I never knew that indoor air quality could be an issue until I moved to New York City from Northern California in 2009.

After settling into my shoebox-sized Manhattan apartment, I discovered that my elderly neighbor across the hall spent most of his time chain-smoking with his door open. Clouds of smoke poured into our hallway and seeped under my door. As a freelance writer, Id spend hours sitting at my desk, trying to work, and struggling to breathe.

It was a bumpy introduction to big-city living. But what I hadnt realized was that I was far from alone in suffering from poor air quality inside my home.

Most of us are familiar with the kind of air pollution caused by car exhaust or wildfires. The U.S. Environmental Protection Agency found that indoor air can be anywhere from two to five times as polluted as outdoor air . Household air pollution is one of the worlds greatest environmental health risks, according to the World Health Organization. In fact, a 2012 WHO study found that complications from breathing dirty indoor air are responsible for 4.3 million premature deaths each year , mostly in developing countries, where billions of people burn coal or wood to fuel indoor cooking.

Indoor air pollution is one of the unrecognized environmental justice issues of our time, according to Jonathan Levy , an environmental health professor at Boston Universitys School of Public Health. He researches air pollution and equity issues in urban areas, and he notes that the homes of low-income families or public housing units tend to be smaller or not as structurally sound, and will have more indoor air pollution as a result.

While some sources of pollution are out of your hands, plenty of others are a result of factors you can control. Knowing what they are is the first step to improving air quality in your home.

People are not as aware of indoor air pollution, Levy says. If you say air pollution to someone, they think about the large factory or the diesel vehicle, and they dont necessarily think about what is happening inside their homes.