1940 to 1990
Governments and international bodies such as the World Health Organization define healthy air in terms of the air quality at a fixed point outdoors. Air quality standards are specified for each pollutant in terms of its concentration in outdoor air (e.g., in parts per million, or ppm) or its mass in a fixed volume of outdoor air (e.g., in micrograms per cubic meter, or μg/m 3). However, indoor air quality, particularly in the home, is a far better indicator of the impact of air pollution on public health.
First, virtually no one spends an entire day, let alone an entire year, rooted at the same spot outdoors. In fact, most people spend the majority of their time indoors, generally at home. Studies of human activity patterns in the United States indicate that the average person spends about 93 percent of his or her time indoors, 5 percent in transit, and the remainder (2 percent) outdoors. 1 About 70 percent of the average person's time is spent indoors at home. The average homemaker spends an even greater amount of time indoors at home (88.7 percent).
Second, the quality of air is often worse indoors than outdoors. Traditional air pollutants have both sources and sinks indoors. Sources include heating and cooking equipment that use fossil fuels and biofuels (e.g., wood and, in developing countries, dung), smoking, solvents, and various cleaning solutions used or stored in the home. Thus, the relationship between indoor and outdoor concentrations is often weak. For instance, levels of CO, NOx, and TSP are generally higher in homes that use natural gas than outdoors, while levels of SO2 and O3 are higher outdoors, by almost a factor of two to five. 2 Another factor contributing to this weak relationship is that the rate at which outdoor air comes into a building (i.e., the air exchange rate) is relatively low, particularly in winter and, where air conditioning is prevalent, in summer. This precludes equilibrium between outdoor and indoor concentrations.