Exploring chemistry’s great indoors
According to research published in Science earlier this year, air pollution now ranks among top five threats to human health, a list that includes malnutrition, dietary risks, high blood pressure, and tobacco. Yet when we speak of contaminants in the atmosphere, we are invariably referring to outdoor settings, which is where researchers usually collect such data. The environmental reality is that in developed countries we are almost constantly indoors, perhaps as much as 90 percent of the time.
For Jon Abbatt, a professor in the University of Toronto’s Department of Chemistry, this distinction is undoubtedly more significant than most researchers have acknowledged.
“When people talk about the indoor environment, they don’t talk about it in the context of it being a reactive space, or in terms of the chemical processes that might be happening,” he says. “The question in my mind is: what chemistry happens indoors?”
In his own analysis of this subject, also published in Science earlier this year, Abbatt offered some key examples to illustrate the expansive research frontier represented by the great indoors. Much of the relevant chemistry appears on surfaces, he points out, including walls, ceilings, or carpets that serve as the media for reactions with gaseous molecules generated by smoking, cooking, and cleaning.
The walls of a smoker’s dwelling are commonly covered in nitrosamines, carcinogenic compounds that form when nitrous acid in the air combines with nicotine from a cigarette. Polycyclic aromatic hydrocarbons, which are also toxic, can likewise arise as combustion products generated by a cook stove using solid fuel, a common practice in many parts of the developing world. Even the seemingly virtuous practice of disinfecting surfaces with bleach can release reactive chlorinated gases that can oxidize and then be broken apart by ultraviolet light to form harmful reactive radicals.
“We can have much more of an effect than just the local effect that we think we’re having when we use a chemical like that,” explains Abbatt. “I wouldn’t advocate that we don’t use bleach, but it’s also true that if you use bleach you not only clean the surfaces that you’re physically wetting, you also evaporate a lot of oxidizing molecules into the gas phase, which can then go on to oxidize other surfaces or be good oxidants in the gas phase itself.”
Another area of interest is volatile organic compounds, which have been increasingly studied in outdoor contexts while forms such as terpenes emitted by foods or cleaning agents can actually be found at much higher levels indoors. For Abbatt, such observations make it all the more important for building scientists to develop more robust chemical research priorities. At the same time, he cautions that depending on their design, indoor settings present an extremely dynamic environment, something else that complicates its study.
“What we’re exposed to is a superposition of air that’s been brought in from outside and what gets emitted indoors or gets formed indoors,” he explains, adding that a modern house might exchange its air entirely on an hourly basis.
That can become much longer in winter, when we minimize the flow of outside air in order to retain heat, and likewise in summer, as air-conditioned home-owners work hard to keep heat out. In either instance, though, the effect turns our homes into ever more tightly enclosed chemical vessels. Abbatt therefore maintains that it is worth grappling with the challenges of indoor chemistry, which could turn out to become crucial to assessing environmental health threats.
“Atmospheric chemistry has spent a lot of time trying to understand the chemistry of forest regions versus that of desert regions versus polluted urban regions or Arctic regions,” Abbatt concludes. “This is just another region to understand — the indoor space.”