Furnace Filters 101

— Written By and last updated by Jessica Griffin

The US Environmental Protection Agency warns that the air inside our homes is generally unhealthier than the air outside. Dust, allergens, irritants, and other inhaled particles cause a lot of harm over time, especially for those who struggle with asthma and other related illnesses. Furnace filters remove particles from the air that can affect the lungs.

Furnace filters are a very important consideration when looking to reduce indoor air pollution. Furnace filters protect the furnace but they also contribute to good health by cleaning the air circulating through the house. Filter designs and rating systems vary. Some filters remove large, heavy particles from the air but allow smaller particles to flow through. Large particles (from 6 to 100 microns in size) include lint, pollen, and mold spores. Medium particles (from 0.3 to 6 microns) include dust, animal dander, and bacteria. The smallest particles (0.3 microns and below) include smoke, smog, and viruses. By comparison, a human hair is between 3 to 200 microns. Fiberglass or cellulose pad filters are low cost but only effective at removing up to 2% of submicron particles.

  • Washable/reusable filters can be washed with a hose and reinstalled. They catch smaller particles and if washed monthly may last 3 to 5 years. These filters are effective at removing up to 6% of submicron particles.
  • Pleated polyester filters provide more filtering capacity than flat filters. Many are made with electrostatically charged fibers that attract small particles. They last up to 3 months and remove up to 11% of submicron particles.
  • Other filters that may be used in special circumstances include deep-pleated, high-efficiency air filters. These filters remove up to 25% of submicron particles. Some have electrostatically charged fibers embedded in the filtering media. Pleated electrostatic filters remove up to 49% of submicron particles.
  • Electronic filters require electricity to operate. Air is directed through a high-voltage grid, which applies a positive charge on particles in the air stream. The particles are attracted to a negatively charged element. This type of filter should be should be washed monthly. High initial cost and maintenance are disadvantages as is the cost of repair if the power unit fails. These filters remove up to 94% of submicron particles.
  • The following factors affect the life of a filter: the presence of house pets, tobacco smoke, remodeling and woodworking in home, and continuous fan operation. Filters should be replaced more often when any or all or these factors exist. Always follow the manufacturer’s recommendations on the filter package for replacing filters.
  • Replacing old filters for new ones is critical for the best operation of your heating and air conditioning system. Make a note on the calendar as a reminder to change filters in your home. Also, write the date you replace the filter and you will not have to guess when it comes time to replace it again.

Resource: American Lung Association