Air Cleaning

Indoor air cleaning is the process of intentionally removing pollutants from indoor air, or from the outdoor air as it enters a building. Particle air cleaning removes particulate pollutants and gas-phase air cleaning removes gaseous pollutants. Most air cleaners use fans to move air through an air cleaning media that removes pollutants. The amount of pollutant removal depends on the air flow rate and on the fraction of pollutants removed as the air flows through the media. Technologies commonly used in air cleaners are described in the supporting information. This section of the web site addresses the relationship of air cleaning to health and perceived air quality, focusing on application of air cleaning to buildings outside of the health care and industrial sectors.

Particle Air Cleaning and Allergies and Asthma

Many studies have evaluated whether use of portable particle air cleaning systems in homes improve the health of people with asthma and allergies. The overall finding is that particle-removal air cleaning devices sometimes modestly reduce adverse allergy and asthma health symptoms and improve related measured signs of health, particularly in people living in homes with pets. Asthma and allergy symptoms are not always improved, and even when they are improved the amount of improvement is generally less than 25%. There is some limited evidence that systems delivering filtered air to breathing zone of sleeping allergic or asthmatic people can be more consistently effective in improving allergy and asthma health effects than room air cleaners or whole-house filtration systems. Air cleaning has not been proven effective in reducing the allergy and asthma symptoms from dust mite allergens.

Particle Air Cleaning and Health Effects of Outdoor Air Particles

Air cleaning can reduce the levels of particles in the indoor air that come from outdoors. Higher levels of these outdoor air particles are linked to a range of respiratory and cardiovascular health effects or markers of adverse health effects including emergency room visits, hospital admissions and death rates. Modeling indicates that particle air cleaning will reduce these adverse effects; however, significant measured data are not available to confirm the model predictions. The results of two studies finding that indoor particle air cleaning reduced biomarkers predicting future adverse coronary events are consistent with the model predictions of reduced hospital admissions and death rates when air cleaning is used to reduce indoor exposures to particles of outdoor origin.

Particle Air Cleaning and Sick Building Syndrome Symptoms

The very limited available evidence suggests that particle filtration in buildings is not very effective in reducing acute health symptoms, often called sick building syndrome symptoms, in subjects without allergies and asthma.

Particle Air Cleaning and Respiratory Infections

The results of model predictions indicate that particle air cleaning will decrease the incidence of communicable respiratory infections such as common colds and influenza that are transmitted, in part, by removing small airborne particles containing infectious bacteria or viruses. However, the model inputs and models themselves have a high level of uncertainty and experimental data are not available to confirm the model predictions.

Air Cleaning for Gaseous Air Pollutants and Health

Some air cleaners are designed to reduce indoor air concentrations of selected types of gaseous pollutants, such as ozone or some of the volatile organic compounds present in indoor air. Various technologies for gas phase air cleaning are employed. At present, limited data are available on the long term effectiveness of these systems in reducing indoor pollutant concentrations and very few data are available on the effectiveness of these air cleaning systems in reducing adverse health effects.

Using Ozone for Air Cleaning

Ozone is an important outdoor air pollutant. Higher outdoor air ozone concentrations have been linked to a variety of adverse health effects including respiratory system irritation, asthma, chest pain, and damage to the lungs. Ozone is very chemically reactive, thus, when it is present indoors it reacts with some types of other indoor air pollutants and with some indoor materials such as carpeting. These chemical reactions can produce potentially harmful pollutants including formaldehyde and very small particles. Ozone generators are devices that intentionally produce ozone and release it to the indoor air. They have been marketed as air cleaners because ozone can chemically destroy some types of other indoor air pollutants. However, ozone air cleaners are not effective as air cleaners and can pose risks to health. Even though most ozone generators do not produce sufficient ozone to substantially reduce concentrations other air pollutants, the ozone that they produce is likely to increase the risk of the same adverse health effects associated with higher outdoor air concentrations of ozone. Several other types of electronic air cleaners can also unintentionally release ozone into the indoor air, usually at a lower rate than ozone generators, but the increases in indoor ozone concentrations may still pose health risks.

Using Ultraviolet Germicidal Lights for Air Cleaning

Ultraviolet (UV) germicidal light, produced by special lamps, is sometimes used to inactivate the bacteria and viruses present in airborne particles, making the bacteria and viruses unable to cause infectious disease in people. With a sufficient UV light intensity, some fungi can also be inactivated. The lamps producing the UV light are normally installed near the ceilings of rooms or in ducts that supply air to rooms. Systems installed in ducts often also irradiate with UV light the wet cooling coils and drain pans present in air conditioning systems. These systems can be very effective in reducing the growth of mold and bacteria on the surfaces of coils and drain pans. The UV germicidal lamp systems must be designed so that occupants receive minimal exposure to the ultraviolet light. The results of studies of the effects of ultraviolet germicidal systems on people's health have been inconsistent. There is clearly enough evidence of potential health benefits to warrant further research. Ultraviolet germicidal systems appear to be more likely to reduce respiratory infections when applied in crowded spaces, and when there are few opportunities for infection away from the spaces containing the ultraviolet equipment.

Implications for Good Building Practices

Particle filtration technologies are well established and widely available. Currently, most building HVAC systems include filters with a relatively low efficiency rating. For example, U.S. commercial buildings often use filters with a Minimum Efficiency Reporting Value (MERV) of 7, with a low efficiency in removing the small particles most important to health. The filters used in the HVAC systems of houses often have an even lower efficiency. The incremental costs of using higher efficiency filters, e.g., those with a MERV 11 rating, are modest — less than a U.S. dollar per person per month [11, 62] and the predicted health benefits are substantial. Consequently, routine use of higher efficiency filters in HVAC systems represents a good building practice. Stand-alone fan filter systems may also be employed to reduce indoor particle concentrations, particularly in homes of people with allergies or asthma and containing pets or other strong sources of airborne allergens. The energy consumption of fan filter systems varies widely. Energy efficient units, sometimes with very efficient brushless direct current fan motors, will cost less to operate and are preferable when considering the impacts of energy use on climate change.