Effect of Air Filtration Systems on Asthma: A Systematic Review of Randomized Trials

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1 Effect of Air Filtration Systems on Asthma: A Systematic Review of Randomized Trials Ellen McDonald, Deborah Cook, Toni Newman, Lauren Griffith, Gerard Cox and Gordon Guyatt Chest 2002;122; DOI /chest The online version of this article, along with updated information and services can be found online on the World Wide Web at: CHEST is the official journal of the American College of Chest Physicians. It has been published monthly since Copyright 2007 by the American College of Chest Physicians, 3300 Dundee Road, Northbrook IL All rights reserved. No part of this article or PDF may be reproduced or distributed without the prior written permission of the copyright holder ( ISSN:

2 Effect of Air Filtration Systems on Asthma* A Systematic Review of Randomized Trials Ellen McDonald, RN; Deborah Cook, MD, FCCP; Toni Newman, BA; Lauren Griffith, MS Biostatistics; Gerard Cox, MD; and Gordon Guyatt, MD, FCCP Study objectives: To systematically review the evidence of randomized trials evaluating the effects of residential air filtration systems on patients with asthma. Data sources: We searched for published and unpublished studies using MEDLINE, the Cumulative Index to Nursing and Allied Health Literature, and the Cochrane Collaboration. We reviewed all reference lists for additional articles of relevance, and contacted experts in the field and air filter manufacturers. Study selection: We identified 10 relevant randomized controlled trials that examined the influence of a residential air filtration system on patients with asthma. Data extraction: In duplicate and independently, we abstracted data on the methodologic quality, population, intervention, and outcomes. Data synthesis: Five of 10 studies enrolled adults only. One study included children only. The sample size ranged from 9 to 45 participants in each study, for a total of 216 patients across all studies. Two studies reported a statistically significant decrease in airway responsiveness associated with air filter utilization. Air filters were associated with significantly lower total symptom scores (weighted mean difference of 0.47; 95% confidence interval [CI], 0.69 to 0.25) on a 10-point scale, and lower sleep disturbance score (weighted mean difference of 0.93; 95% CI, 1.44 to 0.42); however, heterogeneity of results weakens the inferences from these trials. Air filtration systems were not associated with any differences in medication use or morning peak expiratory flow values. None of these trials employed validated scales to measure clinical symptoms or quality of life. Conclusions: Among patients with allergies and asthma, use of air filters is associated with fewer symptoms. Rigorous sufficiently powered randomized clinical trials are needed to more precisely define the influence of air filtration on health-related quality of life and symptom control for asthmatic patients. (CHEST 2002; 122: ) Key words: air filtration system; air quality; allergy; asthma; environment; quality of life Abbreviations: CI confidence interval; HEPA high-efficiency particulate air; PEF peak expiratory flow Asthma is a prevalent and disabling disease worldwide. 1,2 The prevalence of asthma has markedly increased over the past 2 decades in adults and in children. 3,4 Habbick et al 5 estimated a lifetime prevalence of asthma among children in two Canadian cities to be 19%. *From the Departments of Clinical Epidemiology and Biostatistics (Ms. McDonald, Ms. Newman, Ms. Griffith, and Drs. Cook and Guyatt) and Medicine (Dr. Cox), McMaster University, Hamilton, ON, Canada. This work was funded by the Hamilton Community Foundation and the Father Sean O Sullivan Research Center, St. Joseph s Hospital. Manuscript received October 22, 2001; revision accepted April 29, Correspondence to: Ellen McDonald, RN, Critical Care Research Office, St. Joseph s Hospital, 50 Charlton Ave, East Hamilton, ON, Canada; emcdonal@mcmaster.ca The cornerstone of asthma management is administration of anti-inflammatory medications and bronchodilators. 6 In addition, there are adjunctive therapies that may contribute to the overall control of asthma. Some of these therapies, despite showing For editorial comment see page 1509 early promise, have not always proved effective. A Cochrane Collaboration systematic review has shown that limited asthma education is unlikely to improve health outcomes in adults. 7 However, interactive education is more effective in changing the behavior of clinicians and patients than passive education. 8 Alternative interventions also demonstrated to be ineffective include chiropractic manipulation. 9 CHEST / 122 / 5/ NOVEMBER,

