The skinny on indoor ozone

SAN FRANCISCO — Smog’s ozone isn’t just a problem outdoors. This respiratory irritant seeps into homes and other buildings. Indoor concentrations tend to be far lower than those outside, largely because much gets destroyed as the gas molecules collide with surfaces and undergo transformative chemical reactions. New research identifies a hitherto ignored surface that apparently plays a major role in quashing indoor ozone: It’s human skin — or, more precisely, the oils in it.

While the removal of ozone from the air in homes and offices is good, what takes its place may not be. Indeed, some byproducts created when ozone hits skin oils are probably more toxic than the starting ozone, according to scientists here at the American Chemical Society spring national meeting.

Charles Weschler of the University of Medicine and Dentistry, New Jersey, in Piscataway, described stumbling onto skin’s newfound role in driving the chemistry of indoor air while working on the Indoor Environment and Children’s Health Study in Denmark.

Part of the program required analyzing pollutants in dust from the bedrooms of 500 children living on the island of Fyn. The highest volume constituent was diethylhexyl phthalate, better known as DEHP, a hormone-mimicking chemical found in plastics (including those used by hospitals to dispense blood and blood products), fragrances and many other items. Its quantities came as little surprise; phthalates are, after all, everywhere.

But cholesterol, the second most prominent chemical — accounting for around 300 micrograms of every gram of dust — was initially a bit of a head-scratcher. Until, that is, the aerosol chemists realized what the third most common chemical pollutant in the dust was: squalene, a fatty substance that accounts for 10 to 12 percent of the oil found in human skin — but few other places within a home.

As cholesterol also occurs in skin (accounting for about four percent of the oils there), the human body suddenly began to look like a likely source of both.

The body sloughs off its entire outer surface, in bits and pieces, once every two to four weeks, Weschler notes. Each corn-flake-shaped piece of skin weighs perhaps 2 billionths of a gram. Which doesn’t sound like much until you consider that the average person sheds 30 to 90 milligrams of oil-bearing skin flakes per hour. Over time, these skin shards end up coating surfaces throughout a room — every room that people enter.

In Denmark, squalene accounted for, on average, around 30 micrograms per gram of house dust collected from preschoolers’ bedrooms and 10 µg/g in daycare centers’ dust. And that piqued Weschler’s curiosity, because squalene suddenly started to suggest an explanation for a long-standing conundrum.

“We’ve known that the ozone indoors is being gobbled up,” Weschler says. “But we really didn’t know what’s doing the gobbling.” It’s now clear, he told me after his talk, “This squalene is just great at chewing up ozone.”

Cholesterol isn’t as good an ozone gobbler as squalene is or nearly as prevalent in skin oil, but Weschler and his colleagues found scads of it in house dust. In kids’ bedrooms, for instance, the dust contained about 300 µg/g. Even daycare dust hosted some 100 µg/g. Clearly, this didn’t correlate with the human occupancy of the rooms, Weschler noted; it was way too high. So there must be other sources, such as foods perhaps.

In any case, he says it now appears our skin oils might be largely determining indoor ozone concentrations.

And that’s because shed skin’s adherence to other surfaces tends to make the reactivity to ozone of those underlying surfaces more like squalene’s. The skin essentially homogenizes what had been vastly different ozone reactivity potentials of paint, glass, furniture, wood, metal and carpeting.

For instance, Weschler pointed to a window and noted that “when this glass started out clean, ozone would just look at it — nothing would happen. But once I put my fingerprints on it [as he pressed his hand onto the window], the ozone [that strikes it] will now be gobbled up by all of the skin oils I’ve just left behind.”

And it’s possible, Weschler notes, that the relatively high volatility of squalene means some of the oil may just enter the air from intact skin and then plate out over time onto walls and other surfaces.

The big issue, of course, is what the ozone-breakdown products mean for human health. And no one knows for sure yet because the families that form were just described at the meeting — and will be published soon by Weschler and Armin Wisthaler of Leopold-Franzens University in Innsbruck, Austria, in the Proceedings of the National Academy of Sciences. That paper is available online, ahead of print.

In it, the pair describe the chemicals spawned during small scale chemical-mixing experiments using skin oils and ozone in addition to a study where people were exposed to ozone concentrations typical of air. A host of novel pollutants emerged, including some carbonyls — at least one of which is a chemical cousin to diacetyl, the carbonyl responsible for popcorn-worker’s lung disease.

That ozone-bred carbonyl in indoor air is 4-oxopentanal, or 4-OPA. Not only was it not recognized as an indoor air pollutant, Weschler says, but until now, “there were absolutely no tox[icology] studies on 4-OPA.” Such studies are getting under way at the National Institute of Occupational Safety and Health. Ray Wells noted that preliminary data by his team at this federal research center in Morgantown, W. Va., suggest 4-OPA might indeed be nasty.

They used something called a “local lymph node assay” to evaluate the chemical’s irritancy. And “what the local lymph node work that we did showed is that 4-OPA is a very high sensitizer to irritation and inflammation,” Wells says.

What’s really new in the work Weschler just presented “is showing how important human occupants are in reacting with ozone,” says William Nazaroff, an indoor-air quality expert at the University of California, Berkeley.

“Ozone’s harmful, so reducing its level is good,” he says. But it appears that when squalene and other skin oils interact with ozone, “the tradeoff is a losing proposition. For every molecule of ozone that you get rid of — and get some health benefits from — you are producing byproducts. And these byproducts, molecule for molecule, are more harmful than the ozone was.” Or at least at first blush, they appear to be. He too expressed special concern about the newly identified formation of carbonyls like 4-OPA.

But if these are short-lived pollutants, the byproducts might not be so bad? “They are short-lived,” Nazaroff says. “But they’re not short-lived enough. So you’re going to be breathing them.”

In homes with a few occupants, concentrations of these compounds may not be too bad. But their production, he says, might pose problems, at least for sensitive segments of the population, while they’re in densely packed spaces — schools, perhaps, or planes and crowded subway cars.