Thirty-one years ago this month, residents in Dauphin County, Pa., awoke to news that a reactor at the Three Mile Island nuclear plant was reportedly leaking. More than 140,000 people voluntarily evacuated the area amid fears that a radioactive cloud had been released into the environment. Today counties are confronting an "airborne toxic event" that's subtler - but with bigger potential ramifications. Under new smog rules proposed by the Environmental Protection Agency (EPA), hundreds of once-compliant counties may find themselves thrown into violation of the Clean Air Act overnight.

Cities are the largest emitters of ozone, the main ingredient in smog, and urban areas have been struggling to meet air quality standards while rural counties have generally remained below smog pollution limits. But under the new standard - which will set the allowable concentration of ground-level ozone between 60 parts per billion and 70 parts per billion - rural counties in Oregon, the Dakotas, Idaho, Nevada, Kansas, Minnesota and Iowa will suddenly find themselves forced to cut smog pollution for the first time. For many of these counties, which have very few cars and little industry, the pollution in their backyards isn't even theirs - it's being blown in from hundreds of miles away.

This challenge comes as many counties are facing budget shortfalls. When the George W. Bush administration set the current standard of 75 parts per billion in 2008, rural counties predicted that the limit would stunt economic development. "They worried the new standards would severely restrict their ability to attract new business," says Julie Ufner, an associate legislative director for the National Association of Counties.

Though it's too early to tell if those fears were warranted, the concerns haven't changed in 2010. The EPA says the new standard could cost between $19 billion and $90 billion per year by 2020. Failure to follow through on a smog reduction plan, however, could be costlier. Counties that fail to comply face fines, the loss of federal highway funding and additional environmental requirements on new building or expansion projects. For most of these rural counties, already struggling to attract growth, any added burden on development could be crushing.

To understand how strict these new proposals are, consider this: Under the current standard, 322 counties of the 675 that monitor ozone levels are out of compliance. If the 70-parts-per-billion limit is adopted, 515 counties would be out of compliance, and only 15 counties would meet the 60-parts-per-billion standard.

The EPA plans to select a specific figure by August, and rural counties have until then to voice their concerns. The one demand they'll surely insist be met? That regulations be set in place to force corrective agreements between the counties producing the smog, and the counties downwind of the smog. Otherwise, come August, counties and states will have up to 20 years to meet the new limits.