3 Public concern has been raised recently about environmental influences on asthma control. The National Institutes of Health guidelines for the diagnosis and management of asthma address indoor and outdoor air quality. 10 The Canadian Asthma Consensus Conference panel concluded that indoor irritants, particularly tobacco smoke, represent a greater health risk than outdoor air pollutants. 11 Among several environmental interventions, indoor air filters appear to be effective in reducing some airborne irritants. The high-efficiency particulate air (HEPA) filter has higher efficiencies for both larger and smaller particles than other filters, with a minimal particle removal efficiency of 99.97% for particles of 0.3 m in diameter. 12 The HEPA filter has been associated with reduced airborne levels of cat allergen in a case-control study of 50 homes with a cat and 50 homes without a cat. 13 Other observational studies have shown that HEPA filters reduce the burden of particles from mold spores 14 and cigarette smoke. 15 In 1992, a narrative review recommended that patients with severe allergies or asthma should use an air-filtering vacuum cleaner and an air cleaner with a HEPA filter. 12 However, the influence of air filtration on patient-centered outcomes has not been established. 11 Evidence about the use of residential air cleaners was graded as level 3C by the 1999 Canadian Asthma Consensus Conference, indicating poor evidence to support a recommendation for or against use. 16 Subsequently, investigators have suggested that future research should address the influence of improving air quality on asthma control. 5 The objective of this systematic review is to critically appraise and summarize the current randomized trial evidence about the effect of residential air filters on signs and symptoms of asthma, and to inform patients, clinicians, and researchers of our findings. Data Sources Materials and Methods We searched the computerized databases MEDLINE and Cumulative Index to Nursing and Allied Health Literature from 1976 to 2000 using the following text words and key words: asthma, quality of life, air filter, indoor air quality, and randomized controlled trials. We also searched the Cochrane Collaboration Controlled Trials Registry. To identify additional potentially relevant studies, we corresponded with experts in the field of asthma research, manufacturers of air filtration systems, and the authors of the primary studies included in this review. We also reviewed the citation lists or bibliographies of all the relevant studies and reviews, and retrieved any article that looked relevant to this systematic review. We had no language restrictions. Study Selection Using criteria determined a priori, two of the authors (E.M., T.N.) independently reviewed the first literature search to identify primary research studies that addressed the question posed. Based on title and abstract, all citations identified as potentially relevant by either reviewer were then retrieved for full review. To be included in the systematic review, the studies needed to meet the following criteria: (1) design, randomized controlled trials; (2) population, children or adults with a diagnosis of asthma; (3) intervention, use of a residential air filtration system; and (4) patient-oriented outcomes, as reported in each study such as asthma signs and symptoms, physiologic, laboratory, and other end points (ie, measurement and documentation of particulate). We did not consider observational studies, surveys, asthma classification documents, and practice guidelines for this review. We excluded studies of patients with poor asthma control following hospitalization. Data Extraction: Validity Assessment and Clinical Characteristics In duplicate, independently, we abstracted data to describe the methodologic quality and clinical characteristics of these trials. 