."

The bill would bar the EPA from regulating farm dust "unless there's scientific proof that it causes harm." Johanns said that no such proof existed.

EPA's April 2011 Policy Assessment for the Review of the Particulate Matter National Ambient Air Quality Standards recommends doubling the severity of dust regulation, according to a statement from Johanns's office.