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In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.

Article: Most people are aware that outdoor air pollution can damage their health,but many do not know that indoor air pollution can also have significant health effects.Environmental Protection Agency studies indicate that indoor levels of pollutants may be 3~5 times,and occasionally more than 100 times,higher than outdoor levels.These levels of indoor air pollutants may be of particular concern because most people spend about 90% of their time indoors. There are many sources of indoor air pollution in any home.These include sources such as oil,gas,coal,wood,and tobacco products,building materials,wet or damp carpet,and furniture made of certain pressed wood products,products for household cleaning and maintenance,personal care,or hobbies,central heating and cooling systems. Immediate effects may show up after a single exposure or repeated exposures.These include _ of the eyes,nose,and throat,headaches,dizziness.Such immediate effects are usually shortterm and treatable.Sometimes the treatment is simply removing the person's exposure to the source of the pollution,if it can be identified. The likelihood of immediate reactions to indoor air pollutants depends on several factors.Age and preexisting medical conditions are two important influences.In other cases,whether a person reacts to a pollutant depends on individual sensitivity,which varies tremendously from person to person.Some people can become sensitized to biological pollutants after repeated exposures,and it appears that some people can become sensitized to chemical pollutants as well. Certain immediate effects are similar to those from colds or other viral diseases,so it is often difficult to determine if the symptoms are a result of exposure to indoor air pollution.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.Therefore,further research is needed to better understand the effects of indoor air pollution and to find efficient ways to protect our health.
Answer: Refrigerators.
Output:
According to the author,which may NOT be the source of indoor air pollution?