17 Methodologic features we report include the method of treatment allocation, masking of treatment allocation (concealment), Table 1 Methodologic Quality of Randomized Trials of Air Filtration Systems* Source Design Concealment Blinding of Patients Blinding of Outcome Assessors Postrandomization Exclusions Intent-to-Treat Analysis Zwemer et al 20 (1973) RCT crossover NR Yes Yes 6 of 18 excluded No Villaveces et al 21 (1977) RCT crossover NR Yes Yes No No Kooistra et al 22 (1978) RCT crossover NR Yes Yes No Yes for symptoms, no for allergens Verral et al 23 (1988) RCT crossover NR Yes Yes 3 of 16 excluded No Reisman et al 24 (1990) RCT crossover NR Yes Yes 11 of 40 excluded No Antonicelli et al 25 (1991) RCT crossover NR No Yes No Yes Warburton et al 26 (1994) RCT crossover NR Yes Yes 1 of 13 excluded No van der Heide et al 27 (1997) RCT NR Yes Yes No Yes Wood et al 28 (1998) RCT NR Yes Yes 3 of 38 excluded No van der Heide et al 29 (1999) RCT crossover NR Yes Yes 2 of 22 excluded No *RCT randomized clinical trial; NR not reported. Two patients enrolled twice Clinical Investigations

4 Table 2 Clinical Characteristics of Randomized Trials of Air Filtration Systems* Source (Funding) Population Duration of Exposure (Total) Treatment Assignment Results Zwemer and Karibo 20 (not reported) Villaveces et al 21 (Envirco Company) Kooistra et al 22 (not reported) Verral et al 23 (Ontario Ministry of Health, Hepacron, Inc) Reisman et al 24 (Environmental Air Controls, Inc) Antonicelli et al 25 (Anallergo-Firenza, Italy) Warburton et al 26 (National Safety Associates of America) van der Heide et al 27 (Phillips Domestic Appliances and Personal Care) Wood et al 28 (Honeywell Corporation, Eudowood Foundation of Maryland) Symptomatic asthmatics 6to16yrold(n 18) Asthmatics 7 to 15 yrs old (n 13) Hay fever or asthma patients 15 to 68 yr old (n 20) Asthmatics with primarily house dust mite allergy 7to27yrold(n 13) Rhinitis or asthma patients 16 to 61 yr old (n 29) Mild asthmatics with rhinitis 10 to 28 yr old (n 9) Asthmatics 19 to 64 yr old (n 12) Allergic asthmatics 18 to 45 yr old (n 45) Cat-allergic adults with asthma or allergic rhinitis 18 to 65 yr old (n 35) 2 wk (4 wk) 1. Hyposensitization via avoidance and containment methods plus laminar air flow HEPA filter 2. Hyposensitization plus sham filter 2 wk (4 wk) 1. Hyposensitization and education with or without cromolyn sodium plus laminar HEPA filter 2. Hyposensitization and education with or without cromolyn sodium plus sham filter Significantly less uninterrupted sleep (p 0.001), lower overall symptom scores (p 0.001), and lower daytime wheezing scores (p 0.006) Significantly fewer symptoms (awakening, wheeze, cough, dyspnea, sputum, asthma attacks, rhinitis, restrictive activity, or medication use) No difference in PEF 4 wk (8 wk) 1. HEPA filter No difference in daytime sneezing, nasal congestion, itchy eyes, or medication use 2. No filter No difference in ragweed or Alternaria concentrations Nocturnal symptoms reduced (p ) 3 wk (12 wk) 1. HEPA filter No difference in symptoms, IgE, or PEF 2. Sham filter Significant decrease in medication use (p 0.05) improved histamine-induced airway responsiveness (p 0.05) 4 wk (8 wk) 1. HEPA filter No difference in congestion, discharge, eye irritation, cough, airway twitching, asthma, or medication use 2. Sham filter 8 wk (16 wk) 1. HEPA and home cleaning No difference in rhinitis, cough, dyspnea, or use of salbutamol or terfenadine 2. Cleaning alone No difference in FEV 1, PEF, PD 20 No difference in floor allergens HEPA, 30.3 d; sham, 24.0 d 1. HEPA filter No differences in cough, phlegm, wheezing, breathlessness, chest tightness, nocturnal wakening, or bronchodilator use 2. Sham filter No difference in PEF No difference in dust concentrations, gram positive bacteria or fungi 6 mo 1. HEPA filter alone No differences in FEV 1, FVC and PEF, serum IgE, eosinophils, skin tests, airway hyperresponsiveness 2. Sham filter and mattress No difference in floor dust and pillow covers 3. HEPA filter and mattress and pillow covers 3 mo 1. HEPA filter No difference in congestion, rhinorrhea, sneezing, cough, wheezing, chest tightness, sleeping difficulty, or medication use 2. Sham filter No difference in PEF cat RAST, or methacholine challenge *PD 20 provocative dose of methacholine causing a 20% fall in FEV 1 ; RAST radiollergosorbent test. CHEST / 122 / 5/ NOVEMBER,

