Introduction

On the average, North Americans and Europeans spend 80 to 90 percent of their time indoors; thus, the air we breathe is mostly indoor air. Although that may appear to be obvious, until recent years most health studies did not take this fact into account. In the past, studies concerned with the effects of air pollutants on human health considered only the exposure to outdoor pollutants, but indoor pollutants are a distinct and diverse group of their own.