When used correctly, air and HVAC filters can help reduce airborne contaminants, including viruses, in a small building or space. By itself, cleaning or filtering the air is not enough to protect people from COVID-19. In the home, the risk of contracting COVID-19 through drafts or air conditioning units is no more likely to spread the virus through close contact or touching contaminated surfaces. While no portable air filtration system has been proven to filter COVID-19, research supports an air purifier's ability to filter common airborne viruses, provided that the system captures infected air. Consequently, there is a possibility that the air conditioner is a possible transmission route that absorbs virus particles exhaled by an infected person and then expels those infectious particles in the same room or even in another room several floors away.
In fact, other infectious diseases such as measles, tuberculosis, chickenpox, influenza, smallpox and SARS have been shown to spread through heating, ventilation and air conditioning systems. For many years, homeowners have had the option of installing ultraviolet lights in their HVAC system, where the air that is distributed through the home is first treated inside sealed air ducts. In fact, research shows that changing the air in a room several times an hour with clean or filtered outdoor air (using a window fan, using higher MERV filters in a heating, ventilation and air conditioning (HVAC) system, using portable air cleaning devices, and even simply opening a window) can reduce the risk of transmission of COVID-19, with studies showing that five air changes per hour reduce the risk of transmission by 50 percent. Your first line of defense against overhead transmission, and perhaps the easiest to upgrade, is your home's air conditioning filter.
The fact that aerosolized viral droplets can move in drafts in this way means that if you are in a room with an infected person and there is no fresh air circulating, even if you are social distancing to maintain a minimum distance of 6 feet, you may not be sure. The restaurant also had no windows and therefore had no ventilation, which brought fresh air and diluted virus particles in the air. There are currently no other tests documenting the possibility of transmission of COVID-19 through an air conditioning unit. When you close doors and windows to keep warm air out, you essentially eliminate the flow of fresh air so that everyone in the room breathes and breathes the same air again.
The first thing worth noting is that most air conditioners recirculate air within a space (meaning they don't draw fresh air from outside, like an open window would). This recirculation also helps to remove particles from the air when the recirculated air passes through the filters before being returned to the conditioned spaces. Whether you're opening windows, running your air conditioner, or doing some combination of both, a good HEPA filter or an air purifier should help too. After examining video footage of diners who were infected and simulating transmission of the virus, the scientists concluded that the small outbreak was caused by strong drafts from the air conditioning unit on diners which was blowing aerosols containing the virus from an infected person to those close by.
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