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If you walk into a building and it is hot, stuffy, and crowded, chances are there is not enough ventilation. Turn around and go.

Masks do a decent job to prevent the virus from spreading in the environment, but if an infected person is inside a building, inevitably a 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 indoor airborne infectious diseases, and I was asked by my own university, my children’s schools, and even the Alaska State Legislature for advice on how to secure indoor spaces 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. building.

It’s all about fresh, outdoor air

The safest interior space is one that constantly has a lot of Outside air replace the stale air inside.

In commercial buildings, the outside air is usually pumped by heating, ventilation and air conditioning (HVAC) systems. In houses, the outside air enters through windows and open doors, in addition to seeping through various nooks and crannies.

Simply put, the cooler the outside air inside a building, the better. Bringing in this air dilutes any contaminants in a building, be it a virus or something else, and reduces exposure of anyone indoors.

Environmental engineers like me quantify the amount of outside air entering a building using a measurement called the air exchange rate. This number quantifies the number of times the air inside a building is replaced by outside air in an hour.

Although the exact rate depends on the number of people and the size of the room, most experts consider roughly six air changes per hour be good for a 10ft by 10ft room with three to four people. In the event of a pandemic, this is expected to be higher, with a 2016 study suggesting that an exchange rate of nine times per hour reduces the spread of SARS, MERS and H1N1 in a Hong Kong hospital.

Many buildings in the United States, especially schools, do not follow the recommended ventilation rates.

“It was chaos! Students suspended for posting pictures of crowded rooms – now high school closes after 9 people infected

Fortunately, it can be quite easy to get more outside air into a building. Keeping open windows and doors is a good start. Boxing a fan in a blowing window can also dramatically increase air exchange. In buildings that do not have opening windows, you can change the mechanical ventilation system to increase the amount of air it pumps.

But in any room, the more people there are, the faster the air needs to be replaced.

Using CO2 to measure air circulation

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

Every time you breathe out, you release CO2 in the air. Since the coronavirus is most often spread by breathing, coughing, or speaking, you can use CO2 Levels to see if the room fills with potentially infectious exhales. The CO2 level allows you to estimate whether enough fresh outside air is entering.

Outdoors, CO2 levels are just over 400 parts per million (ppm). A well ventilated room will have around 800 ppm of CO2. Higher than that and it’s a sign the room might need more ventilation.

Last year, researchers from Taiwan reported the effect of ventilation on a tuberculosis epidemic at the University of Taipei. Many rooms in the school were poorly ventilated and had CO2 levels above 3,000 ppm.

When engineers improved airflow and achieved CO2 levels below 600 ppm, the epidemic has completely stopped. According to research, increased ventilation was responsible for 97% of the decrease in transmission.

Since the coronavirus spreads through the air, higher CO2 levels in a room likely mean there is a more chance of transmission if an infected person is inside. Based on the study above, I recommend trying to keep CO2 levels below 600 ppm. You can buy good CO2 meters for about $ 100 online; just make sure they are accurate to within 50 ppm.

Air filters

If you find yourself in a room that cannot get enough outside air for dilution, consider an air purifier, also commonly known as an air purifier. 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.

The US Environmental Protection Agency says air purifiers can do it for the coronavirus, but not all air purifiers are created equal. Before you go out and buy one, there are a few things to keep in mind.

The first thing to consider is how efficient is the filter of an air purifier. Your best option is a cleaner that uses high efficiency particulate air (HEPA), because they remove more than 99.97% of all particle sizes.

The second thing to consider is the power of the cleaner. The larger the room, or the more people, the more air needs to be cleaned. I worked with colleagues at Harvard to develop a tool to help teachers and schools determine the power of an air purifier you need for different class sizes.

The last thing to consider is the validity of the claims made by the company producing the air purifier.

The Home Appliance Manufacturers Association certifies air purifiers, so the AHAM verified seal is a good place to start. In addition, the California Air Resources Board has a list of air purifiers which are certified safe and effective, although not all use HEPA filters.

Keep the air fresh or go outside

Both World Health Organization and U.S. Centers for Disease Control and Prevention say that poor ventilation increases the risk of coronavirus transmission.

If you are controlling your indoor environment, be sure to circulate enough fresh air from outside into the building. A CO2 monitor can help you know if there is enough ventilation, and if CO2 levels start to rise, open a few windows and take a break outside. If you can’t get enough fresh air into a room, an air purifier may be a good idea. If you buy an air purifier, be aware that it does not remove CO2, so even though the air is safer, CO2 levels can still be high in the room.

If you walk into a building and it is hot, stuffy, and crowded, chances are there is not enough ventilation. Turn around and go.

By paying attention to air circulation and filtration, improving them where you can, and staying out of places where you can’t, you can add another powerful tool to your tool kit. anti-coronavirus tools.

This article was published with permission from The Conversation. To understand new developments in science, health and technology, every week. Subscribe to The Conversation scientific newsletter.

Shelly Miller is professor of mechanical engineering at the University of Colorado at Boulder.

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