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Editor’s note: This article was originally published on Oct. 28, 2020, and has been updated for the 2021 season. 

As cases spike and cold weather arrives in Colorado, people are spending more time indoors and once again debating if and how they should travel for the holidays. With COVID-19 cases on the rise nationally, it remains important to reduce one’s risk of contracting or spreading this airborne virus. 

Here is updated advice from CU Boulder’s Shelly Miller, professor of mechanical engineering and expert in indoor air quality, about the ways we can all help reduce our risk and keep our communities safe during the winter season. 

Professor Shelly Miller. (Credit: Glenn Asakawa, CU Boulder)

What is the main thing people should keep in mind going into this winter season to lower their risk of contracting or spreading COVID-19? 

Update: Full vaccination of two or three doses is an important layer of protection against COVID-19. On Nov. 11, 2021, Gov. Jared Polis signed an executive order that made all adults in Colorado eligible for booster shots, an order which formally declares that the entire state is at risk from COVID-19. 

What I would really like to stress is that the best way to keep yourself safe is to use a layered approach. Any one layer will only reduce your risk by maybe 50%. These layers include: the amount of time you spend indoors, the number of people you spend time with, and how far you are from them. Are you always wearing a mask? Is the space well-ventilated? What comes later are the easy things, like hand hygiene and surface cleaning. You have to consider them all before you make your decision and assume that your risk is low enough. 

If a person chooses to travel home for the holidays, how can they significantly reduce their risk? 

Update: According to Miller and The Centers for Disease Control and Prevention (CDC), a person does not need to quarantine before or after flying, but doing so can add a valuable layer of protection for those at high risk of COVID-19. According to the CDC, if exposed to COVID-19, fully vaccinated people should get tested five to seven days after their exposure even if they don’t have symptoms and wear a mask indoors in public for 14 days or until their test result is negative. 

Staying in motels and hotels are OK, but I would recommend trying to find one where you can park outside and then just go into your room, where there are no shared hallways. I ask, how long has it been since somebody else has been in the space? Ideally, it’s 24 hours, so the aerosols have settled and begun to inactivate. 

When riding in a car with other people, wearing masks and opening the windows for short periods of time reduces the chance of transmission.

Opening windows in your home for a short time every hour will bring fresh air inside and help make an indoor space safer. 

For the times when you need to or have to be indoors, how can you tell if a space is well ventilated or safe enough to be in for a time? 

Update: According to the CDC, masks are not required if all people indoors together are fully vaccinated. 

The first thing I’m looking for is whether they have windows or doors open. Even if windows and doors are not open, they can still allow outside air to infiltrate an indoor space because it's an opening in the building shell. Bigger volumes, like large spaces with high ceilings, are also better because there's more volume for the virus to mix around in, diluting it.  

The second thing I look for is how many people are in the space, and are they all wearing masks? If there are people in there who aren’t wearing masks, I will leave. If you’re having people over at your own home and talking, you’re going to be generating a ton of aerosol. The only way to keep yourself safe in that space is to wear a mask. You have to wear a mask if you want to hang out and talk with other people and share their air. 

Is it worth it to open a window in the winter? 

You can open a window for 10 or 15 minutes, then turn on your heater and close the window, and repeat that every hour. And yes, it will get cold, but you've now brought fresh air in and you've exhausted warm air. Then you can close it up. 

If you are outside with a propane heat lamp or a gas-fired heater, you might be generating a little bit of carbon monoxide. Make sure you have enough ventilation to dilute the carbon monoxide that you're building up. 

How does turning on the heat in your home affect its ventilation? 

In most homes, when you turn on the heat, you're recirculating the indoor air through a coarse filter and it can also increase the infiltration of air outside coming in. So, in general, running your heat is going to be a helpful thing to do to keep your indoor environment clean. You can also run your exhaust hoods in the bathroom and over your stove, which will take inside air and throw it out. Then, outside air will have to infiltrate into your house and it will increase your ventilation.

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