"While pollutants commonly found in indoor air are responsible for many harmful effects, there is considerable uncertainty about what concentrations or periods of exposure are necessary to produce specific health problems. People also react very differently to exposure to indoor air pollutants.
  Further research is needed to better understand which health effects occur after exposure to
the average pollutant concentrations

 found in homes and which occurs from the higher concentrations that occur for short periods of time."

"An EPA report stated
that indoor air pollution is
one of the nation's most important environmental
health problems."