Indoor pollutants and asthma in children

WHAT DO WE KNOW? In many children asthma is closely associated with allergy, and many asthmatic children are allergic to dust mite and pet allergens. For them, reduction of exposure is likely to be beneficial. But does this mean that mite and pet exposure causes asthma? This issue is surprisingly complex. Whereas studies suggest that exposure to specific allergens increases sensitisation to those allergens, this does not necessarily mean that such exposures also increase the incidence of asthma. 2 Some studies even suggest that early life exposure to pets (and other animals) may reduce the incidence of asthma. Compared with the already complex role of exposure to allergens and other biological contaminants, the evidence with respect to chemicals such as nitrogen oxides, sulfur oxides, and particles produced by combustion appliances, environmental tobacco smoke, and volatile organic compounds (VOCs) is even less clear. In this issue of Thorax two new studies from Australia attempt to provide further evidence for the role of unvented heating appliances and of VOCs, respectively. Phoa et al retrospectively collected data on unvented heater exposure in a sample of children aged 8–11 years. Early—but not current—exposure to unvented space heaters was found to be associated with airway hyperresponsiveness and wheeze. Rumchev et al conducted a case-control study in young children and found that children aged 6– 36 months, discharged from a hospital emergency department with a diagnosis of asthma, had higher concentrations of VOCs in their homes than control children living in the same community. WHAT EVIDENCE CAN WE GLEAN FROM THESE NEW STUDIES? Effect of gas heaters on development of childhood asthma The study by Phoa et al was questionnaire based and grouped all sorts of ‘‘fume emitting’’ space heaters into one category—including gas heaters, kerosene heaters, open fireplaces, and wood stoves, each of which have distinctly different patterns of emissions of noxious substances. No measurements of pollutants were performed in the homes. Flued gas heaters, open fires, and wood stoves were all included in the fume emitting category, whereas it is likely that these are connected to chimneys and flues which would prevent most of the emissions from entering the indoor space. Adjustment for socioeconomic status (a likely covariate of flueless heater use) was not attempted, so some uncertainty remains about the extent to which the findings could be attributed in part to differences in socioeconomic status. The authors reported no association between asthma and current use of flueless heaters, but the analysis presented adjusted for the use of flueless heaters early in life. If there was a substantial correlation between early and current use of flueless heaters, this may mean that the lack of association with current heater use is to some extent due to overadjustment. Unvented space heaters, fireplaces, and wood stoves have been studied before by several investigators. In a small study Cooper and Alberti found no effect on the lung function of healthy residents of homes with kerosene heaters. In a study in small children Triche et al found a slight increase in cough with the use of kerosene heaters and wood stoves and slightly more wheeze with the use of unvented gas heaters in the home. In a study from Seattle neither asthma nor wheeze in 5–9 year old children was found to be related to the use of either gas heaters, wood stoves or kerosene heaters. A prospective study from Tasmania did find a relationship between gas heater usage in the first year of life and asthma at the age of 7 years. A large study from Finland found a negative association between the use of wood stoves and asthma outcomes, but argued that this was because wood stoves were used more on traditional farms. The association disappeared after adjustment for living on a farm. Wood smoke is a well known respiratory irritant, and the use of wood stoves was found to be related to lower respiratory tract illness in a case-control study in small children. A study conducted among a panel of asthmatics found that subjects had more symptoms of cough and shortness of breath on days with reported use of gas stoves, fireplaces or wood stoves. Although the evidence is not entirely consistent, it is clear that unvented combustion appliances increase the exposure of inhabitants to combustion products to a sufficient extent to cause exacerbations and, perhaps, induction of respiratory diseases and symptoms. The study by Phoa et al adds to this evidence, albeit with less specificity than previous studies.