are found at the top of the energy ladder, which are
cleaner but also more expensive, have a much more
recent history.
The exploitation of fossil fuels that are integral to
modern living has been part of the rapid technological,
social, and cultural changes of the past 250 years.
Although such changes have brought about undeniable
benefi ts, they have also contributed to the pollution
of local and regional environments due to the release
of a great number of chemicals.3 People living in
industrialized countries have other exposures due to
more energy-effi cient houses built from a variety of new
building materials, plus chemicals and pets, in addition
to a more sedentary lifestyle spent mainly indoors.
Total human exposure to air pollutants is determined
by the concentrations found outdoors and indoors
in the different microenvironments and the time
spent in each of these environments, commonly called