A rarely mentioned class of air pollution particles known as “secondary organic aerosols” (SOAs) has more mass and is more dangerous than previously understood. That’s the conclusion of a new study released yesterday by scientists at the University of California, Irvine. The new research is causing top air-quality officials to question whether their current pollution-control strategies are effective. It’s left some wondering exactly how clean the air in urban areas is – or isn’t. “If the authors’ analysis is correct, the public is now facing a false sense of security in knowing whether the air they breathe is indeed safe,” Bill Becker, the head of the National Association of Clean Air Agencies (NACAA), told the New York Times. NACAA is the nation’s leading association of air-quality management agencies.

If you are already using a high-efficiency air purifier, take some comfort in knowing your mechanical filtration system is theoretically stopping these aerosol particles as effectively as it stops other particles of the same size in the air. What makes secondary aerosol particles, formed by the interaction of pollutants and natural chemical compounds, unique is they can be as much as 10 times denser than ordinary pollution particles. Previously, the tiny aerosol droplets were thought to dissipate quickly through evaporation. But the new research describes gases that essentially convert to particles and then remain particles.

“They check in and they can’t check out,” lead researcher and chemist Barbara Finlayson-Pitts told the Orange County Record. The effect of these secondary pollution particles helps explain why many computers provide a large underestimate of smog-particle masses.

Air purifiers that are effective against pollution particles are not necessarily effective against gases, although systems such as the IQAir HealthPro Plus include separate filtration media for particles and gases. Gases are generally tinier than particles and effective filtration requires an air purifier with a specialized filtration medium such as activated carbon. The research on SOAs was conducted at the University of California, Irvine, by a team of researchers known as “AirUCI.” Scientists at AirUCI study a variety of air-quality topics, including the effectiveness of indoor air purifiers on air quality. To conduct the current study, the researchers mingled pinene, an organic gas found in Pine-Sol and other household cleaners, with nitrogen oxides and ozone, a known lung irritant.