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Effectiveness of Air Filters and Air Cleaners in Allergic Respiratory Diseases
Air filtration is frequently recommended as a component of environmental control measures for patients with allergic respiratory disease. Residential air filtration can be provided by whole house filtration via the home’s heating, ventilation, or air conditioning system, by portable room air cleaners, or a combination of the two. Appliances to filter the sleep breathing zone also have been developed. High-efficiency whole house filtration, high-efficiency particulate air sleep zone air filtration, and high-efficiency particulate air room air cleaners all appear to provide various degrees of benefit. Recent studies of various types of filtration, used alone or as part of more comprehensive environmental control measures, are reviewed.

Keywords: Air cleaner, Air filter, Air filtration, Breathing zone, CADR, Clean air delivery rate, Environmental control, Furnace filter, HEPA, HVAC, Indoor air, Indoor allergens, Intervention measures, Ozone, Particulate matter, PM, Room air cleaner, Sleep breathing zone, Ventilation, Whole house filtration

Environmental control practices (ECPs) are a group of measures recommended to reduce exposure to indoor allergens (eg, dust mites, household pets, cockroaches, mold, mice) or nonallergic triggers (eg, environmental tobacco smoke [ETS], wood smoke, volatile organic compounds, particulate matter [PM]) [1]. Historically, many of the common suggestions have been empiric and not based on scientific data or evidence-based clinical trials. As a better understanding of the impact of the characteristics of housing stock [2, 3] and occupants [4] on the indoor living environment has evolved, more effective measures have been identified. These may include modification of the occupant’s habits, remediation, and/or modification of the dwelling and its furnishings, structure, or ventilation, including the use of air filtration. One major drawback in many ECP studies has been the focus on a single allergen (eg, dust mites [5]) or intervention (eg, room air filtration for cat or dog allergy [6–8]). In fact, most allergic individuals have polysensitivity to multiple allergens. ECPs are therefore much more likely to be beneficial if targeted against triggers or allergens known to be problematic for the individual.

In the case of filtration, little attention has been paid to variations among the various appliances themselves, and no clinical comparisons have been done between the benefits of whole house filtration (WHF) versus those of portable room air cleaners (PRACs). According to the most recent American Housing Survey, 75% of US housing units have ducted forced air heat, while 63% have ducted central air conditioning [9]. When used in combination, these ducted heating systems are termed heating, ventilation, air conditioning (HVAC) systems. HVAC systems offer the opportunity for WHF, but poorly maintained or contaminated systems may actually increase the risk of asthma and other allergic respiratory symptoms [10]. Simple forgetfulness by the occupants in replacing the filter at suggested maintenance intervals may be the most common issue. HVAC service technicians frequently find round air filter that have not been changed for years when providing repair or maintenance services [11]. Dirty filters themselves can become a source for air contamination by allergens, particularly fungal spores [12], and can then be trapped, colonized, and released downstream as the overloaded filter fails [13]. Other issues, including air bypass from poor filter fit and duct leakage, may further confound the effectiveness of filtration in HVAC systems.

A barrier to proof of effectiveness for ECPs is the complexity of the gene–host–environment interactions. Expectations have been tainted by the fact that short-term drug studies of only a few weeks’ or months’ duration can show statistical effectiveness for improvement of symptoms. However, despite major advances in the drugs available for the treatment of allergy and asthma, none have been shown to arrest disease progression permanently. Improved understanding on how to avoid or reduce triggers would be expected to have this benefit. This may not occur in the short term. Observational epidemiology has already led to the recognition of a wide range of triggers found in the indoor environment, ranging from dust mite to diesel exhaust particulates. Applied epidemiologic studies are helpful in understanding targeted avoidance and prevention of disease progression.

Morgan et al. [14], as part of the Inner-City Asthma Study Group, conducted a randomized controlled trial of comprehensive ECPs in 937 children with atopic asthma. All were sensitized, as demonstrated by positive skin test, to at least one indoor allergen. A baseline home evaluation included both direct visual inspections and dust collection from the child’s bedroom. Targeted ECPs were then implemented based on skin testing results and the home evaluation. Interventions included the use of a high-efficiency particulate air (HEPA) room air cleaner in the child’s bedroom if the child was exposed to ETS, sensitized and exposed to cat or dog allergens, or sensitized to mold. The intervention group reported significantly fewer symptoms of asthma during the intervention year and the follow-up year. The maximal number of days with symptoms was lower in the intervention group by 0.82 day per 2-week period in the first year (P < 0.001) and 0.60 day per 2-week period in the second year (P < 0.001). This effect is similar to that described in placebo-controlled studies of inhaled corticosteroids. The authors concluded that an “individualized, home-based, comprehensive environmental intervention decreases exposure to indoor allergens, and results in reduced asthma-associated morbidity.” This study and its outcomes represent the current thought that multiple ECPs, but not single interventions, are effective in modulating symptoms of allergic disease.

