People spend about 90 percent of their time indoors, which increases the exposure to dangerous particles, according to the U.S. Consumer Product Safety Commission. Any given volume of space teems with unseen molecules that could damage tissue when breathed in, so with important differences between indoor and outdoor air quality.
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Indoor air has a higher concentration of gases and particles compared to outdoor air, typically due to inadequate ventilation combined with high temperature and humidity levels, which can hold a greater concentration of gases. Homes built to minimize the amount of air that leaks in will have particularly poor ventilation.
Because of the heightened concentration levels, the worst exposure to carcinogens and dangerous pollutants tends to occur in indoor environments. Most gases and particles outdoors become diluted or minimized instantly. Pollutants that have particular power outdoors usually remain close to the source. Many of these pollutants also pose problems indoors too.
Outdoor air quality decreases if the amount of pollution overwhelms the large volumes of air. For instance, big cities with a lot of vehicles or factories typically have high concentrations of carbon monoxide and nitrogen oxides. Pollen also presents a concern near flowers. These particles become even more dangerous indoors, however. Poorly ventilated indoor buildings may contain tobacco smoke, household chemicals, pesticides, biological contaminants such as mold and pollen, gases such as radon and carbon monoxide and building materials such as asbestos, formaldehyde and lead, states Medline Plus.
The acceptable level of a gas or a particle depends upon its potency. Radon, for instance, is measured by a unit called picocuries per liter, which is the amount of disintegrations per second in one liter of space. The average outdoor radon level is 0.4 pCi/L. The average indoor radon level, on the other hand, is 1.3 pCi/L, which is enough to give 2 people out of 1,000 lung cancer over the course of a lifetime, assuming that they've never smoked. Outdoor air will diffuse radon, but indoor air will concentrate it.
Openings, joints and cracks in the wall or floor can act as meeting grounds where outdoor and indoor air exchange with each other. This depends on differences in temperature between outdoor and indoor air. When outdoor air infiltrates the home, it can bring in pollutants. If the indoor air is instead ventilated, pollution from within the home may dilute into the outdoor atmosphere.