We may not think of our living rooms and offices as chemical factories, but reactions occurring within them can produce a dangerous array of toxic air pollutants, scientists say.
In some cases, these chemicals are formed by the same reactions that produce urban smog, says Sasho Gligorovski, a physicist and atmospheric chemist at the Chinese Academy of Sciences in Guangzhou, China, and coauthor of a review article on the subject in the journal Science.
For example, he says, indoor environments can contain highly reactive hydroxyl (OH) radicals at levels comparable to those in outdoor air. That’s a surprise because in urban smog, hydroxyl radicals are produced by photochemical reactions involving ultraviolet radiation from the sun and nitrogen-containing pollutants from such sources as car exhaust. Finding similar levels of hydroxyls in indoor air, he says, means that enough ultraviolet is penetrating windows to produce the same reactions indoors.
And while this doesn’t mean indoor air is turning into smog, it does mean that a lot of chemical reactions are possible within it.
“The high concentration of OH radicals indoors makes the indoor environment [into] a reaction chamber,” Gligorovski says.
In addition, while we may think of our air-conditioned indoor air as cleared of pollutants, we are constantly adding materials into it, ranging from household cleaners to hairspray, cooking fumes, and scented candles. Some, like cigarette smoke, have long been known to be toxic. Others might be surprising. For example, Gligorovski says, burning incense is associated with increased risk of lung, throat, and mouth cancer, especially among people who are frequently exposed to it, as in Buddhist temples.
Even our own bodies can be sources of such chemicals. A paper in the journal Proceedings of the National Academy of Sciences, Gligorovski says, found that oils on our skins will react with ozone in indoor air, producing a host of potentially dangerous byproducts.
“The chemistry is sufficiently fast that many of the chemically reactive oils on our skin are transformed into more oxidised molecules on timescales of tens of minutes,” he says.
Other scientists note that such reactions aren’t the only processes at work in indoor air — and may not be the most dangerous ones. It’s long been known that in some geological settings, buildings can collect enough radioactive radon gas seeping up from the ground to significantly increase the risk of lung cancer.
Also, dampness and mould are clearly associated with respiratory health effects, such as asthma, says William Fisk, a mechanical engineer who heads up the Indoor Environment Group at Lawrence Berkeley National Laboratory, Berkeley, California.
“Indoor allergens from dust mites, cockroaches, rodents, and pets [also] contribute to allergy and asthma symptoms,” he says.
How the new findings about indoor air chemistry fit into this context isn’t yet known, but “clearly, there are many chemical reactions occurring indoors,” Fisk says. “The significance of these for human health is not well understood and should be investigated.”
Hugo Destaillats, an environmental chemist in the same research group as Fisk, adds that chemical reactions of the type discussed by Gligorovski don’t occur only in the air. They also occur on surfaces — important because indoor environments have a much higher surface to volume ratio than the great outdoors.
Not to mention that many of our indoor materials have complex three-dimensional structures through which air can percolate.
“Think about the thick layer of gypsum compressed in the wallboard commonly used in the USA,” he says. “Or about the fibres in carpet, or the polyurethane foam in upholstery and bedding.”
Not only do these materials have large surface areas exposed to the air, but they may accumulate pollutants today, only to release them in the future.
Meanwhile, Gligorovski says, it’s important to learn more about indoor air chemistry. Partly that’s because we spend a lot of time indoors, but it’s also because energy-efficient homes and offices have reduced ventilation, increasing the degree to which indoor air pollutants can build up.
“Considering that people spend on average 80 to 90% of their life indoors, indoor air quality is of major importance,” Gligorovski says.
But that doesn’t mean the situation is hopeless. “Understanding indoor chemistry can help us design materials that effectively sequester and eliminate pollutants from indoor air,” Destaillats says. “There are already some products on the market with these advanced functionalities. I anticipate that we will see more in the near future as we learn more about how to control indoor chemistry to our benefit.”