WASHINGTON -- Several studies have concluded that tens of thousands of deaths are being caused in the United States each year by a form of air pollution that for the most part falls within current legal limits: tiny particles of soot that are inhaled.
Rough calculations emerging from studies at the Environmental Protection Agency and the Harvard School of Public Health suggest that 50,000 to 60,000 deaths a year are caused by the particle pollution, a far larger number than any other form of pollution and one that rivals the death toll from some cancers.
The deaths occur mostly among children with respiratory problems, people of all ages with asthma, and the elderly with illnesses such as bronchitis, emphysema and pneumonia.
The new findings, disclosed gradually over the last 2 1/2 years, will require environmental scientists and industries to re-evaluate their approaches to air pollution and will probably require the federal government to rewrite its standard for particle pollution. The troublesome particles are mainly thrown into the air by industrial plants, with a small proportion coming from the exhausts of diesel vehicles.
Paul Portney, vice president of Resources for the Future, a not-for-profit environmental research group, said that of the $35 billion a year the nation spends for all its scrubbers, catalytic converters and other air-pollution control efforts, only a fraction is aimed at the small particles, which are 10 microns or smaller in diameter. (By comparison, a human hair is about 75 microns in diameter.)
Only one-third of all air-pollution expenditures go to remove particles, and most of that goes to decades-old efforts like devices on power plants that catch only large particles, those that are larger than 10 microns in diameter.
Regulatory effort has been focused more on other types of pollution, such as ozone and sulfur dioxide, that have been shown to damage health; it is uncertain whether they cause death. It is believed that indoor air pollution, such as secondhand cigarette smoke and radon, cause the greatest health damage among pollutants, apart from the particles. Each is estimated to cause 5,000 or more cancers a year.
Rep. Henry A. Waxman, D-Calif., who heads the subcommittee on energy and the environment, said: "The EPA must now undertake a thorough review of the particle standard. The agency under the Bush administration never even got started with the legal requirements that this standard be reviewed. By law, it should have been finished last year."
Ronald H. White, director of environmental health for the American Lung Association, said the evidence for deaths from particle pollution is now great enough that the association will soon sue the EPA in hopes of getting the agency to revise its standard and strictly enforce it.
But some scientists say the evidence is not yet good enough to make policy decisions. The evidence is based on epidemiological studies, which cannot conclusively establish a cause-and-effect relationship between a substance and its health effect; for that, detailed biological studies of the effect of substances on the tissues themselves are needed.
Dr. Jonathan Samet of the University of New Mexico said: "What is missing is understanding the biological basis of the effect they are describing. We shouldn't make major policy decisions about causes of death based on the kind of evidence we have so far.
"The effects are not massive, not like the London fog of 1952 in which thousands of people died, but are more subtle. Before we make the big decision to control particles more, with the costs of doing that, we need to fill out the story more. We need to do actual exposure studies with people to see how particles affect them."
Dr. Samet said he believed there still may be other ways of explaining the deaths linked to pollution -- for example, combinations of factors, such as respiratory illness coming with a heat wave at the same time as a pollution episode.
Rob D. Brenner, chief of policy in the EPA's air pollution office, said: "We have a real concern about the new data. If it turns out that recent analyses show that particles are the much more significant problem than they seemed to be, our efforts to control particles and our acid rain initiatives will not be enough. We will have to do more."
The EPA missed last year's deadline to update the particle standard.
In early budget estimates, the EPA has not set aside money to review the particle standard for the next fiscal year, but the agency is planning to spend $10 million to revise the ozone standard.
Particles have been among the most obvious and longest-known forms of pollution. During times of stagnant air, the particles make a haze or fog. Such pollution has been recorded as early as the 17th century in England. Among the most famous episodes was the London pea soup of 1952 that killed 4,000 people in a few days.
But particle pollution has been relatively neglected and difficult to study. Particles of all kinds are present in air, so the polluted substance that must be studied is "urban air," a messy hodgepodge.