Dust and soot in the air contribute to between 20 and 200 early deaths each day in America's biggest cities, according to the largest coast-to-coast scientific study of the problem. Ill health from particulates, tiny specks smaller than the width of a human hair, is spread across 20 of the largest cities in the United States--including Los Angeles, Santa Ana-Anaheim, San Bernardino and three other California areas--which are inhabited by about 50 million people, the new research indicates. Elderly people are the most frequently harmed.

For years, researchers have known that microscopic particles can lodge deep in the lungs. They have known, as well, that high levels of particles in the air are associated with respiratory illness, heart attacks and premature deaths. But whether the particles are actually the cause of illness has been in dispute. The Environmental Protection Agency has been attempting to tighten limits on emissions of particles. Critics in the business community are challenging those new rules in the Supreme Court, arguing that the regulations are too costly and that the scientific evidence behind them is too sketchy.

The new study, conducted by a team of researchers at Johns Hopkins University in Maryland and published in the current edition of the New England Journal of Medicine, is likely to bolster the EPA's case. The researchers found strong evidence that dust and soot particles, not other factors suggested by industry, appear to be causing the harmful effects. And they found that the ill effects occur even in cities that meet existing national air pollution standards--suggesting that stronger controls would protect public health.

The researchers examined daily changes in air pollution and mortality between 1987 and 1994 and made allowance for other factors that could skew the results, including access to health care, influenza outbreaks, socioeconomic status, weather and the presence of other pollutants. The EPA currently sets the maximum allowable concentration of microscopic particles, called PM10, at 150 micrograms per cubic meter of air. All the cities tested had average air pollution levels well below that level. But the researchers found that even at those existing levels, if the amount of particles rose by 10 micrograms per cubic meter, the rate of death increased about 0.5%. Based on those existing levels, the researchers estimated that 20 to 200 deaths per day nationwide are caused by the particles.

"When we look nationally, we see an effect of particles on mortality that suggests there is a public health problem. The science continues to indict particles and their role in mortality," said Dr. Jonathan M. Samet, lead author of the study and chairman of the epidemiology department at Johns Hopkins. "The higher the PM10 levels, the higher the health effects, so if you are exposed to high levels, the risk is greater," said Jean Ospital, health effects officer for the South Coast Air Quality Management District, which is charged with cleaning up smog in the Los Angeles region.

Other studies have estimated that particulate pollution may cause 1% of heart disease fatalities in the United States, which translates to about 10,000 deaths per year or about 28 per day. In Los Angeles County, 77 residents die from cardiovascular disease from all causes daily. Some scientists have questioned whether particle smog merely hastens by a day or two the demise of people who are elderly or very ill. The new study does not conclusively answer that question, but the researchers noted that the evidence is "heavily in the direction that the deaths are among people of old age."

Some of the nation's worst particle smog is found in California, both in the Inland Empire and in parts of the east Bay Area and the San Joaquin Valley. In addition to the three Southland cities, the other California metropolitan areas in the study were San Diego, San Jose and Oakland. Elsewhere, the researchers looked at New York, Philadelphia, Pittsburgh, Atlanta, Miami, Cleveland, Detroit, Chicago, Minneapolis, Dallas-Forth Worth, Houston, San Antonio, Phoenix and Seattle.