5 Table 2 Continued Source (Funding) Population Duration of Exposure (Total) Treatment Assignment Results van der Heide et al 29 (Phillips Domestic Appliances and Personal Care) Cat or dog-allergic asthmatics yr old (n 22) [ SD] 3 mo (6 mo) 1. HEPA filter alone No difference in wheezing, dyspnea, cough, or medication use 2. Sham filter No difference in FEV 1, PEF Airway hyperresponsiveness to histamine significantly reduced blinding of the patients, clinicians and outcome assessors, whether cointerventions were described, the proportion of patients who were excluded after randomization, and intention-totreat analysis. We also report source of funding. Clinical characteristics we report include the population, sample size, duration of exposure, and outcomes. Differences between abstractors were resolved by consensus. Data Synthesis Symptoms and medication use were originally reported in different units in each trial, precluding combining these data as reported. To address this issue, we transformed and standardized those data to allow for comparison across the trials. 18 To combine individual symptom results from each study, we created a total symptom score that included nighttime symptoms when available. Second, we combined nasal symptom scores using the same methodology. Third, we created a nocturnal symptom score by combining symptoms experienced at night, such as awakening and sleep disturbance. We then created a total medication score by combining maintenance and as-needed medications among studies reporting total medication use. As reported in some studies, we also generated a combined symptom and medication outcome score. The symptom scores were indexed on a 10-point scale. Peak expiratory flow (PEF) values were analyzed as reported using morning values in milliliters per minute. For statistical synthesis, we used both random-effects and fixedeffects models. 19 The random-effects model assumes that the studies included in the review are a random sample of a population of studies addressing the clinical question. The randomeffect model takes into account both within-study variability and variability in results beyond what is attributable to within-study variability. The fixed-effects model assumes that there is a single true value underlying all the study results, and that the observed estimates of effect differ from each other only because of random error. The fixed-effects model only takes into account withinstudy variability. Study Selection Results Our search strategies identified 10 randomized trials evaluating the effect of air filtration systems in patients with asthma Four of 10 authors of the original studies responded to our request for further studies or additional information. Agreement on studies to be included was 100%. Validity Assessment The validity of included trials is summarized in Table 1. Eight of the 10 trials used a crossover design ,29 Masking of treatment allocation (eg, concealment of treatment allocation prior to randomization) was not documented in any trial. After randomization, patients were blinded to whether their air filter was active or not in 9 of 10 studies ,26 29 Blinding of health-care workers was conducted in two trials. 24,29 Blinding of outcome assessors occurred in all 10 trials. Of the 216 total patients enrolled in these 10 studies, 26 patients were unavailable for follow-up. Intention-to-treat analysis was reported in two trials. 25,27 Clinical Characteristics Five of 10 trials enrolled exclusively adults, 22,24,26 28 while 1 trial enrolled exclusively children 29 (Table 2). The sample size ranged from 9 to 45 participants in each study, for a total of 216 patients across all studies. Outcomes Multiple outcome measures were used to determine the effectiveness of air filters in each study (Table 2). Five studies reported lung function. 23,25 27,29 All but one study used a subjective measure of symptoms. 27 However, none of these trials employed validated scales to measure clinical symptoms or quality of life. Five studies reported allergen levels. 22,25 28 Air filtration systems were not associated with any differences in medication use or symptom/ medication scores (Table 3). We found no improvement in morning PEF values whether the fixed- or random-effects model was used for data pooling (Fig 1). Two trials showed that air filters were associated with significantly fewer symptoms. 20,22 Two studies reported a statistically significant decrease in airway responsiveness associated with air filter exposure. 23,29 We present the meta-analysis of study results in Table 3. There was a trend toward lower total symptom scores as shown by a weighted mean difference of 0.76 (95% confidence interval [CI], 2.17 to 0.65; p 0.29) on a 10-point scale using the random-effects model. Using the fixed-effects 1538 Clinical Investigations

