The following was written by Public Health Insider, the blog for the Seattle-King County Health Department:

As the season changes, cooler temperatures mean more time indoors. The risk from COVID-19 increases with indoor gatherings compared to outdoors, but there are ways to reduce the spread and stay healthy. We talked to Health Officer Dr. Jeff Duchin about COVID-19 risks indoors and what to do about it.

Dr. Duchin, you’ve said “outdoors is safer than indoors.” Why is that?

Socializing outdoors is less risky than indoors for two main reasons: First, natural outdoor airflow rapidly dilutes viruses floating in the air. Second, sunlight kills viruses. But, as we head into fall and winter, we spend less time outdoors and more time indoors, where the risk of COVID-19 spread is higher.

Why does being indoors present a higher risk?

Closed windows and doors decrease fresh airflow which can increase risk, especially when you have more people inside. Dryer, less humid air from heating may also increase the risk.

It’s important to remember that COVID-19 often spreads from people before they develop symptoms or recognize that they are ill – people who look and feel well. And although COVID-19 spreads mainly through close contact with an infected person, studies have shown that COVID-19 can at times spread further than 6 feet through the air.

Although these situations have been relatively uncommon, spread can be a problem where COVID-19 can build up in the air, such as in crowded, enclosed settings, with longer time spent indoors, when ventilation is limited, when people are not wearing masks, and, when doing activities that involve speaking loudly, singing or exercising (where we exhale more virus-containing particles into the air).

Realistically though, when it gets colder we are going to spend more time inside. What can we all do to reduce our risk indoors?

Cloth face masks should be worn at all times in public including indoors when others are present regardless of the distance between people. You do not need to wear a mask indoors at home with your household members.

What makes an effective mask?

Cloth masks should be well made and fit well. The mask should have at least 2 layers of fabric and a snug fit with no gaps around the face. Air should go through, not around, the mask when you breathe. You can tell the mask fits well when it moves in and out a bit with each breath. The Centers for Disease Control and Prevention has more information about masks.

What else can we do when indoors with people from outside the household?

Reduce the number of people you’re around as much as possible and the time spent around others indoors. Stay as far apart as possible, even with masks on. Remember, the guidance is not “mask up OR stay 6 feet or more of distance.” Rather, the safer thing to do is to wear a mask AND stay 6 feet or more apart from others.

You’ve said airflow is important. How can we increase airflow indoors?

Do what you can to improve ventilation in indoor spaces, including opening windows when possible. More fresh air means lower risk. If possible, adjust the ventilation system to increase the intake of outdoor air. Consider consulting with a building or HVAC engineer or home heating expert for advice. They can potentially improve air exchange in the building allowing outside air to come in or adjust filtration to better clean the air. Portable air purifiers can also be useful in some settings and special germicidal UV lights can be considered in certain circumstances, please consult an expert for these options. See the links below for more information from CDC and EPA.

It sounds like we need to take several actions to reduce the risk of COVID-19 indoors, right?

Exactly – we can’t depend on any one measure such as masks, distancing or ventilation alone. Instead, we need to use a combination of strategies together to most effectively reduce the risk from COVID-19. These steps include wearing a mask that fits well, staying at least 6 feet away from others, improving ventilation, avoiding crowded places, limiting interactions with others outside the home, and, practicing good hygiene and environmental cleaning.

Are the recommendations for workers different?

They can be, depending on the type of work and work setting. Cloth masks should generally be worn in most workplaces but workers should follow Washington Labor & Industries guidance for personal protective equipment. When outdoors, masks should be worn if 6 feet of distance from others cannot be maintained unless a higher level of protection is required.

Employers and business owners should also review CDC guidance for workplaces and buildings.

Where can we go for more information about lowering risk indoors?

You can find more information on indoor air and reducing COVID-19 risk at the EPA Indoor Air and Coronavirus website.