Air pollution is responsible for 7 million premature deaths around the world every year, according to the World Health Organization. That's roughly the same number of people who die from smoking or malnutrition. The damaging effects of air pollution are caused mainly by the small airborne particles produced from traffic and coal-fired plants or through chemical reactions with compounds in the atmosphere. In indoor environments, the smallest particles (so called nanocluster aerosols), can result from cooking, burning candles, 3D printing and—as the research team in question has found—humans simply being exposed to ozone. In their study, the scientists discovered that human bodies can be a source of nanocluster aerosol emissions in places where the indoor air contains ozone.