When it's smoggy outside, the ozone (O3) responsible for the murk slips indoors, too, wafting through doors or ventilation systems. Once inside, the volatile oxygen molecule reacts with carpets, chemical cleaner residue and human skin.

In fact, according to new research published last week in Proceedings of the National Academy of Sciences, ozone coming into contact with human skin and hair, specifically the oils on each of them, sets off a whole lot of chemistry, some of it possibly of concern.

"Ozone and humans have been interacting since the dawn of man," says chemist Charles Weschler of the Environmental and Occupational Health Institute in New Jersey, lead author of the study. But "we found that when ozone reacts with skin oils you get a series of products, some of which have not been previously identified."

One of those products, known as 4-oxopentanal, or 4-OPA, is structurally similar to diacetyl, perhaps better known as the popcorn-butter flavoring chemical that has caused serious lung issues for factory workers, known as "microwave popcorn lung". "We don't know how toxic some of these compounds are," Weschler says. "4-OPA's structure is similar to other dicarbonyls that we do know have adverse effects."

It all started in airplanes
Weschler and colleague Armin Wisthaler of the University of Innsbruck in Austria first studied the issue of ozone in aircraft cabins. Whereas many larger planes have special filters to eliminate ozone, smaller planes, such as Boeing 737s, typically do not. The researchers hoped to determine what kinds of compounds were being produced when ozone interacted with the people and parts inside a jet.

On examination, the ozone proved to be largely interacting with one of the primary components of skin oils, the natural organic compound known as squalene, which is used as an adjuvant and moisturizer. Inside the body, squalene is the precursor of cholesterol, but the sebaceous glands in the skin churn out the stuff as is and ozone is drawn to it like a magnet, Weschler says.

In fact, it appears that squalene is the primary antioxidant protecting the skin from ozone, not vitamin E or other chemicals. "I'm not a dermatologist," Weschler says. But "it's a very good thing we have squalene and these unsaturated fatty acids in our skin. I think it keeps other bad things from happening."

In the new study, Weschler and Wisthaler first used proton transfer reaction mass spectrometry—a tool for detecting volatile organic compounds in air at levels of as little as a few parts per trillion—to determine what appeared after the squalene and ozone interacted. The interaction immediately produced acetone, geranyl acetone, hydroxy acetone and a compound known as 6-MHO—none of which are considered cause for health concern.

But some of those products go on to interact with yet more ozone in the air to form dicarbonyls—the aforementioned 4-OPA, plus 1,4-butanedial, 4-MON and 4-MOD. And it is these that might be cause for concern. "We did not find these carbonyl products in the absence of ozone," Weschler notes. "It's not something we naturally emit."

And all of them are possible lung irritants.

Testing pseudo–office workers
More than 35 million Americans complain of eye, nose or throat irritation when working indoors, costing the U.S. economy at least $20 billion annually in lost productivity. Some researchers have pinned the blame on indoor air pollution and a study last year showed that when outdoor ozone levels rose, the number of people inside suffering from so-called "sick building syndrome" also increased.

So Weschler and Wisthaler simulated a typical office environment at the Technical University of Denmark in Copenhagen—two people in a carpeted 28.5–cubic meter room at a temperature of 23 degrees Celsius with two small stainless steel tables, two chairs, two flat-screen LCD monitors, two headsets, one walkie-talkie, one small mixing fan, a few books, two laptops, two bottles of water and ozone concentrations that reached roughly 32 parts per billion, an average exposure for a hot, smoggy day.

The squalene on these pseudo–office workers' skin soon began to interact with the ozone in the chamber. In fact, just one person in a similar size room removes as much as 25 percent of the ozone in the air, according to the results of the tests, turning it into various by-products. And, in cases where ozone preceded the people into the room, 4-OPA reached levels as high as two parts per billion after just four hours.

The question is: is that cause for concern?

No one knows, but the National Institute for Occupational Safety and Health (NIOSH) has begun testing. "Do dicarbonyls in general have similar effects as diacetyl?" asks NIOSH chemist Ray Wells of the compound responsible for popcorn lung's chemical cousins. "They are being formed but the concentrations that people are being exposed to we don't really know yet."

NIOSH immunotoxicologist Stacey Anderson has begun testing dicarbonyls, 4-OPA in particular, to see if they provoke an immune reaction in mice or human lung cells. "We have some promising results from that work in 4-OPA and others, some traditional markers for irritation," such as cytokine levels, she says.

Weschler adds: "The jury's still out on some of these oxidation products. Four-OPA might be of concern, some products that remain on skin might be irritating." And it may be that the products have some kind of additive effect.

"It underscores the necessity to control ozone entry into buildings in regions with even moderate ambient ozone levels," says environmental health scientist Michael Apte of Lawrence Berkeley National Laboratory in California. "Likely, the findings in this paper contribute to an explanation of symptoms in the upper respiratory system and mucosa, and the lower respiratory system. It may explain skin-irritation symptoms."

If that's the case, and a lot more research needs to be done to determine that, then there's a simple solution to sick building syndrome: "You can shut this chemistry down by simply getting rid of the ozone," Weschler notes, which can be accomplished with a simple charcoal filter in HVAC (heating, ventilating and air-conditioning) systems—a remedy currently being considered as a guideline by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. "If they wanted, they could take ozone out—and I'd like to see that happen."