Shelly Miller is a professor of mechanical engineering at the University of Colorado Boulder. Much of her work is focused on how to control the transmission of airborne infectious diseases indoors, making her just the person to ask for advice when it comes to stopping the spread of Covid-19 inside buildings.

So, how can you make the indoor spaces you inhabit as safe as possible?

First and foremost, Miller says 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 something else and reduces the exposure of anyone inside. Environmental engineers 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 America don’t meet these requirements, but 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.

If you are in a room that can’t get enough outside air for dilution, consider an air cleaner, also commonly called air purifiers. These machines remove particles from the air, usually using a filter made of tightly woven fibers. They can capture particles containing bacteria and viruses and can help reduce disease transmission.

Want the full guide on how to use ventilation and air filtration to prevent coronavirus from spreading indoors? Look no further.