Particle Air Cleaning and Allergies and Asthma
Many studies have experimentally investigated whether use of particle air cleaning systems, often called particle filtration systems, in homes reduces self-reported symptoms of allergies or asthma, or improves related objectively-measured signs of health, such as forced expiratory volume (a measure of lung function) or biomarkers of inflammation, in people who are allergic or asthmatic. Most of these studies have employed stand-alone (portable) fan filter systems incorporating HEPA (high efficiency particle air) filter media, often with air cleaners installed in the bedroom of the allergic or asthmatic persons, or in main living areas of the house. Recent studies have had strong study designs with people's health during periods of air cleaner operation compared to their health during periods of ineffective placebo air cleaning. There have been several critical reviews and analyses of the related literature [1-7], most with similar conclusions. In 2000, after reviewing the available literature, the Institute of Medicine  concluded that "overall these studies suggest that air cleaners are probably helpful in some situations in reducing allergy and asthma symptoms, but air cleaning, as applied in these studies, is not consistently and highly effective in reducing symptoms". Wood (2002) published a review of a subset of the same set of papers. He concluded that air cleaners "have relatively little value in the control of dust mite allergens" and "they make the most sense for animal allergy". He characterized the benefits as "far from definitive" but suggested that use of air cleaners makes sense to "allergic pet owners who refuse to remove the offending pet from their homes". Reisman  reviewed four of the studies considered by the Institute of Medicine and concluded that the four studies "show minimal, if any, effectiveness in treatment of allergic respiratory disease." In 2002, McDonald et al.  published the results of a statistical meta-analysis of ten randomized trials of the use of stand-alone residential air filtration systems in homes of subjects with asthma. Nine of ten studies in the meta-analysis were a subset of the studies reviewed by the Institute of Medicine. Nine of ten studies had subjects with perennial allergic disease, often associated with indoor allergens as opposed to seasonally-variable outdoor plant pollens. Overall, the analysis indicated that particle filtration was associated with statistically significant improvements in total symptoms and sleep disturbance. On average, when filter systems were employed symptom scores improved by approximately 5% to 8% and sleep disturbance scores improved by approximately 10%. There were no overall statistically significant improvements in nasal symptoms, medication use, or peak expiratory flow (a measure of lung function). The most recent critical review  considered the results of 16 newer studies plus the results of prior reviews and concluded "particle filtration can be modestly effective in reducing adverse allergy and asthma outcomes, particularly in homes with pets." In most cases, when symptoms or signs of allergy and asthma improved, the improvement was less than 25%.
Some of the studies have used particle filtration systems that deliver filtered air to the breathing zone of the allergic or asthmatic persons while they sleep. Two of the critical reviews [1, 6] included language indicating that these systems may be more consistently effective in improving allergy and asthma health effects than use of room or whole-house filtration system. Further research is warranted as many of the more recent studies of these systems were supported by the suppliers of the air cleaning devices.