In a 1987 study, the U.S. Environmental Protection Agency (EPA) ranked indoor air pollution 4th in cancer risk among the 13 top environmental problems analyzed.

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What factors contribute to the high risk from indoor air pollution? First, people spend most of their time indoors. A recent ARB-sponsored study found that Californians spend an average of 87% of their 24-hour day indoors. If pollutants are present indoors, people will almost certainly inhale them. Second, indoor air pollutant levels are often higher than those outdoors. Research by the ARB, the EPA, and others has shown that indoor levels of some pollutants, such as formaldehyde, chloroform, and styrene, range from 2 to 50 times higher than outdoor levels. Exposure to pollutants such as environmental tobacco smoke and radon occurs almost entirely indoors. For most of us, the amount of air pollution that we breathe is primarily determined by what is in the indoor air. Reference: California Air Resource Board, May 2, 2001

On October 15th, 2003, World Health Organization (WHO) warned that health conditions in Asia's booming cities will suffer dramatically unless governments take urgent measures to slash air and water pollution. WHO estimates 1.5 billion urban dwellers face air pollution above the maximum recommended limits!
(source:The News Straits Times, 15th Oct 2003)

Here are more facts for you to consider:

According to the American College of Allergies, 50% of all illness is aggravated or caused by polluted indoor air.   The Environmental Protection Agency (EPA) themselves declare that indoor air is anywhere from 2 to 10 times more hazardous than outdoor air.   The EPA also warns us that the indoor air quality is the United States' number one environmental health problem.   Today's homes and buildings are built air-tight, and contain a long list of pollution sources. As a result, natural air-cleansing agents such as ozone and negative ions are kept out, while contaminants are kept in. A recent study found that the allergen level in super-insulated homes is 200% higher than it is in ordinary homes.   According to Scientific America, a baby crawling on the floor inhales the equivalent of 4 cigarettes a day , as a result of the outgassing of carpets, molds, mildews, fungi, dust mites, etc.   Most people spend well over 90% of their time indoors. In which case, indoor air is going to impact our health far more than outdoor air.   The EPA informs us that 6 out of 10 homes and buildings are "sick", meaning they are hazardous to your health   Asthma cases have increased by more than 100% since 1976. About 1 in 9 children now have asthma. Death rates due to asthma have tripled, and quintupled in children ages 5 to 9, since 1976. Hospitalization rates and doctor visits are still continuing to increase dramatically.

Oct 15, 2003, World Health Organization (WHO) warned that health conditions in Asia's booming cities will suffer dramatically unless governments take urgent measures to slash air and water pollution. WHO estimates 1.5 billion urban dwellers face air pollution above the maximum recommended limits!. In Asia, more than 1.5 million people die every year from pollution-related diseases, including 1/2 million deaths attributed to particulate matter and sulphur dioxide. Bringing particulate matter down to safe levels could save up to 700,000 lives annually, WHO said.

The question we need to ask now is ...