Analysis & Opinions - Belfer Center for Science and International Affairs, Harvard Kennedy School

Indoor Ventilation Strategies to Reduce the Spread of COVID-19 at Home

| Apr. 05, 2021

The COVID-19 pandemic continues to sweep across the globe, with its associated death count totaling nearly 2 million lives to date. Household transmission of COVID-19 remains a high priority given the ease of how the virus, SARS-CoV-2, that causes COVID-19 can spread in closed, confined spaces. Herein, we outline environmental health strategies to fortify home air ventilation and filtration systems, and these recommendations dovetail perfectly with our group’s previous human factors guidance to prevent the spread of COVID-19 within your home using the 3 S approach (see excerpt below):

“More and more households are faced with the challenging situation of caring for a loved one with COVID-19 while trying to avoid spreading the infection to others within the home. Transmission of SARS-Co-V-2 increases by 10 fold among household members when living with a person infected with the virus compared to other contacts. However, it is possible to care for a household member who has COVID-19 and not catch the virus yourself. To reduce household transmission, a multi-pronged mitigation approach needs to be adopted including prompt isolation of persons with suspected or confirmed COVID-19, adjustments made to shared living space (i.e.: separate bedroom and bathroom when possible), mask-wearing inside home when distancing is not feasible and more. Here are some practical strategies households can take to minimize the risk of spread.”

To provide more context on the need to create healthy home environments and curb the spread of SARS-CoV-2 within the household requires emphasis on ventilation given aerosol transmission via inhalation of suspended viral particles that concentrate in the air. Settings where such airborne transmission via aerosols and/or droplets appears to be linked with outbreaks include indoor spaces, prolonged exposure to respiratory particles, under ventilated areas, and poor use of masking.

Below, we outline ventilation strategies to curb the spread of COVID-19 through modification of your home’s air quality and flow. These strategies should be taken in combination with best COVID-19 prevention practices such as regular hand washing, surface sanitation, masking, and physical distancing of at least 6 feet. The goal of indoor ventilation strategies is to increase the number of air exchanges, which refers to the number of times air moves in and out of a space within a given time frame. A typical home in the U.S. gets about 0.5 air changes per hour. That’s not enough for the purposes of infection control. We want to aim for an air change rate of 4-6 times per hour by using fresh, filtered, and flowing air.  

Figure 1: Indoor Ventilation Strategies to Prevent the Spread of COVID-19 at Home.


1) Fresh air brings the outdoors into your buildings as a new source of virus-free air. Opening windows or doors is one strategy to improve indoor air exchange. Natural ventilation can achieve excellent air exchange, even with windows open only slightly to a gap of 6 inches. Another strategy is to increase the volume of outdoor air per minute utilized in your home’s Heating and Ventilation Air Conditioning (HVAC) system. A HVAC technician would be able to assist you in determining if this type of modification is applicable and appropriate for your building’s HVAC system. Importantly, additional filtration (see strategy #2) may be required when opening windows in areas of high air pollution. In a similar manner, use caution when opening windows and doors in cases where outdoor air quality or temperatures may exacerbate preexisting health conditions (e.g.: asthma, COPD, severe allergies).

2) Filtered air refers to the removal of contaminants from the indoor air space. Consider getting your HVAC system inspected by a technician that can ensure a proper filter seal, installation, and model that balances functionality with efficiency (usually a MERV-13 or highest compatible filter equivalent). You may also consider disabling any HVAC demand-control ventilation (DCV) settings that regulate the air ventilation rate based on room temperature and occupancy. The HVAC filtration system can also be supplemented by adding portable air purifiers to the space. Air portable cleaners not only play a key ancillary role, but they can also act as critical standalone filtration in buildings without a central HVAC system. Make sure your portable air cleaner contains a high-efficiency particulate air (HEPA) filter, and look for a high Clean Air Delivery Rate (CADR) that should be reported by the manufacturer. A ‘rule of thumb’ is to select a device with a CADR of 300 for every 500 square feet of space. You don’t want, or need, any unnecessary add-ons with your device – just look for HEPA and that high CADR.

3) Flowing air refers to the circulation of air throughout the indoor environment. The overarching principle of airflow is to prevent the concentration of infectious particles in the air, ideally with the routing of contaminated particles out of the enclosure. In multistory buildings or homes, try to open windows or doors on different levels as this will improve circulation. Next, you can use fans as a resource to improve airflow and exchange. Many buildings have kitchens, bathrooms, and HVAC systems that are equipped with built-in fans. Set these fans to run continuously whenever possible. For example, many people open windows and run exhaust fans while using the bathroom, only to turn them off when they leave the room. We advise you keep these fans running! Additionally, you can supplement built-in airflow systems with portable fans placed strategically throughout the household or building. A great spot for a fan is in the window, which can facilitate air exchange. For example, indoor-facing window fans can augment the flow of natural air into the building while outdoor-facing window fans can facilitate the removal of contaminated air from the space. Avoid placing fans in an orientation where they might directly blow air particles from one individual onto another, which could inadvertently facilitate the spread of COVID-19.  Finally, there’s more you can do to create a healthier home beyond the COVID-19 pandemic: check out our report on 36 tips for a healthier home to learn more.

For more information on this publication: Belfer Communications Office
For Academic Citation: Thomasian, Nicole, Syra Madad and Joseph G. Allen.“Indoor Ventilation Strategies to Reduce the Spread of COVID-19 at Home.” Belfer Center for Science and International Affairs, Harvard Kennedy School, April 5, 2021.

The Authors

Joseph G. Allen