One of the main factors that determine indoor air quality are Suspended Particles
Coarse, fine and ultrafine particles in ambient air are known to cause adverse health effects, including on the respiratory and cardiovascular systems. Particles may in part come from outdoor pollution, but can also form indoors by the burning of fuels for heating and cooking, and by reactions between ozone and some volatile organic compounds (VOCs). In addition, man-made nanoparticles, that are increasingly used in consumer products, may have an impact as indoor air pollutants.
(GreenFacts- Facts on Health and the Environment)