6 Table 3 Meta-analysis of Randomized Trials of Air Filtration Systems Outcome* Trials, No. Patients, No. Weighted Mean Difference Random Effects (95% CI) Random Effects p Value Weighted Mean Difference Fixed Effects (95% CI) Fixed Effects p Value Heterogeneity p Value Total symptoms (0 10 score) ( ) ( ) Nasal symptoms (0 10 score) ( ) ( ) Sleep disturbance ( ) ( ) Medication use ( ) ( ) Symptoms/medication score ( ) ( ) Morning PEF rate, ml/min ( ) ( ) *A negative difference indicates a better outcome for the active filter group in all cases except PEF. Overall meta-analysis results difference. Significant differences among study results. Weighted mean differences are reported in effect size units. model, the symptom improvement was statistically significant (weighted mean difference of 0.47; 95% CI, 0.69 to 0.25; p 0.01). However, the magnitude of this apparent benefit on symptoms differed across studies (p value for heterogeneity 0.01). These results are graphically depicted in Figure 2. The most conservative estimate of the effect of HEPA filters on symptoms is shown by the random-effects model and the associated wide CI. Air filters were not associated with any improvement in nasal symptoms. We also found a trend toward less sleep disturbance associated with air filters, as shown by a weighted mean difference of 1.08 (95% CI, 2.78 to 0.62; p 0.21) using a random-effects model, or 0.93 (95% CI, 1.44 to 0.42; p 0.01) using a fixed-effects model. However, these study results were also heterogenous, weakening the inferences we can draw from this meta-analysis (p value for heterogeneity 0.01). Discussion In this systematic review of 10 randomized clinical trials among adults and children with asthma and allergy symptoms, we found a small but statistically significant difference in total symptoms and sleep Figure 1. Weighted mean difference for morning PEF. This figure shows the results of four randomized trials of domiciliary air filtration systems with respect to morning PEF values. The pooled analysis using both the fixed- and random-effects analysis show no benefit of air filtration on PEF values. CHEST / 122 / 5/ NOVEMBER,

7 Figure 2. Weighted mean difference for total symptoms. This figure shows the results of four randomized trials of domiciliary air filtration systems with respect to total asthma symptoms. Overall, a modest improvement in symptoms is shown by the pooled data. The fixed-effects analysis shows a significant reduction in symptoms, while the random-effects analysis shows a trend toward improvement. disturbance associated with use of domestic air filters. We did not identify any benefit conferred by air filters with respect to nasal symptoms, medication use, or PEF values. We adhered to rigorous systematic review methods 17,30 and transparent reporting 31 in this review. Jadad and colleagues 32 previously summarized the clinical, methodologic, and reporting aspects of systematic reviews and meta-analyses on the treatment of asthma, highlighting how serious methodologic flaws limited their usefulness. Strengths of this systematic review include a focused clinical question, a comprehensive search for published and unpublished research, explicit selection criteria, validity assessments conducted in duplicate independently, and reporting of the heterogeneity of study results. In critically appraising review articles, it is important to distinguish between the quality of the review methods and the quality of the studies included in the review. Accordingly, we evaluated and reported the randomized trial methodology in detail, 17 thereby allowing readers to make their own inferences about the primary evidence. None of the studies explicitly reported on concealment of treatment allocation. Few studies reported strategies to maintain the blinding of participants, caregivers, clinicians, and outcome assessors; however, eight studies used sham air filters in the control period (Table 2). Of the 10 randomized trials included in this systematic review, 9 trials evaluated asthma symptoms, but none included a validated generic or disease-specific quality-of-life instrument. At the time that many of these trials were conducted, few such instruments were available. Some simple symptom measures used in these trials may be insensitive to detect clinically important improvements due to environmental modifications. This hypothesis is supported by the observation that a reduction in airborne particulate matter associated with air filters did not always correlate with an improvement in symptoms. 24 Currently available valid and reliable disease-specific quality-of-life or symptom tools in this field include the Asthma Quality of Life Questionnaire, 33,34 the Living With Asthma Quality of Life Questionnaire, 35 and the Asthma Control Questionnaire 36 for adults, and the Pediatric Asthma Quality of Life Questionnaire 37 for children. The dearth of randomized trials evaluating the effect of air filters in children merits comment. Only one study enrolled exclusively children, 29 despite the high and growing prevalence of asthma in this 1540 Clinical Investigations

