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“The cloud of smoke which hangs over the best part of Chicago night and day the year round, raining down soot and cinders on the washed and unwashed, is not generated spontaneously in the heavens. It comes from somebody’s chimney.”

This lament was printed in the Chicago Tribune in 1888. Historically speaking, outdoor air pollution has been pretty easy to spot. In those fevered years of the Industrial Revolution, coal smoke blackened streets and buildings. When automobiles commuting from far-flung suburbs became the norm in the 1950s and ’60s, exhaust smog hung thick in the air. To mitigate what would be recognized as a widespread health crisis, the first federal Clean Air Act was passed in 1970.

But these days, we breathe a lot less of that cleaned-up outdoor air than you would expect. According to the Environmental Protection Agency, we spend an average of 90 percent of our time inside—in our homes, in our offices, in our schools. And it turns out that pollution generated inside these buildings, while more difficult to see than coal smoke, poses significant health issues too.

The idea of indoor air quality, or IAQ, isn’t exactly new. In their coal- or wood-heated homes, our forefathers knew the importance of ventilation. Benjamin Franklin wrote about it in the late-18th century: “Another grand mean of preserving health is to admit a constant supply of fresh air into your chamber,” he wrote. “A more sad mistake was never committed than that of sleeping in tight rooms.” And yet tighter rooms became the norm following the energy crises of the 1970s. “People started to tighten up the building envelope to save on energy costs, and that caused all sorts of sick building syndrome-type issues,” says Scott Steady, who manages UL Environtment‘s GREENGUARD certification program. Asthma, drowsiness, dizziness, headaches, even more serious diseases like cancer have been linked to poor IAQ. Worker productivity, too, was shown to decrease within these types of office environments.

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