Aaron Lee knows more about his bronchial tubes than most 11-year-olds. When he awakens at 3 a.m., struggling for breath in his Inglewood home, he gives himself a home breathing treatment--even though he sometimes falls back to sleep, spilling medicine on his pajamas.

Aaron, who has suffered from asthma for nine years, is among 100 children in the Los Angeles area helping researchers solve one of the thornier air quality mysteries of the 20th Century:

Why do microscopic particles in the air--a problem nationwide but most serious in Southern California--make respiratory illnesses worse? How do these particles, which are less than half the thickness of a human hair, kill?

Particulate pollution is a chemical grab bag that contains items as simple as desert sand when it is crushed into unseen powder by traffic, and as complex as the matter formed when chemicals in air pollution interact in Los Angeles.

As early as 1952, it was blamed for contributing to an estimated 4,000 deaths during a major fog episode in London. In the 1990s, it has been fingered by researchers as the culprit in as many as 60,000 deaths each year in the United States, 1,600 in the South Coast Air Basin.

Air quality officials throughout California are moving to deal with this confounding form of pollution, creating anxiety among builders and farmers at whom new regulations are aimed.

In the last three years more than a score of studies have linked non-crisis levels of the pollutants to increased respiratory and heart disease, school absenteeism, hospital visits, cancer in women and premature death.

Throughout the 1970s and '80s, ozone and other pollutants captured research money and scientific attention, diverting them from the study of particulates, which were mainly thought to mar visibility. Today, particulates are believed to be more damaging than ozone and are elbowing their way toward the top of a long list of particularly harmful pollutants.

But the limited understanding of particles and how they inflict physical damage has caused a gap in regulating killer dust and other forms of particulate pollution. Federal standards significantly more lax than those in California add to the question of just how strictly particulates need to be regulated.

In addition, particulate pollution is harder to control than ozone. It is not the direct result of manufacturing, but an indirect effect of human activities such as driving and construction.