8 population. 5 The Seattle-King County Healthy Homes Project, which surveyed low-income urban caregivers of children with asthma, found that 12% used a vacuum with a HEPA quality filter. 38 However, the random-effects model meta-analysis in this review that gives smaller studies proportionally greater weight in the pooled estimate, and results in more conservative interpretation of the effect of HEPA filters, suggests no overall benefit in terms of symptoms and sleep disturbance. These findings, and the fact that disease-specific outcomes were not measured, precludes making guidelines or policy recommendations about the use of air filters. 39 However, the epidemiologic trend of increased asthma and allergy symptoms, the growing importance of patient empowerment through symptom control, and the need to measure outcomes that are important to patients suggest that further large rigorous randomized trials of environmental interventions such as air filters are warranted. References 1 Centers for Disease Control and Prevention. Asthma mortality and hospitalization among children and young adults: United States, MMWR Morb Mortal Wkly Rep 1996; 45: Sears MR. Epidemiological trends in asthma. Can Respir J 1996; 3: Burney PGJ, China S, Rona RJ. Has the prevalence of asthma increased in children? Evidence from the National Study of Health and Growth BMJ 1990; 300: Ninan TK, Russell G. Respiratory symptoms and atopy in Aberdeen school-children: evidence from two surveys 25 years apart. BMJ 1992; 304: Habbick BR, Pizzichini MMM, Taylor B, et al. Prevalence of asthma, rhinitis, and eczema among children in 2 Canadian cities: the International Study of Asthma and Allergies in Children. Can Med Assoc J 1999; 160: Haahtela T, Jarvinen M, Kava T, et al. Comparison of a 2 -agonist, terbutaline, with an inhaled corticosteroid, budesonide, in newly detected asthma. N Engl J Med 1991; 325: Gibson PG, Coughlan J, Wilson AJ, et al. The effects of limited (information only) patient education programs on the health outcomes of adults with asthma. In: Cochrane Collaboration. Cochrane Library. Issue 3. Oxford, UK: Update Software, Davis DA, Thompson MA, Oxman A, et al. Changing physician performance: a systematic review of the effect of continuing medical education strategies. JAMA 1995; 274: Balon J, Aker PD, Crowther ER, et al. A comparison of active and simulated chiropractic manipulation as adjunctive treatment for childhood asthma. N Engl J Med 1998; 339: National Asthma Education and Prevention Program. Expert panel report 2: guidelines for the diagnosis and management of asthma. Bethesda, MD: National Institutes of Health, 1998; publication No Ernst P, Fitzgerald JM, Spier S. Canadian Asthma Consensus Conference summary of recommendations. Can Respir J 1996; 3: Evans R. Environmental control and immunotherapy for allergic disease. J Allergy Clin Immunol 1992; 90: Custovic A, Simpson A, Pahdi H, et al. Distribution, aerodynamic characteristics, and removal of the major cat allergen Fel d 1 in British homes. Thorax 1998; 53: Maloney MJ, Wray BB, DuRant RH, et al. Effect of an electronic air cleaner and negative ionizer on the population of indoor mold spores. Ann Allergy 1987; 59: US Environmental Protection Agency. Residential aircleaning devices: a summary of available information. Washington, DC: Office of Air and Radiation, US Environmental Protection Agency, Boulet LP, Becker A, Berube D, et al revision of the Canadian Asthma Consensus Guidelines: Asthma Consensus Editorial Committee. Can Respir J 1999; 6: Guyatt GH, Sackett DL, Cook DJ. Users guides to the medical literature: II. How to use an article about therapy or prevention; A. Are the results of the study valid? Evidence- Based Medicine Working Group. JAMA 1993; 270: Fleiss JL. The statistical basis of meta-analysis. Stat Methods Med Res 1993; 2: DerSimonian R, Laird N. Meta-analysis