BioEd Online

A potentially harmful smog can form inside homes through reactions between air-fresheners and ozone, say researchers at the US Environmental Protection Agency (EPA). The reactions generate formaldehyde, classed as a probable carcinogen, and related compounds that many experts believe are responsible for respiratory problems.

The researchers studied the reactions between ozone gas and fragrance molecules such as pinene and limonene, which are emitted by air-fresheners that plug into electrical outlets. Ozone, produced at ground level when vehicle exhaust emissions react with sunlight, is a common urban pollutant, and environmental bodies have set limits on outdoor levels of it.

"If you open a window on a high-ozone day, you could trigger these reactions," says Mark Mason, an environmental scientist at the EPA's National Risk Management Research Laboratory, North Carolina. Mason led the study, which is published in Environmental Science and Technology¹.

Some people actually use ozone generators in their homes to remove unwanted odours and 'clean' their air, which could create indoor ozone levels that are much higher than those in the study. There is currently no regulation of household ozone levels.

"If you are concerned about indoor air, you should not introduce any extra chemical sources to your home, and that includes volatile organic compounds and ozone," advises Frank Princiotta, director of the EPA's Air Pollution Prevention and Control division.

"This EPA study is only preliminary because it is based on work in a room-sized test chamber rather than a house," cautions Ken Giles, public information officer at the US Consumer Product Safety Commission, which regulates products such as air-fresheners.

"But we do not think that 'freshening' air is a good way to deal with air pollution," he adds. It is better to prevent the smells you are trying to disguise in the first place, rather than covering them up with more chemicals, he argues.

Particle exposure

Mason's team found that mixing ozone and air-freshening chemicals generated particles of formaldehyde-related compounds at a concentration of about 50 micrograms in each cubic metre of air. This is close to the EPA's outdoor particle limit. But in comparison, a noticeably smoky room will have more than 100 micrograms of particles per cubic metre, says Ken Donaldson, a toxicologist at Edinburgh University, UK.

"The study finds the same sort of exposure that you might get from painting a room, but in the long term the effects may add up," says Donaldson. "Basically, this is yet more particle exposure, which you do not want."

Similar particles are belched out by vehicle exhausts and are known to cause respiratory problems, says Donaldson. As a rough estimate, an increase of ten micrograms of particles per cubic metre of air will lead to a 1% increase in deaths from conditions such as asthma, he calculates.

"We now have reasons to be concerned, but we need specialist health studies before we think about regulation," says Bill Nazaroff, an environmental engineer at the University of California, Berkeley. "I think it is a bad idea to have [ozone generators and air-fresheners] in the same room, but I also think ozone air cleaners are a bad idea, period," he says.

There are potential solutions to the problem, however. "Air-freshener manufacturers could limit these reactions by changing their formulations," suggests Mason. Nazaroff agrees: the room fragrances could be concocted from less reactive chemicals, much as gasoline has been reformulated to produce less smog, he says.