WASHINGTON -- While some truckers congratulate themselves for being early adopters of new emission-busting diesel engines, additional stringent air-quality rules could be around the bend for many more U.S.-bound carriers.

The U.S. Environmental Protection Agency recently proposed to tighten health standards for smog, a move that could lead to more state and local regulations of trucking. 

Under the EPA's new smog proposal, the agency wants to set the "primary" standard for ozone, between 0.060 and 0.070 parts per million. Existing ozone standards are set at 0.075 parts per million, mandated in March 2008.

While post-2007 diesel engines virtually eliminate soot-causing particulate matter and 2010 model engines drastically reduce NOx, which creates smog, the new EPA mandate could affect older trucks down the road in jurisdictions that choose to enact their own stricter emissions regulations.

California and New York are two states that commonly legislate their own air quality standards, which place additional demands on some truckers operating there.

The California Air Resources Board's Truck and Bus Rule, for example, requires most truck owners to install diesel particulate filters on their rigs by Jan. 1, 2011, with nearly all vehicles upgraded by 2014. And a rule requires reefer units to also have their own DPFs installed.

"EPA is stepping up to protect Americans from one of the most persistent and widespread pollutants we face," said Lisa P. Jackson, EPA administrator. "Smog in the air we breathe poses a very serious health threat, especially to children and individuals suffering from asthma and lung disease. It dirties our air, clouds our cities, and drives up our health care costs across the country."