There are currently about seven billion people on earth. If we took the 5 x 1018 kilogram mass of the earth’s atmosphere and divided it up among those 7 billion people, each person would have their own 7.14 million kilograms of atmosphere. (To put that in perspective, a pineapple weighs about 1 kilogram!) That sounds like lot of air per person, however, we humans tend to stay at the surface of the Earth. Even within that space, we like to hang out in cities and urban centers. So our individual share of the atmosphere is a lot less than that.
And there’s a lot of interesting and complicated chemistry happening in that air. Understanding the chemistry of the outdoor environment is crucial for solving many of the issues we face as a society, from air pollution, to climate effects, to heart disease.
If we added up the mass of all indoor air, on the other hand, it would be about 20,000 times lighter than outdoor air on a per-person basis. In the United States there are about 100 billion kilograms of indoor air, including both commercial and residential buildings. If we divide that mass among the 320 million residents of the United States, we get roughly 350 kilograms of indoor air per person. Compare this to the 7.14 million kilograms of outdoor air per person, and you can see why researchers have historically focused on the chemistry of the outdoor environment. If there’s so much more outdoor air than indoor air, then why should we study the chemistry of the indoor environment?
Let’s consider time instead of mass.
From a mass perspective, the outdoors dominate. But from a health perspective, we must also consider where humans spend most of their time. Think about your typical day. You probably wake up at home after a full eight hours of rest and cook something for breakfast. Then perhaps you get in your car or walk to the bus stop and commute to an office space. You spend most of your day at the office, and then you return home. If you’re motivated, maybe you spend an hour or so at the gym. If you’re feeling social, you may head to a restaurant or bar. How much of your average day is spent outdoors? If you’re like the typical human, you likely spend about 90% of your time inside. Our homes, businesses, exercise facilities, cars, and gathering places are all examples of indoor environments where we’re constantly breathing the air. Therefore, it is crucial to understand the chemistry occurring in these spaces and what it means to our health and well-being.
This doesn’t mean that the chemistry of outdoor environments is unimportant. In fact, our buildings are regularly exchanging air with the surrounding atmosphere, and thus many of the chemical processes we care about outside are also relevant inside. However, the nature of indoor spaces allows for unique chemistry to occur. In relatively small, enclosed environments, chemical reactions occurring on walls, fabrics, humans, and even pets become much more important. Acts such as cooking a meal, applying perfume, or mopping the floors can influence the chemistry around us and impact the quality of the air we breathe. The chemistry of indoor environments is incredibly important, yet poorly understood. More research is needed to explore the chemistry occurring inside our living, working, and recreational spaces.
Adapted from Nazaroff, W. W. and Goldstein, A. H. (2015), Indoor chemistry: research opportunities and challenges. Indoor Air, 25: 357-361. doi:10.1111/ina.12219