WASHINGTON — Emissions from old diesel engines cause more than 20,000 Americans a year to die sooner than they would have otherwise, an environmental group estimated Tuesday.

An industry group criticized the findings as outdated and misleading. The metropolitan areas with the highest number of early deaths from diesel engines were New York, Los Angeles, and Chicago, according to the Boston-based Clean Air Task Force. The study included the surrounding suburbs, so New York's estimated total of 2,729 deaths included parts of New Jersey and Connecticut.

The states with the most deaths were New York with 2,332, California with 1,784, and Pennsylvania with 1,170, according to the group.

The group said it based its figures on the most recent government emissions data - from 1999 - and from public health studies of the effects of various types of air pollutants.

Conrad Schneider, co-author of the report, said regulations designed to make new diesel engines cleaner don't affect millions of older trucks, buses and construction engines.

"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 Schneider, advocacy director for the Clean Air Task Force, a coalition of regional and local groups.

The Environmental Protection Agency last year required new diesel engines on trucks and buses to cut in half the amount of nitrogen oxides produced. In 2007 emissions are to be cut further.

Since many older diesel engines can run for 30 years, more action is needed by federal, state, and local governments to retrofit existing diesel engines to run more cleanly, the group said.

Retrofits for a typical transit bus can cost about $5,000 to $7,000 US.

The head of a Washington-based industry group criticized the report's assumptions and conclusions.

"I think they have overstated the risk here using data that's six years old," said Allan Schaeffer, executive director of the Diesel Technology Forum.

Schaeffer said it takes eight modern tractor trailer engines to produce the same amount of pollution generated by one such engine made twelve years ago, and that diesel exhaust comprises just 4.4 per cent of fine particle pollution.

"Our industry is getting cleaner faster than most other industries out there," Schaeffer said.

Diesel pollution is blamed for contributing to asthma, respiratory diseases, and heart attacks. The study estimates the risk of health complications from diesel exhaust for people living in cities is three times higher than the risk for those in rural areas.