As Nevada fights the "Brown Cloud" of pollution over Las Vegas, diesel smoke is coming under more intense scrutiny.

The Las Vegas Review-Journal reports that a state committee next month will consider recommending that Nevada tighten its relatively lax standard for diesel smoke. Nevada is one of a number of states that test for opacity, or thickness, of diesel smoke using a special opacity meter. In Nevada and Utah, diesel smoke is allowed to be no more than 70 percent opaque. That's 30 percentage points higher than in most states. The reason for the lax standard, according to the paper, is that environmental regulators classified Nevada as a high-altitude state, where less dense air makes it harder for engines to achieve a cleaner-burning fuel/air mix. But the "Brown Cloud" hangs over the low-lying Las Vegas Valley.
In addition, Clark County's new Air Quality Management Department is expected to launch a $300,000 study of how much diesel contributes to the Las Vegas Valley's air pollution. "Certainly diesel emissions are an area we want to focus on as an agency," air department head Christine Robinson told the paper.
Pollution is an issue because the Las Vegas Valley has violated an upcoming federal ozone standard on three days this year.