Will Air Cleaning Reduce Health Effects from Indoor Air Pollutants?

Air-cleaning devices may help reduce levels of smaller airborne allergens, particles, or, in some cases, gaseous pollutants in a home. However, air cleaners may not decrease adverse health effects particularly in sensitive populations such as children, people with asthma and allergies, and the elderly.

 

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Clinicians frequently recommend that patients who have asthma or allergies use HEPA air filters in HVAC systems or in portable air cleaners. Regardless of how efficient and effective air-cleaning devices are in removing pollutants, a question still remains about their ability to reduce adverse health effects.

How effectively air-cleaning devices alleviate allergic and other health symptoms remains uncertain. Strong data linking air-cleaning devices to reduced health symptoms do not exist. Many studies have associated air-cleaning devices with reductions in airborne indoor pollutant concentrations, but more clinical studies are needed to determine whether air cleaners significantly affect health outcomes. A literature review documented only a limited number of studies that attempted to evaluate the clinical outcomes of air cleaner use. These studies focused on more sensitive groups, such as asthmatic and allergic individuals, children, and the elderly. A number of the studies had important limitations, such as small study size, short duration, and lack of blinding (i.e., subjects and scientists were aware of air cleaner operation), which may result in a placebo effect. The results were also more suggestive than conclusive.

Many indoor pollutants related to asthma and allergies are either airborne particles or irritants, such as the gaseous components of secondhand smoke or nitrogen dioxide, chemicals linked with gas cooking appliances, fireplaces, wood stoves, and unvented kerosene and gas space heaters. Most studies involving subjects who have perennial and seasonal allergy or asthma symptoms tested portable air cleaners equipped with HEPA filters.

 

Few studies tested gas-phase filtration and air cleaners using UV light technology, such as ultraviolet germicidal irradiation (UVGI) cleaners - and PCO cleaners. Photocatalytic oxidation (PCO) cleaners use a UV lamp along with a substance, called a catalyst, that reacts with the light; and are intended to destroy gaseous pollutants by converting them into harmless products, but are not designed to remove particulate pollutants. The scarcity of data results in little scientific evidence that these devices are associated with a reduction in health symptoms.

The effects of particle air cleaners on allergy and asthma symptoms have been reviewed by the Institute of Medicine (IOM) Committee on the Assessment of Asthma and Indoor Air of the National Academy of Sciences. The IOM concluded that:

• The results of existing experimental studies are inadequate to draw firm conclusions regarding the benefits of air cleaning for asthmatic and allergic individuals … Air cleaners are helpful in some situations in reducing allergy or asthma symptoms, particularly seasonal symptoms, but it is clear that air cleaning, as applied in the studies, is not consistently and highly effective in reducing symptoms.
• The use of air cleaners may help reduce levels of smaller airborne allergens or particles, but should not be expected to effectively reduce health symptoms.

Several factors should be considered in evaluating whether an air cleaner is beneficial in alleviating health effects.

• Many studies on the health benefits of air cleaning involve multiple interventions and thus are not useful in determining the effects of air cleaners alone.
  

The health benefits of air cleaners are often studied along with other interventions such as mattress and pillow covers, exclusion of pets from the bedroom, weekly baths for pets, or vacuum cleaning. Studies that consider air cleaning concurrently with other interventions have relatively little value in determining the clinical outcome resulting from the use of air cleaners because it is not clear if any improvements demonstrated are due to the air-cleaning devices or to the other interventions.

• An air cleaner’s ability to remove some airborne pollutants, including microorganisms, is not, in itself, an indication of the air cleaner’s ability to reduce health symptoms.
  

Pollen, dust mite and cockroach allergens, some mold spores, and animal dander carried on large particles settle rapidly before they can be removed by filtration. Because these particles do not remain airborne, air-cleaning devices are relatively ineffective in their removal. Therefore, effective allergen control requires routine cleaning and dust control including the weekly washing of bed sheets, frequent vacuuming of carpets and furniture, and regular dusting and cleaning of hard surfaces. (For more on allergen control, visit www.epa.gov/asthma.)

A significant fraction of cat and dog allergens and a small portion of dust mite allergens associated with mite feces are carried on small particles. Consequently, they are more easily dispersed throughout a house, remain airborne longer, and are more likely to be removed by air cleaners. Although there is evidence that some air cleaners can remove a portion of smaller particles from the air, there is little evidence that these reductions in particle levels alleviate health symptoms. This lack of improvement in symptoms may be due in part to the fact that, once sensitized, allergic and asthmatic individuals respond to much lower levels of pollutants.

 

There is little clinically confirmed support for the effectiveness of UVGI cleaners in reducing health symptoms in either airstream or surface applications. Despite UVGI’s ability to deactivate some surface-grown microorganisms, data linking the effectiveness of UVGI systems to reduced health symptoms in sensitive populations such as children, asthmatic and allergic individuals, and the elderly are not available for residential settings.

• Some air cleaners may produce new, potentially toxic pollutants or may re-disperse old ones.

A limited number of studies report that irradiation by UVGI lamps reduce vegetative bacteria and molds that are either airborne or growing on moist HVAC surfaces. However, the dead mold spores may still cause allergic reactions in some people.

High moisture and elevated temperatures can promote bacteria and mold growth in particulate filter media. Air filters may re-emit bacteria and mold spores during HVAC startup and off-and-on operations when air velocity suddenly increases.

 

Ozone generators sold as air cleaners for use in occupied indoor spaces produce ozone, a lung irritant. Electronic air cleaners, such as ion generators and electrostatic precipitators, have the potential to emit potentially dangerous levels of ozone. Contamination of electrode surfaces in electronic air cleaners may cause increased ozone levels. There also have been reports of electronic air cleaners producing fine particulate material from the reaction of ozone produced in the corona discharge with other chemicals indoors.

 

Liquid tobacco smoke particles trapped by an air filter may give off odorous organic gases. Saturated sorbent filters may also release trapped gaseous pollutants back into the airstream. If a PCO system’s design parameters do not match the pollutant targeted for decomposition, the PCO cleaner may create, as a result of the oxidation process of certain VOCs in indoor air, by-products that are indoor air pollutants.

Current evidence suggests that air cleaning may have a useful role when used in conjunction with source control and ventilation with clean outdoor air.

 

[Editor's note: HHI maintains that non-ozone-producing air cleaners are helpful in many cases - despite the lack of definitive proof of health benefits - especially when paired with eliminating sources of pollution and ensuring good ventilation.]

 

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