You don't have to be able to smell or see air pollution to die from it.
A study of the nation's 20 largest cities confirms that small amounts of particles less than one-fifth the width of a human hair are enough to raise the death rate. And the death rate climbs steadily along with the number of these fine particles.
The study, conducted by researchers at the Johns Hopkins School of Public Health, supports Environmental Protection Agency standards that were set in 1987 and revised in 1997, said Bob Perciasepe, EPA assistant administrator for air quality.
The findings should squelch criticism that earlier research at the EPA, Harvard and elsewhere was inconclusive, said James H. Ware, dean of the Harvard School of Public Health.
He said it shows that the fine particles, and not the weather, certain chemicals or other factors, drive increases in the daily death rate.
The study, published in Thursday's New England Journal of Medicine, looked at death rates and at the amount of ''fine particulate pollution'' - that is, particles less than 10 microns across. A micron is one-thousandth of a millimeter.
Such particles come from just about everywhere - cars, power plants, construction, agriculture and road-related pollution, such as bits of brakes and tires.
The study deals in amounts almost staggeringly small: micrograms - ten millionths of a gram, or about four ten-millionths of an ounce - per cubic meter of air.
Under EPA rules, the maximum allowable level of 10-micron particles in 24 hours is 150 micrograms per cubic meter. All 20 cities averaged levels of one third or less of the maximum.
For each 10 micrograms of particles per cubic meter of air over a 24-hour period, the death rate from all causes rose just over one-half of a percentage point, the researchers said.
To put it another way: If you take a large city where about 100 people die each day, and the fine particle pollution rises by 20 micrograms per cubic meter over 24 hours, you can add one death to the daily rate. If it rises 40 micrograms, you can add two deaths.
Los Angeles averaged 148 deaths a day from 1987 through 1994, when the study took place. New York averaged 190.9, and Chicago 113.9.
''Total mortality is a relatively crude indicator of population health. Even though we are using this relatively crude measure, we are still finding an effect,'' said Dr. Jonathan M. Samet of Johns Hopkins.
The EPA rules on fine particulate pollution are now before the Supreme Court, but the findings have no direct bearing on the case, which looks at the evidence the EPA used to make its decision in 1997. The main question before the high court is whether pollution regulations must consider the costs of compliance.