BREATHING OF POLLUTED AIR is as old as mankind,
particularly since the domestication of fi re. Evidence
of fi re accompanied hominid remains from
500 000 years ago in China,1 and offered people then
a survival advantage through cooking foods, heating,
and keeping bugs and beasts at a distance. When people
built shelters for dwellings, they also brought pollutants
into the indoor living space.2
Today, burning biomass—principally wood, crop
residues, and dung—remains an important source of
exposure to a variety of toxins, mainly in the rural
areas of developing countries. Humans have cooked
in a similar manner for thousands of years. Fuels that