Published: Oct. 28, 2020 By

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

CU Boulder Today spoke with 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 this winter season. 

Please check your state and county and city guidelines and public health orders where you are and where you are traveling to, follow other public health guidance, including wearing a mask in public, washing hands frequently, and keeping physical distance of at least 6 feet from others outside your household. Coloradans are strongly encouraged to limit gatherings and gathering sizes. On Oct. 23, the Colorado Department of Public Health and Environment (CDPHE) amended its Safer at Home public health order to limit personal gatherings in all counties at all Safer at Home levels. The revised order reduces the size of personal gatherings to no more than 10 people from no more than two separate households. 

Shelly Miller

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? 

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? 

You need to plan ahead and to quarantine 10 to 14 days on the front end, as well as when you get there. And if you can get one, get a test before you travel home, and even get another one after you get home because you can be infectious around a two-week period. 

If traveling in a car with other people, I would always wear masks and keep the windows open at least a little bit. 

Staying in motels and hotels are okay, 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. 

You flew recently for the first time since the U.S. COVID-19 outbreak began. What precautions did you take and what are the risks? 

Flying is a reasonable thing to do, provided that people are all wearing masks and the occupancy is lower than normal. I chose to fly an airline that I knew was decreasing occupancy. I also made sure I wasn't near anybody while boarding the aircraft, because socially distancing while you're boarding is important. I got up and moved when the person behind me started talking a lot—I don't want to sit by someone who's talking. I wore a K95 mask—a very well fitting, efficient mask—and I turned on all the air flows, and I stayed quiet. My backup plan was to ask to be reseated. 

The aircraft generally has good ventilation and filtration. But all the crowding in the boarding area is dangerous, and you don’t want to have to fly in a full plane, or with people without masks who are talking a lot. 

Man driving with maskOpen window in winter

Top: When driving or riding in car with other people, always wear a mask and open the windows when possible—even a little bit helps.
Bottom: 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? 

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.