Almost one-third of the heavy trucks on U.S. roads now use the most stringent emission-control systems, which radically reduce their emissions of nitrogen oxides, soot, and other pollutants.
Commercial trucks log much higher annual mileage than passenger cars, especially the semi tractors that haul long-distance freight across the country.
That makes reducing their emissions a hugely important piece of the air-quality and climate-change puzzle, especially globally.
California, the North American leader for more than half a century in cutting vehicle emissions, and several other states want those trucks to do better still.
In June 2016, government agencies from Arizona, California, Connecticut, Delaware, Nevada, New Hampshire, New York, Ohio, and Washington petitioned the EPA to look into tightening the truck-emission standards that went into effect for 2010.
Massachusetts joined the list just days later.
The proposal calls for instituting "ultra-low nitrogen oxides (NOx) exhaust emission standards for on-road heavy trucks and engines."
Specifically, the request wants NOx emission standard for new trucks lowered from the 0.2 grams per horsepower-hour to 0.02 g/hp-hr. That would represent a 90-percent reduction from current emission levels.
Last December, the environmental agency responded that it acknowledged "a need for additional NOx reductions from this category of vehicles and engines, particularly in areas of the country with elevated levels of air pollution."
The EPA said it intended to initiate a rulemaking process, in consultation with stakeholders, that would revise existing federal "on-highway heavy-duty NOx emissions control program."
It also said it would work to harmonize federal and California standards, since the state alone among all states has the legal right to set its own vehicle emission standards.
The agency suggested the necessary technical work to propose new standards would take roughly 24 months, with a target of new regulations to take effect for the 2024 model year.
Whether such work will survive under new EPA administrator Scott Pruitt, a climate-science denier who has proposed a budget cut of more than 30 percent to the agency he runs, remains unknown.
[hat tip: CCM591]