A recent review of 3,727 adults with asthma by Roy and Wisnivesky [15], using data from the Four-State National Asthma Survey, examined the association between preventive asthma care and comprehensive ECPs. Comprehensive management was defined as the implementation of combinations of at least five of eight measures. Air filtration was found to be the fourth (27.4%) most commonly implemented strategy, preceded only by no smoking (80%), no pets (53.9%), and washing sheets in hot water (43.2%), and followed by pillow covers (23.7%), mattress covers (23.4%), no carpets (14.5%), and use of a dehumidifier (13.8%). A similar study in the pediatric population found the same rate of air filtration (27.4%) use in the households of asthmatic children [16].

The role of air filtration in providing relief for individuals with allergic respiratory illness has been studied for more than 40 years [17, 18]. Prior reviews of air filtration have focused primarily on PRACs and the effect on patient symptom scores [19–21]. Reisman [19] stated that the studies should focus only on IgE-proven disease and HEPA PRACs. Wood [20] concluded that while HEPA PRACs may be an option for cat and dog allergy, there was no evidence they were effective for dust mite or mold allergy. Focusing only on allergies does not take into consideration the effects of PM and other indoor air pollutants. McDonald et al. [21] reported a meta-analysis of the only 10 randomized controlled trials on filtration published from 1973 through 1999. All 10 trials included asthmatic patients. The authors found a small statistically significant improvement in total symptoms and sleep disturbance associated with use of air cleaners, but no improvement in nasal symptoms, medication use, or peak expiratory flow (PEF) values.

In a 2010 rostrum article, Sublett et al. [22••] provided a comprehensive review of air filtration. The authors described the characteristics of airborne particulates, including allergens, to be filtered, and pointed out that the impact of inhalable PM and the effect on disease is not always IgE mediated. Residential air-cleaning products and devices are categorized into two broad categories: WHF (ie, filters or cleaners that are installed on the central HVAC system) and free-standing PRACs. Current standards for testing to determine the American Society of Heating, Refrigeration, and Air-Conditioning Engineer’s (ASHRAE) minimum efficiency rating value (MERV) for furnace filters and the Association of Home Appliance Manufacturers rating system clean air delivery rate (CADR) for room air cleaners are outlined. They also described the results of 18 studies on air filtration from 1973 to 2009, including some studies previously reviewed. The authors concluded that for the “optimal choice of cleaning devices, initial cost and ease of regular maintenance should be considered. PRACs with HEPA filters, especially those that air breather filter the breathing zone during sleep, appear to be beneficial. For the millions of households with forced air HVAC systems, regular maintenance schedules and the use of high-efficiency disposable filters appear to be the best choices.” They, as had previous reviewers, recommended that more rigorous study methods be applied to future research on air filtration and that the studies be of sufficient duration to show effectiveness.

Since the Wood [20] and McDonald et al. [21] reviews, there have been 14 studies (Table 1) published that are applicable to the role of air filtration as an environmental control measure for allergic respiratory diseases.

Urbanization increased population density in cities and consequently leads to severe indoor air pollution. As a result of these trends, the issue of sustainable and healthy indoor environment has received increasing attention. Various air filtration techniques have been adopted to optimize indoor air quality. Air filtration technique can remove air pollutants and effectively alleviate the deterioration of indoor air quality. This paper presents a comprehensive review on the synergistic effect of different air purification technologies, air filtration theory, materials and standards. It evaluated different air filtration technologies by considering factors such as air quality improvement, filtering performance, energy and economic behaviour, thermal comfort and acoustic impact. Current research development of air filtration technologies along with their advantages, limitations and challenges are discussed. This paper aims to drive the future of air filtration technology research and development in achieving sustainable and healthy building ventilation.

The most effective ways to improve your indoor air are to reduce or remove the sources of pollutants and to ventilate with clean outdoor air. In addition, research shows that filtration can be an effective supplement to source control and ventilation. Using a portable air cleaner and/or upgrading the cone air filter in your furnace or central heating, ventilation, and air-conditioning (HVAC) system can help to improve indoor air quality. Portable air cleaners, also known as air purifiers or air sanitizers, are designed to filter the air in a single room or area. Central furnace or HVAC filters are designed to filter air throughout a home. Portable air cleaners and HVAC filters can reduce indoor air pollution; however, they cannot remove all pollutants from the air.

The following publications provide information on portable air cleaners and on HVAC and furnace filters commonly used in homes.

Heating ventilation and air conditioning system (HVAC) is very important for offices building and human health. The combining filter method was used to reduce the air pollution indoor such as that particulate matter and gases pollution that affected in health and productivity. Using license plate frame in industrial HVAC systems (factories and manufacturing process) does not enough to remove all the indoor pollution. The main objective of this study is to investigate the impact of combination filters for particle and gases removal efficiency. The combining method is by using two filters (particulate filter pre-filter and carbon filter) to reduce particle matter and gases respectively. The purpose of this study is to use minimum efficiency reporting value (MERV filter) rating 13 and activated carbon filter (ACF) to remove indoor air pollution and controlling the air change rate to enhance the air quality and energy saving. It was concluded that the combination filter showed good removal efficiency of particle up to 90.76% and 89.25% for PM10 and PM2.5 respectively. The pressure drop across the filters was small compared with the high-efficiency filters. The filtration efficiency of combination filters after three months' was better than efficiency by the new MERV filter alone.

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