Which activities have the highest and lowest risk during the COVID-19 pandemic? - USA TODAY

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The CDC updated its definition of "close contact" on Wednesday. But what does that mean for you? The agency now says that being within 6 feet of an infected person for 15 cumulative minutes over a 24-hour period is enough to count. 

Long story short: Even brief, fleeting interactions add up, and can expose you to COVID-19.

The CDC change aside, a recent study calls into question 6-foot social distancing guidelines themselves. The article, published in The BMJ, challenges historical standards of physical distancing when dealing with a respiratory viruses such as COVID-19. It claims the standards are based on outdated notions of respiratory droplet size which lack consideration of turbulent gas clouds that can carry the virus further.

In addition to questioning physical distancing standards, the study also presents a new framework of graded recommendations that better reflect the multiple factors that combine to determine relative risk of activities.

We wanted to find to what this means for you, so we spoke to one of the scientists behind the study, Lydia Bourouiba, Associate professor at the Massachusetts Institute of Technology and Director of The Fluid Dynamics of Disease Transmission Laboratory.

Single-rule recommendations are not always appropriate in all settings, according to Bourouiba. She thinks that understanding the nuance and relative risk in various settings is a better approach.

"As we reopen society, we have to be more resilient and resilience comes from adaptation. The framework we present distills extremely complex factors and research to their essence," Bourouiba says. "Various factors of risk combine to define a relative risk in one situation verus another."

The chart below, based on the study, highlights the relative risk of activities based on factors such as mask wearing, setting, occupancy, room ventilation, time spent, and speaking types from silent to shouting.

Based on the factors above, we picked a sample of real-life activities and asked Professor Bourouiba to rate the risk associated with them. Please remember, there are many ways to make activities more or less dangerous. Here are a few common scenarios:

Best-case scenario: Outdoors, low occupancy

• Low occupancy
• Wearing face coverings for prolonged time
• Physically distanced seating, particularly if unmasked eating/drinking is involved 
• Outdoors and well-ventilated
• Speaking, shouting, and singing

Risk: Low

"A lot of air circulation in an outdoor space along with wearing masks and keeping one's distance is required for an event with shouting and singing to remain low risk. Distancing should increase with the degree of violence of exhalation in order of breathing, talking, shouting, and singing," Bourouiba says. 

Risky scenario: Indoor wedding

• High occupancy
• Wearing face coverings for prolonged time
• Indoors and well-ventilated
• Talking, shouting, and singing

Risk: High

An indoor venue with more people quickly increases the risk associated with this activity. Even if people wear masks and the room is well-ventilated, the prolonged time makes this troublesome. Professor Bourouiba adds, "there have been reports of indoor wedding events under such risk conditions that ended up becoming superspreading events, including some that resulted in COVID-19 related fatalities." 

Best-case scenario: Off hours

• Low occupancy
• Physically distanced seating
• Wearing face coverings for prolonged time
• Indoors and well-ventilated
• Silent

Risk: Low

If you can go off-hours and avoid the crowd, the risk decreases. If all people maintain social distance and pray silently while wearing masks and the indoor space is well-ventilated, consider this the safest option. 

Risky scenario: Peak time/crowded

• High occupancy
• No face coverings for prolonged time
• Indoors and poorly ventilated
• Speaking, shouting, and singing

Risk: High

Large crowds gathering, speaking and singing produce more airborne droplets and aerosols that travel greater distance. Even if the room has improved ventilation, the chance of spreading the virus increases if people don't wear masks.

Best-case scenario: Outdoor tennis

• Low occupancy
• No face coverings for prolonged time
• Outdoors and well-ventilated
• Speaking, shouting and heavy breathing

Risk: Medium

When playing singles tennis you generally keep a safe distance from your opponent at all times. The fact that you are outdoors reduces the risk. "Face coverings should still be advised as much as possible and overlap of breathing zones from heavy exhalations when getting close to the net should be avoided," Bourouiba says. 

Risky scenario: Indoor basketball

• Low to high occupancy
• No face coverings for prolonged time
• Indoors and well-ventilated
• Speaking, shouting, and heavy breathing

Risk: High

"Heavy breathing increases the range of the exhaled cloud and the typically close person-to-person coverage means that players tend to be in the direct breathing zone of each other," Bourouiba says. 

Risk: Medium to High depending on precautions taken by the dental office.

Going to the dentist poses a risk because the office is indoors. You'll have to remove you mask and the simple act of cleaning your teeth or performing dental procedures increases the chance for respiratory droplet and aerosol emission.