in clinical trials. Control Clin Trials 1986; 7: Zwemer RJ, Karibo J. Use of laminar control device as adjunct to standard environmental control measures in symptomatic asthmatic children. Ann Allergy 1973; 31: Villaveces JW, Rosengreen H, Evans J. Use of a laminar air flow portable filter in asthmatic children. Ann Allergy 1977; 38: Kooistra JB, Pasch PE, Reed CE. The effects of air cleaners on hay fever symptoms in air-conditioned homes. J Allergy Clin Immunol 1978; 61: Verral B, Muir DCF, Wilson W, et al. Laminar flow air cleaner bed attachment: a controlled trial. Ann Allergy 1988; 61: Reisman RE, Mauriello PM, Davis GB, et al. A double-blind study of the effectiveness of a high efficiency particulate air (HEPA) filter in the treatment of patients with perennial allergic rhinitis and asthma. J Allergy Clin Immunol 1990; 85: Antonicelli M, Bilo MB, Pucci S, et al. Efficacy of an air-cleaning device equipped with a high efficiency particulate air filter in house dust mite respiratory allergy. Allergy 1991; 46: Warburton CJ, Niven RM, Pickering CAC, et al. Domiciliary air filtration units, symptoms and lung function in atopic asthmatics. Respir Med 1994; 88: van der Heide S, Kauffman HF, Dubois AEJ, et al. Allergen reduction in houses of allergic asthmatic patients: effects of air-cleaners and allergen-impermeable mattress covers. Eur Respir J 1997; 10: Wood RA, Johnson EF, Van Natta ML, et al. A placebocontrolled trial of a HEPA air cleaner in the treatment of cat allergy. Am J Respir Crit Care Med 1998; 158: van der Heide S, van Aalderen WMC, Kaufman AF, et al. Clinical effects of air cleaners in homes of asthmatic children sensitized to pet allergens: J Allergy Clin Immunol 1999; 104(2 pt 1): Cook DJ, Sackett DL, Spitzer WO. Methodologic guidelines for systematic reviews of randomized control trials in health care from the Potsdam consultation on meta-analysis. J Clin Epidemiol 1995; 48: Moher D, Cook DJ, Eastwood S, et al. Improving the quality of reports of meta-analyses of randomised controlled trials: CHEST / 122 / 5/ NOVEMBER,

9 the QUORUM statement. Quality of Reporting of Metaanalyses. Lancet 1999; 354: Jadad AR, Moher D, Browman GP, et al. Systematic reviews and meta-analyses on treatment of asthma: critical evaluation. BMJ 2000; 320: Juniper EF, Guyatt GH, Epstein RS, et al. Evaluation of impairment of health related quality of life in asthma: development of a questionnaire for use in clinical trials. Thorax 1992; 47: Juniper EF, Buist S, Cox FM, et al. Validation of a standardized version of the Asthma Quality of Life Questionnaire. Chest 1999; 115: van der Molen T, Postma DS, Schreurs AJM, et al. Discriminative aspects of two generic and two asthma-specific instruments: relation with symptoms, bronchodilator use and lung function in patients in patients with mild asthma. Qual Life Res 1997; 6: Juniper EF, O Byrne PM, Guyatt GH, et al. Development and validation of a questionnaire to measure asthma control. Eur Respir J 1999; 14: Juniper EF, Guyatt GH, Feeney DH, et al. Measuring quality of life in children with asthma. Qual Life Res 1996; 5: Krieger JW, Song L, Takaro TK, et al. Asthma and the home environment of low-income urban children: preliminary findings from the Seattle-King County Healthy Homes Project. J Urban Health 2000; 77: Bero LA, Jadad AR. How consumers and policymaker use systematic reviews for decision making. Ann Intern Med 1997; 127: Clinical Investigations

10 Effect of Air Filtration Systems on Asthma: A Systematic Review of Randomized Trials Ellen McDonald, Deborah Cook, Toni Newman, Lauren Griffith, Gerard Cox and Gordon Guyatt Chest 2002;122; DOI /chest This information is current as of December 17, 2007 Updated Information & Services References Citations Permissions & Licensing Reprints alerting service Images in PowerPoint format Updated information and services, including high-resolution figures, can be found at: This article cites 36 articles, 11 of which you can access for free at: This article has been cited by 1 HighWire-hosted articles: Information about reproducing this article in parts (figures, tables) or in its entirety can be found online at: Information about ordering reprints can be found online: Receive free alerts when new articles cite this article sign up in the box at the top right corner of the online article. Figures that appear in CHEST articles can be downloaded for teaching purposes in PowerPoint slide format. See any online article figure for directions.