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Aug. 11, 2020, 3:19 p.m. EDT

Here’s how to keep your home, business or classroom safe from COVID-19

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The vast majority of SARS-CoV-2 transmission occurs  indoors , most of it from the  inhalation of airborne particles  that contain the coronavirus.

The best way to prevent the virus from spreading in a home or business would be to simply keep infected people away. But this is hard to do when an estimated  40% of cases are asymptomatic  and asymptomatic people can  still spread the coronavirus to others .

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.

I am a  professor of mechanical engineering  at the University of Colorado Boulder. Much of my work has focused on how to control the transmission of  airborne infectious diseases indoors , and I’ve been asked by  my own university , my kids’ schools and even the Alaska State Legislature for advice on how to make indoor spaces safe during this pandemic.

Follow the latest updates on the coronavirus pandemic.

Once the virus escapes into the air inside a building, you have two options: bring in fresh air from outside or remove the virus from the air inside the building.

The safest indoor space is one that constantly has lots of  outside air  replacing the stale air inside.

In commercial buildings,  outside air is usually pumped in  through heating, ventilating and air-conditioning (HVAC) systems. In  homes, outside air gets in  through open windows and doors, in addition to seeping in through various nooks and crannies.

Simply put, 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 .

Environmental engineers like me quantify how much outside air is getting into a building using a measure called the  air exchange rate . This number quantifies the number of times the air inside a building gets replaced with air from outside in an hour.

While the exact rate depends on the number of people and size of the room, most experts consider roughly  six air changes an hour  to be good for a 10-foot-by-10-foot room with three to four people in it. In a pandemic this should be higher, with one study from 2016 suggesting that an exchange rate of nine times per hour  reduced the spread of SARS, MERS and H1N1  in a Hong Kong hospital.

Many buildings in the U.S.,  especially schools , don’t meet recommended ventilation rates.

‘It was chaos!’ Students get suspended for posting pictures of packed halls — now the high school is closing after 9 people were infected

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.

But in any room, the more people inside, the faster the air should be replaced.

So how do you know if the room you’re in has enough air exchange? It’s actually a pretty hard number to calculate. But there’s an easy-to-measure proxy that can help.

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