NEWS | May 1, 2018

High-efficiency filters reduce indoor air pollution but not asthma symptoms

Potential benefits of high-efficiency air filtration in homes studied in Fresno and Riverside


A new study focused on households with asthmatic children finds that the use of high-efficiency filtration, either as part of a central air system or standalone air cleaner, improves indoor air quality. The study also found that while participants did not report reduced asthma symptoms, they did have fewer visits to doctor’s offices and hospitals for asthma care.

Study results were recently presented at the California Air Resources Board (CARB).

The two-year study, “Benefits of High Efficiency Filtration to Children with Asthma,” was conducted by a team of UC Davis public health researchers led by Professor Deborah Bennett. CARB funded the study because other research has linked exposure to fine particles with premature death, asthma, heart disease and respiratory problems.

For the current study, researchers evaluated the extent to which high-efficiency air filtration in homes reduces indoor exposure to air pollution — and symptoms in asthmatic children. The study involved 191 asthmatic children age 6-12 years old from 172 households in and around Fresno and Riverside.

The outcomes showed:

  • Indoor air quality was improved with the use of high-efficiency filtration, with a 48 percent reduction in the mean indoor concentrations of fine particulate matter (PM 0.2 and 2.5) for homes with high-efficiency filters compared to those with no or low-efficiency filtration controls. Smaller reductions were observed for coarse (PM 10) particles (31 percent). (See data table below.)
  • Air quality improvements in homes were greater with two continuously operating standalone air cleaners than with central-system filtration, in part because central systems did not run continuously. Larger reductions were observed when participants kept their windows closed and ran the air cleaners for the full duration specified in the study protocol.
  • A slight reduction in health care visits for participants, especially for those characterized as severe asthmatics.
  • Participants with air cleaners in their bedrooms reported waking fewer times from asthma-related symptoms if they also kept their bedroom doors closed at night.

Percent Reductions in Indoor Particulate Matter (PM) Concentrations:

Particulate Matter Size Fraction (microns)




All Homes: Control vs. Filtration




Central Filtration Homes:  Control vs. Filtration




Air Cleaner Homes: Control vs. Filtration




Based on the study outcomes, CARB recommends using high-efficiency air filtration to improve indoor air quality. The agency remains committed to finding ways to improve symptoms for those with asthma.

More information about UC Davis Health and its Department of Public Health Sciences is at

More information about CARB is on the agency's website.

Information on asthma prevalence in California is available from the Department of Public Health.