Fauci warned against Thanksgiving celebrations. How can I stay safe indoors from the coronavirus during cold seasons?

Officials have been able to control COVID-19 transmission rates by implementing policies that encourage residents to eat and drink, exercise and spend time with friends and loved ones at a safe distance outside.

But health experts are concerned cases could spike again as cooler temperatures in the fall and winter force people back indoors.

The nation's leading infectious disease expert Dr. Anthony Fauci also is concerned upcoming holiday celebrations could increase transmission rates and advised Americans to skip any big Thanksgiving plans.

Speaking to “CBS Evening News" Wednesday, Fauci cautioned against “gathering together in an indoor setting” with large groups of out-of-town guests. “It is unfortunate because that’s such a sacred part of American tradition – the family gathering around Thanksgiving,” he said. “But that is a risk.”

Some experts suspect indoor transmission is what facilitated the summer surge of COVID-19 cases in southern states as residents retreated to public places with air conditioning to escape the heat. The three most populous states – California, Texas and Florida – each tallied more than 500,000 infections at the height of the surge in August, according to Johns Hopkins data.

“Indoors in public spaces is one of the places where the largest amounts of risk and transmission are likely to be happening,” said William Hanage, associate professor of epidemiology and a faculty member in the Center for Communicable Disease Dynamic at Harvard T.H. Chan School of Public Health.

What makes congregating indoors so dangerous and how can you stay safe?

'A minority of infections leads to the majority of transmission'

Dr. Lewis Nelson, professor and chair of emergency medicine at Rutgers New Jersey Medical School, said one of the main reasons there’s a higher risk of transmission indoors than outdoors is lack of ventilation.

Natural air currents outside disperse virus particles more quickly and effectively than inside. There’s minimal to no air circulation indoors, allowing virus particles to linger in the air or fall on high-touch surfaces.

“If I were to smoke a cigarette (inside), you would see the smoke particles linger,” he said. “Whereas outdoors the smoke kind of leaves.”

Additionally, indoor public places have more surfaces. As respiratory droplets or aerosol particles fall, they land on table tops, chairs, door handles and other objects people frequently touch.

“Outdoors have less surfaces,” Nelson said. “Nobody is touching the ground and then touching their eyes, nose or mouth.”

People also tend to be closer indoors because they’re confined by walls. Hanage said bars are a major source of transmission in communities because people tend to gather there for long periods of time as judgement is impaired by the consumption of alcohol.

According to the U.S. Centers for Disease Control and Prevention, adults with confirmed COVID-19 are twice as likely to have dined out at a restaurant in the 14 days before becoming sick than those who tested negative.

Positive patients also were more likely to report going to a bar or coffee shop when the analysis was limited to those without close contact with people known to have the coronavirus.

“A minority of infections leads to the majority of transmission,” he said. “Obviously if you’re at a bar then that cluster tends to be much larger as more people are gathering together.”

How to increase airflow and ventilation indoors

Experts agree increasing air flow in an indoor setting is important for reducing transmission risk as it prevents virus particles from hanging in the air for too long.

Ventilation rate is the volume of outside air provided per unit of time and air change rateis the ventilation rate of a space divided by the volume of that space, according to Shelly Miller professor of mechanical engineering at the University of Colorado Boulder.

Most air conditioning and heating systems cycle about 20% of fresh air into a building, while recirculating the remaining 80% or so for energy efficiency.

However, ventilation can be increased by opening a window, turning on a fan or even adding a portable air filter to a room. Most portable air filters can’t filter out virus particles if they don’t have HEPA filtration, but they still facilitate air circulation.

"If you can clear any airborne virus out quickly, you’ll reduce the transmission risk," Miller said.

Beyond using a simple fan or portable air filter, experts recommend not investing too much time or money into other cleaning gadgets for the home, such as UVC devices, as they can be harmful if used incorrectly.

Another good way to reduce the risk of transmission is to limit the number of people in a room, which contributes to better indoor air quality overall.

“If I drop the number of students from 35 to 17 now, the ventilation provides twice as much outside air per person and that’s awesome,” Miller said.

Building consultant company BranchPattern developed an online calculator that determines transmission risk by inputting space characteristics such as heating, ventilation, how many people are in the room and for how long. The user also can add parameters such as mask wearing and portable filters.

Getting back to the basics: Masks, social distancing and hand hygiene

Experts say the best way to stay safe indoors is through the three basic mitigation efforts: masks, social distancing and hand hygiene.

“If you take all these things together and put them into decent practice, it should hopefully slow down rates of (transmission),” Hanage said.

Masks are especially important. CDC Director Robert R. Redfield told a Senate panel in mid-September a vaccine may not be available to the American public until summer or fall 2021 and that masks are “the most important, powerful public health tool we have” – possibly even more effective than a vaccine.

Dr. Sunil Sood, infectious diseases specialist at Northwell Health’s South Shore University Hospital in Bay Shore, New York, said diners should wear masks even when they’re eating outside.

“It is tiresome … (but) you just have to do that,” he said. “The only time you should take your mask off is when you’re actually biting and chewing.”

This means keeping the mask on while chatting with other diners, waiting for food and speaking with your waiter.

Dr. Chad Asplund, professor of family medicine and orthopedics at the Mayo Clinic, said these policies also apply to the gym.

He recommends wearing a mask at all times, wiping down machines and washing your hands. He also advises against using some gym equipment, such as yoga mats and blocks.

“If you were doing intervals, it becomes harder to wear a mask,” Asplund said. “You may want to get creative with times you normally go because there are definitely times (where it gets crowded) before work and after work.”

For social distancing, Colorado's state health department developed an online tool that calculates transmission risk using the total square footage of the space and objects in the room to determine how many people can safely be there at one time.

Keep an eye on community transmission rates

Although mask-wearing, social distancing, hand hygiene and increased air flow can reduce the risk of transmission indoors, these mitigation efforts aren't 100% effective, especially if community transmission rates are high.

Barry Bloom, research professor of public health and former dean of Harvard T.H. Chan School of Public Health, recommends residents keep an eye on transmission rates in their area to determine if it’s safe to go to an indoor public setting.

“When (rates are) high, as in many parts of the states, it’s just asking for trouble,” he said.

British Prime Minister Boris Johnson unveiled a new system Monday that set out progressively stricter measures to slow the spread of the virus, three weeks after a nationwide program that banned gatherings of more than six people and required pubs and restaurants to close early.

“It makes a great deal of difference whether you’re in a low transmission environment or high transmission environment how much flexibility you have to stay safe,” Bloom said.