Overview
Indoor exposure to allergens and irritant pollutants plays an important role in asthma pathogenesis. Mounting evidence has suggested that indoor allergens and pollutants contribute significantly to asthma development and exacerbation. Such findings, coupled with the fact that most people spend nearly 90% of their time indoors, make the indoor environment an important area for research and prevention.
Indoor allergens include house dust mites, cockroach and pet allergens, and indoor dampness and mold. Children with asthma are more likely than those without asthma to have an allergic response to household allergens. Asthmatics commonly have a positive skin prick test to protein extracts from cockroaches, house dust mites, cat and dog dander, pollen, and common molds. A report from the Institute of Medicine (IOM) and a recent meta-analysis concluded that indoor dampness is associated with up to a 50% increase in asthma-related health outcomes in children. The report concluded that indoor dampness is associated with higher levels of allergens, such as mold, as well as with inflammatory agents, such as endotoxin.
There is no question that exposure to allergens can trigger an asthma attack in someone who has the disease and is already sensitized to these proteins. Reducing dust mites, indoor dampness, and other allergens in the home has been shown to decrease the severity of respiratory symptoms in sensitized asthmatics. Allergen remediation, especially when the intervention is comprehensive and tailored to an individual's allergen sensitivities, has been associated with improved health outcomes.
Irritant Pollutants
Nonallergic pathways to asthma also exist, and many irritant pollutants have been shown to trigger asthma attacks. Some pollutants have also been associated with an increased risk for new-onset asthma. Exposure to secondhand cigarette smoke has been consistently associated with increased frequency and severity of asthma attacks in both children and adults, and with the development of asthma in children.
Infants whose mothers smoke during pregnancy have reduced pulmonary function and are more likely to have persistent wheezing until at least 6 years of age. Maternal smoking, at the very least, doubles a child's risk for asthma. Risk is associated with prenatal and postnatal exposure to secondhand smoke, and is clearly dose-related -- and increasing in homes that have a greater number of smokers and heavy smokers.
Moreover, smoking and genetics have synergistic effects. Children of nonsmoking parents with asthma and allergies have a 12-fold greater risk of developing early-onset persistent asthma than children of nonsmokers with no family history of asthma or allergies. Maternal smoking in pregnancy interacts with genetic susceptibility to multiply the risk by an additional 3-fold. Cigarette smoke resembles diesel exhaust and industrial emissions, containing a similar mix of tiny particles, toxic chemicals, and respiratory irritants. Exposure to cigarette smoke and outdoor particulate air pollution may therefore cause a similar asthmatic response.
Volatile Organic Compounds
Many volatile organic compounds (VOCs) are found in modern buildings, particularly in those in urban areas. These volatile chemicals include respiratory irritants, such as formaldehyde, toluene, and chloroform. VOCs may enter from the outside and remain trapped in the indoor environment, or may be released from building materials, carpets, and furniture found inside. They also are found in some household products, including glues, paints, fragrances, and cleaning products.
In addition to VOCs, cleaning products contain enzymes and surfactants that can be irritating and cause an immunologic response. Cleaning products can also contain potent airway irritants, such as bleach and ammonia, that can trigger respiratory symptoms at high concentrations. Homes with an attached garage contain VOCs from evaporated gasoline emitted from parked cars. Researchers have theorized that these chemicals may play a role in asthma; some observational studies have suggested an association between VOC exposure and asthma. Very little evidence exists, however, to help determine whether VOCs or detergents are important variables in causing or exacerbating asthma. Other indoor pollutants, including ozone, particulate matter, nitrogen dioxide, and sulfur dioxide, have been associated with asthma.
Outdoor vs Indoor Air Pollutants
Outdoor air pollutants migrate indoors or can be generated from specific sources inside the home, such as cigarette smoke, wood-burning stoves, furnaces, gas stoves, and ozone-generating air "purifiers." Most of the research revealing an association between these compounds and respiratory diseases has examined outdoor sources of exposure. Sulfur dioxide, ozone, particulate matter, and nitrogen dioxide are examples of potent airway irritants that have been clearly associated with increased rates of asthma exacerbations and respiratory disease.
Some studies have shown that indoor concentrations of such pollutants can actually exceed outdoor concentrations. Further, air pollutants may act in conjunction with common allergens to increase sensitivity to pollen or other allergens. In laboratory volunteers, combined exposures to levels of ozone, nitrogen dioxide, or diesel exhaust particles found in urban air and low levels of common allergens, such as pollen, dramatically enhanced asthmatic and allergic reactions. Maintaining adequate ventilation in the home and avoiding indoor sources of these pollutants, such as cigarette smoke, are important ways to reduce exposure.
Recommendations
Although ongoing research continues to further describe the relationship between the development and exacerbation of asthma and the indoor environment, healthcare providers can play a significant role in counseling patients on preventive measures that will help avoid common triggers found in indoor air. Clinicians can evaluate the patient's indoor environment as part of routine asthma management.
Counseling should emphasize the importance of avoiding environmental tobacco smoke, controlling common household allergens, ensuring adequate ventilation in the home, and supporting local and regional measures to improve air quality. Specific measures to control household allergens, such as cockroaches and dust mites, include sealing cracks, cleaning up food and water, using pillow and mattress covers to control dust mites, washing bedding in hot water, and eliminating sources of dampness. There are useful patient and provider resources available online from the American Lung Association and The National Asthma Education and Prevention Program.
Reviewed by Ramaz Mitaishvili, MD
