Conrad Schneider, advocacy director for Clean Air Task Force, praised new US regulations aimed at making new diesel engines cleaner but commented that they did not do enough to account for the high numbers of older diesel vehicles.

"Those are great rules; they will hold new engines to higher standards. ... In the meantime, we're stuck with a legacy of dirty diesel engines," said Mr Schneider,

A report by the US Clear Air Task Force, a coalition of regional and local groups, claims their research indicates diesel emissions could be responsible for over 20,000 Americans dying sooner.

The research concluded that the states with the most deaths per year were New York with 2,332, California with 1,784 and Pennsylvania with 1,170. New Jersey ranked fourth with 880 fatalities.

The scientists based their research on data of diesel pollution from past analysis of general air pollution-related health problems.

The environmental group wants new regulations that encourage retro-fitting of existing diesel engines to run cleaner.