As we continue to learn more about COVID-19, the vast majority of SARS-CoV-2 transmission occurs indoors, most of it from the inhalation of airborne particles that contain the coronavirus. Masks do a decent job at keeping the virus from spreading into the environment, but if an infected person is inside a building, inevitably some virus will escape into the air. So how can you make indoor air safe? A Mechanical Engineer who has advised several universities and the Alaska State Legislature on how to make indoor spaces safe during the pandemic says it is all about fresh, outside air. The more fresh, outside air inside a building, the better. Bringing in this air dilutes any contaminant in a building, whether a virus or a something else, and reduces the exposure of anyone inside. While commercial building HVAC system incorporate outdoors air into buildings and opening windows and doors does the same in our homes, it may not be enough. Thankfully, it can be pretty easy to get more outside air into a building. Keeping windows and doors open is a good start. Putting a box fan in a window blowing out can greatly increase air exchange too. In buildings that don’t have operable windows, you can change the mechanical ventilation system to increase how much air it is pumping. Another option is to consider an air cleaner or air purifier. Your best option is a cleaner that uses a high-efficiency particulate air (HEPA) filter, as these remove more than 99.97% of all particle sizes. Select a machine that is certified by The Association of Home Appliance Manufacturers (AHAM) or reference the California Air Resources Board has a list of air cleaners that are certified as safe and effective. Another important thing to consider is how powerful the purifier is. The larger the room or the more people in a room, the more air needs to be cleaned. Choose an air purifier for the appropriate square footage of a room and replace the filer as recommended. Although fresh air and air purifiers can help clean the air, it isn't foolproof and social distancing, face masks and hand washing should continue to be practiced.