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Improving indoor air quality

Information on this page explains ways to improve indoor air in our homes, businesses, schools, and other places where people gather indoors.

Healthy indoor spaces are key to reducing the spread of airborne diseases, including colds, flus and COVID‑19. Two key strategies are ventilation and filtration:

  • Ventilation pulls fresh outside air into indoor spaces to reduce the amount of airborne pollutants, such as viruses.
  • Filtration removes viruses and pollutants from the air by trapping them in a filter.

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Guide for improving indoor air quality

This guide is for residential settings, operators of congregate settings, schools and childcare services, centers of faith, restaurants, and retail or other settings where people may gather. Download the guide.

To increase ventilation

  • Open windows and doors to bring in more outside air when it is safe.
  • Place portable fans as close as possible to open windows to blow potentially contaminated air out. Be sure the fans are pointing outside.
  • Run exhaust fans like bathroom fans or kitchen hoods whenever the building is occupied.
  • If you have a Heating, Ventilation, and Cooling (HVAC) system:
    • Create a schedule for regular HVAC inspections, cleaning and maintenance. Include filter replacements, and system upgrades or improvements, as needed.
    • Open dampers (moveable vents) to let in more outside air. Try to increase the amount of indoor air it pulls in to 100%.
    • If possible, upgrade the filter in your HVAC system to MERV 13 or the highest rating your system allows.

To increase filtration

  • Use portable HEPA-equipped air cleaners, specifically AHAM verified and/or CARB certified cleaners.
    • Read our HEPA air cleaner guidance to determine the number you may need, or use the Portable Air Cleaner Purification Calculator from the Harvard Healthy Buildings Program.
    • Keep it on – aim for the highest setting to achieve the cleanest air quality, but if it is too loud, turn it down and add additional portable HEPA air cleaners.
    • Create a cleaning and maintenance schedule that includes vacuuming the pre-filter and wiping the unit clean regularly. Additionally, make sure your schedule includes replacing filters according to the cleaner’s maintenance schedule (or sooner, based on use). Regular maintenance will help the cleaner run properly.
    • Worried about energy use? Most HEPA air cleaners, on their highest setting, only use about as much energy as a light bulb.
  • For a low-cost option, use a box fan with a MERV 13 furnace filter attached to the back. Watch our video to learn more, or read our guidance (see pages 4-5 for box fan filter instructions).

Free technical assistance

We offer free technical assistance to help you improve your ventilation and indoor air quality. To request assistance, please complete this form to the best of your ability (form is available in multiple languages). Our staff will respond as soon as possible. If you need help with the form or an interpreter, please contact or 206-477-5166. If you previously received a portable HEPA air cleaner from us, watch our video "Using and maintaining your HEPA Air Cleaner" [한국어 (Korean), Español (Spanish)] for information on setting up and maintaining your cleaner. (Please note, we no longer have portable HEPA air cleaners to provide).

Other strategies

  • Ultraviolet Germicidal Irradiation (UVGI) systems can kill germs in the air but cannot remove pollutants. If you install UVGI we recommend using it in addition to ventilation and air filtration to improve indoor air. This solution is only intended for specific. Work with a specialist to see if it is appropriate to be installed in your space.
  • Carbon dioxide (CO2) monitoring can be a helpful way to tell whether ventilation is adequate. The CDC recommends maintaining CO2 readings below 800 parts per million (ppm) through increased ventilation. When this isn’t possible, increase filtration through additional means such as portable HEPA air cleaners. Use this Harvard Healthy Buildings program online tool (designed for schools) to help determine the target CO2 based on the size of the room, ventilation rates, expected capacity and activities in the space. Note: CO2 does not tell you if indoor air quality is good. Instead, it can tell you if ventilation is not good, or if there are too many people inside your building. Read the CDC’s guidance to learn more about when CO2 can be a helpful thing to monitor.

These strategies work best when used with staying up to date with vaccinations, mask-wearing, hand-washing, social distancing, and other public health recommendations that reduce the risk of spreading airborne diseases like COVID-19.

Guide for improving indoor air quality

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This guide is for residential settings, operators of congregate settings, schools and childcare services, centers of faith, restaurants, and retail or other settings where people may gather. It includes strategies that reduce the spread of COVID-19 in their facilities.

Download the full guide

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