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Dr. Ernst is associated with the Division of Clinical Epidemiology, McGill University Health Centre.
Correspondence to: Pierre Ernst, MD, Division of Clinical Epidemiology, McGill University Health Centre, Hôpital Royal Victoria, 687 Avenue des Pins Ouest, Bureau R4.29, Montréal, PQ, Canada H3A 1A1
Continued exposure to inhaled aeroallergens is associated with increased morbidity in the majority of patients with asthma, those with allergies.1 Therefore, asthma management guidelines appropriately insist on measures to reduce exposure to aeroallergens, especially exposure to the indoor allergens like house dust mites and furred pets, that are most closely linked to an aggravation of asthma.2 3
Effective measures to reduce exposure to the allergens produced by house dust mites are well described.4
Essentially, they consist of encasing the mattress and pillows in mite-impermeable materials, removing reservoirs of allergen such as carpets, upholstered furniture, and soft toys, especially from the bedroom, washing the remaining bedding weekly in hot water (
54.4°C) and maintaining indoor humidity below 50%. Removing carpets allows frequent wet mopping of floors. If carpet removal is not possible, vacuuming should be carried out with a high-efficiency particulate air (HEPA) vacuum cleaner with a double reservoir bag, preferably when the asthmatic patient is not present. Central vacuum cleaners exhausted to the outside are also likely to be effective. Such a multifaceted and concerted approach to reducing levels of dust mite allergen among asthmatic subjects with positive allergy skin test results to mite allergens, if successful, is clearly associated with improvement in the parameters of asthma control—symptoms, need for medication, measures of airflow, and measures of bronchial responsiveness.1
Reducing exposure to allergens derived from furred pets is most reliably done by completely excluding the pet from the home and avoiding exposure to pets elsewhere. Anyone working with patients with asthma who have documented pet allergy, even those patients whose asthma is difficult to control, knows that this is often not possible, at least in the short term. It is important to make patients aware of the choices they are making (more medication, decrease in health-related quality of life) and to reexamine these choices on a regular basis. Patients are often interested in alternatives to excluding the pet from the home, however. What well-founded advice can we provide?
Unfortunately, there is little hard evidence that anything other than complete avoidance can benefit asthmatic patients with a documented pet allergy.5 Since there appears to be a dose-response relationship between parameters of asthma control and exposure to allergen, it seems reasonable to keep the cat out of the bedroom, living room, and playroom. Washing pets has given variable results and must be repeated so frequently as to be impractical in most instances. Dust control measures prescribed for dust mite control, such as removal of carpets or use of a HEPA vacuum cleaner with double-reservoir bags, will also reduce the amount of settled allergen. Cat and dog allergen, as opposed to dust mite allergen, are often found on small particles that are easily airborne.1 This renders the use of air filtration devices attractive. Due to the small particle size of airborne allergens, HEPA cleaners are likely required. In addition to exclusion of the cat from the bedroom, a placebo-controlled, randomized study among adults with cat allergy using such a device installed in the bedroom failed to demonstrate any health benefit, despite a significant reduction in airborne allergen levels.6 In contrast, a randomized, placebo-controlled crossover trial among cat- allergic children found an improvement in bronchial hyperresponsiveness.7 This study installed the air cleaners in both the bedroom and the living room, where allergen levels were higher.
In the current issue of CHEST (see page 1535), McDonald and colleagues report a systematic review of randomized trials, or meta-analysis, of the use of air filtration systems in patients with asthma or allergies. Overall, they found a reduction in symptoms, but not of medication use, and no improvement in measures of peak flow. These muted benefits are not surprising given the small size of the studies as well as the availability of only 10 trials that could be included in the estimate of potential benefits. Furthermore, the inclusion of studies where air filtration devices were installed with the aim of reducing exposure to dust mite allergen may have masked the benefits in patients with pet allergy. Since dust mite allergen is found on large particles less likely to be airborne than pet allergen, dust mite exposure is less amenable to control with air filtration devices.
The more important conclusion reached by McDonald and colleagues is the need for more and larger studies. Given the potential benefit of allergen avoidance and the relative lack of adverse effects, practical methods of reducing allergen in the indoor environment, where we spend the greater part of our time, deserve adequate evaluation and investment. Fortunately, at least one large study of the overall health and financial benefits of dust mite avoidance is underway among newborns, and early results are promising.8 It may be more difficult to justify large studies of expensive or time-consuming ways of reducing exposure to furred pets, since total exclusion of the pet from the home is so simple and effective. For patients with the courage to get rid of their pets, it is important to remind them that the full benefits of their action will be reaped only over many months, since pet allergen, especially cat allergen, can persist for prolonged periods at significant levels in the home.5
In summary, all patients with asthma deserve an allergy evaluation to identify sensitization to common inhaled allergens. Avoidance of allergens to which a patient with asthma is sensitized is an integral and effective part of asthma management. Indoor allergens are of particular importance because of the large amount of time spent indoors. The indoor allergens most likely to be relevant are dust mites, cockroaches, and furred pets. Avoidance measures for dust mites5 and cockroaches9 are well described and are probably effective at improving asthma control if the measures are strictly adhered to. Air filtration devices are unlikely to be important or effective over and above the more usual measures, given the characteristic distribution of these allergens in the home. Air filtration devices are effective at reducing levels of pet allergen in the home and may improve asthma control when combined with exclusion of the pet from the bedroom. This is likely to be much less effective than ridding the home of the pet completely and is therefore difficult to recommend.
References
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