Increased levels of two major indoor air pollutants - nitrogen dioxide and house dust mite - are responsible for worsening symptoms in children with asthma, reports a study in the May Journal of Occupational and Environmental Medicine, official publication of the American College of Occupational and Environmental Medicine (ACOEM).

Led by Monika Nitschke, Ph.D., of University of Adelaide, Australia, the researchers examined the relationship between nitrogen dioxide and dust mite levels and asthma symptoms in 174 asthmatic schoolchildren. Indoor nitrogen dioxide levels were measured at home and school, while levels of house dust mite allergen were measured in dust samples from the children’s mattresses.

For both indoor air pollutants, higher exposure levels were linked to increased asthma symptoms. Children exposed to higher levels of nitrogen dioxide at school were more likely to have breathing difficulties, both during the day and at night, as well as nighttime chest tightness. Higher levels of nitrogen dioxide in home kitchens were also associated with increased asthma symptoms.

Among children allergic to house dust mite, higher levels of mite allergen were also related to increased asthma symptoms. Above a certain cutoff point, children with higher levels of mite allergen exposure had increased rates of nighttime wheezing, daytime cough, and daytime asthma attacks.

The cause of asthma is still uncertain, but environmental factors may play an important role in triggering asthma attacks. House dust mite allergen and nitrogen dioxide are important indoor air pollutants. Nitrogen dioxide is a byproduct of gas combustion - the school air quality measurements were made as part of a study on the effects of replacing unflued gas heaters in classrooms. In schools where unflued gas heaters were replaced, nitrogen dioxide levels declined, followed by decreased asthma symptoms.

“Exposure to nitrogen oxide emitted by unflued gas appliances indoors and exposure to house dust mites in bed can exacerbate asthma in children,” Dr. Nitschke and colleagues conclude. The two pollutants triggered different types of symptoms, reflecting separate components of the asthma disease process. Wheezing and coughing triggered by dust mite allergen are linked to allergic reactions, while the breathlessness and chest tightness triggered by nitrogen dioxide are likely related to airway inflammation.

“In light of the strong evidence that the extent of health effects is dose dependent, exposure [to indoor air pollutants] should be minimized,” the researchers write. The heater replacement study has demonstrated the health benefits of reducing indoor nitrogen dioxide levels - more research is needed to determine whether reducing exposure to house dust mite will reduce asthma symptoms.

ACOEM, an international society of more than 5,000 occupational physicians and other health care professionals, provides leadership to promote optimal health and safety of workers, workplaces, and environments.