Combustion indoors produces both gases (eg, nitrogen dioxide, carbon monoxide) and particulate matter that may affect the development or exacerbation of asthma. Sources in the home include both heating devices (eg, fireplaces, woodstoves, kerosene heaters, flued [ie, vented] or nonflued gas heaters) and gas stoves for cooking.
Home heating devices include both those used as the primary heating source and those used as a secondary source. In areas where central heating is the norm (ie, most of the United States), woodstoves, kerosene heaters, or gas space heaters may be used as a secondary source, allowing the family to heat a specific room or to lower the thermostat and reduce the cost of central heating. Some sources may be used only occasionally, such as fireplaces used for ambiance or space heaters used in case of power failure. In other parts of the world such as parts of Europe and China, burning wood or coal within the living space of a home may represent a primary method of heating. These differences in source use (eg, daily versus infrequently, primary versus secondary) lead to considerable variability in exposures.
Gas stoves are another common source of indoor combustion. Exposures to emissions from stoves can also vary considerably depending on stove and household characteristics. Older stoves with continuously burning pilot lights produce significantly more exposure to nitrogen dioxide and particulate matter than stoves with electronic ignition. Other gas appliances, including gas dryers and hot water heaters, may also be sources of exposure to nitrogen dioxide and other pollutants, although exposures tend to be lower from these sources. In the Netherlands, exposure to gas geysers (a type of hot water heater) is common and their effect on respiratory symptoms has been studied .
Although previous reviews  have considered the association of indoor combustion sources with respiratory symptoms in general, this article highlights the recent literature, focusing specifically on exposure to indoor combustion in association with either the development or exacerbation of asthma. Studies of both the incidence and prevalence of asthma in association with indoor combustion sources are included. Severity of asthma will be measured by increases in asthma symptoms (eg, wheeze, persistent cough, chest tightness and shortness of breath) as well as increases in medication use, physician visits, emergency room visits, and hospitalization. Since asthma is a chronic condition affecting both children and adults, both age groups are included in this review. Heating and cooking devices will be considered separately. Generally, exposure in the home has been investigated. The few studies of children exposed at school  are also reviewed.
The following questions are considered in this article: Is indoor combustion associated with asthma development, or with exacerbation of symptoms among asthmatics? Is exposure from heating devices more harmful than exposure from cooking devices? Are specific fuels (eg, coal), more likely to be associated with risk than others (eg, wood)? Are children more at risk than adults? Is the association of indoor combustion and asthma confounded by poverty or environmental factors associated with substandard housing or older appliances that burn inefficiently?