While this story is intended to help readers with activities and their associated risk, this activity is especially dangerous for the dentist and the hygienist.

Professor Bourouiba adds, "This is an example of an aerosolizing medical procedure. Other such procedures that put healthcare workers and patients at increased risk are intubations and certain types of respiratory support measures. Special precautions such as high-grade medical respirators and extended decontamination protocols of both air and surfaces have to be followed to reduce the risk." 

Risk: Low to Medium depending on measures put in place by the medical office.

For the majority of ailments, you can leave your mask on or remove it for only a short period of time. Many practices are reducing the number of patients that can be in the waiting room and are cleaning surfaces regularly. The Mayo Clinic has a checklist you can follow if you cannot do a telemedicine or video appointment.

"The risk remains low if the air ventilation is tailored to the medical procedures performed and the number of individuals in the occupied space, in addition to strict protocols of decontamination of surfaces and air, and patient triage to not mix potentially infectious patients with others," Bourouiba says. 

Best-case scenario: Low occupancy theater

• Low occupancy
• Face coverings removed during snacking
• Prolonged time
• Indoors and well ventilated
• Silent

Risk: Medium

Lowering occupancy and spacing people out helps to limit risk. However, prolonged time and potential mask removal count against this activity. "Adequate ventilation, low occupancy, distancing, surface decontamination masking, absence of snacking/removal of mask, and silence are key to keeping the risk to medium in this setting," according to Bourouiba.

Risky scenario: Theater-going without social distancing

• High occupancy
• Face coverings removed during snacking
• Prolonged time
• Indoors and well ventilated
• Silent

Risk: High

Assuming high occupancy and mixed use of masks, there is high risk associated with going to the movies. Being silent and improving ventilation helps limit risk, but because you are indoors over a prolonged time, this activity should be avoided, Bourouiba says. 

Best-case risk scenario: Drive-in movie

• Low occupancy
• No face coverings for prolonged time
• Outdoors and well-ventilated
• Silent

Risk: Low

This assumes you are going to the drive-in with people who you live with, such as your family. It also assumes you remain in your car and wear a mask whenever you leave your vehicle. Professor Bourouiba says, "distancing between cars, especially if no masks are being used in the cars and the windows might be open, is critical to maintain breathing zones apart. This also assumes that no shared facilities are being used, for example public bathrooms." 

Best-case scenario: Gym with few people in it

• Low occupancy
• Use of face coverings for prolonged time
• Indoors and well-ventilated
• Speaking, heavy breathing

Risk: Medium to High

Mask regulations vary by state, so check your local gym's website for rules. "In this case, masks should be used at all times, else it is high risk. It is also important that the gym members/staff decontaminate the equipment after each use and that special disinfection protocols and sufficient physical distancing is practiced at all times to keep breathing zones apart," Bourouiba says. 

Risky scenario: Full gym with people close together

• High occupancy
• Mixed use of face coverings for prolonged time
• Indoors and well-ventilated
• Speaking, heavy breathing

Risk: High

Higher occupancy increases the chance you will be near someone who is breathing heavily. If your gym doesn't require mask use while exercising, the threat of transmission increases.

Best-case scenario: Dining outdoors

• Low occupancy
• No face coverings for prolonged time
• Outdoors and well-ventilated
• Speaking

Risk: Medium

Being outdoors certainly does help to limit the risk. However, mask-removal and prolonged time spent offset the benefits of the setting.

Risky scenario: Dining indoors

• High occupancy
• No face coverings for prolonged time
• Indoors and well-ventilated
• Speaking

Risk: High

Poor ventilation and high occupancy can put diners in risk. At a restaurant in China, ten people in three families were infected in one hour without direct physical contact. The risk is reduced in a well-ventilated room, however, "eating involves uncovered interactions, so adequate separation of breathing zones is critical to maintain the risk to be medium." Adequate table separation and management of air-flow patterns is important. No table should be down-wind/vent from another, Bourouiba adds. 

Risky scenario: Crowded supermarket

• High occupancy
• Wearing face coverings for prolonged time
• Indoors and well-ventilated
• Silent, speaking

Risk: Medium

It's better to go to the store alone, maintain distance from other shoppers, and keep your mask on at all times. Many national grocery chains, like Walmart, Target, and Aldi, require customers to wear masks inside their stores. "It is critical that distancing, masking, and venting are used in combination to maintain this at a medium rather than high risk. It would always be best to ensure low occupancy instead of high in such settings, to remain at lowest risk level, and tailor the air change levels per occupant in addition to managing local airflow patterns," Bourouiba says. 

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