When the CDC loosened mask guidance for outdoor activities this week, it did so based on high vaccine effectiveness and on evidence that transmission of SARS-CoV-2 is far less frequent in the open air.
Vaccinated people can now safely gather, mask-less, with friends and family outside, and even unvaccinated individuals can take off their mask while walking, running, or biking with their household, or at small gatherings of fully vaccinated people.
But CDC still warned that vaccinated people should keep their masks on in crowded outdoor settings, such as a parade or concert, and experts caution there are other scenarios where masking outdoors may still be a good idea -- because outdoor transmission isn't impossible.
Published Evidence
Studies have shown that people are far less likely to catch COVID-19 outdoors, but experts say that not all outdoor activities carry the same risk. Factors like crowd density, air flow, and weather patterns can all determine how the virus spreads -- and can be used as metrics to inform people about which precautions they should take in outdoor settings.
Early research established that spending time outside reduces risk of COVID-19 infection. A systematic review published in February in the Journal of Infectious Diseases found that less than 10% of all COVID-19 cases were transmitted in outdoor settings, and that the risk of transmission indoors was almost 20 times higher than it was outside, albeit with a wide confidence interval (OR 18.7, 95% CI 6.0-57.9).
"I think what we did was really confirm what the CDC was saying -- that the risk is lower outside than it is inside," said Nooshin Razani, MD, MPH, of the University of California San Francisco, who authored the review.
Razani added that in some of the studies included in the review, outdoor transmission was probably even lower than 10%. Some of the infections occurred at construction sites or summer camps, which could have been both indoors and outdoors.
Additional data from the Health Protection Surveillance Centre in Ireland showed that just 262 of over 200,000 infections occurred outside -- just 0.1% of all cases.
Linsey Marr, PhD, an expert in airborne viral transmission at Virginia Tech, said in a statement to MedPage Today that "changes in guidelines for outdoor masking are welcome because we know that transmission outdoors is much less likely than indoors."
"The virus spreads most commonly by breathing it in from the air, usually coming from people nearby or sometimes farther away if you are in a poorly ventilated room," Marr stated. "Outdoors, the virus cannot accumulate in the air; it quickly becomes diluted in the atmosphere, like a drop of dye in the ocean."
Marr said that it is reasonable to end outdoor mask mandates for fully vaccinated people, but certain situations may still call for it, such as close, face-to-face conversation or crowded places like a baseball game or long line, where it's hard to keep enough physical distance.
And while these loosened guidelines are welcome, Marr added that the different stipulations are confusing based on whether or not someone is vaccinated. For example, the CDC states that fully vaccinated people can eat at an outdoor restaurant with friends from multiple households. But Marr said all of the people sitting at the table should be vaccinated too.
Transmission Dynamics
Being infected outdoors, particularly for those who aren't vaccinated, isn't impossible, as transmission strongly depends on the amount and the direction of airflow, experts told MedPage Today.
Lydia Bourouiba, PhD, director of The Fluid Dynamics of Disease Transmission Laboratory at MIT, said viral particles travel along airflow patterns and can accumulate in spaces over time.
"The emissions from various respiratory exhalations -- such as coughs, sneezes, or talking -- are concentrated in a gas turbulent cloud that carries forward the droplets of all sizes within it," Bourouiba told MedPage Today.
When a group of people gather indoors (say, in a space with confined walls and not a lot of ventilation) these exhalation clouds are emitted with concentrated viral particles. The clouds can move through a room across different distances, depending on the type of exhalation and if individuals are wearing masks or not. As the clouds move forward and decrease in speed, the particles disperse more slowly, lingering and accumulating in specific areas.
But if that group gathered outdoors, in an open park, for example, that accumulation is negligible, Bourouiba said.
Kiran Bhaganagar, PhD, of the University of Texas in San Antonio, said that there are four factors to analyze when looking at how outdoor conditions affect transmission -- wind speed, local temperature, air moisture levels, and turbulence (that's how much the wind speed fluctuates).
Last fall, Bhaganagar and colleagues used meteorological models to determine how weather conditions affected SARS-CoV-2 transmission. They found that a combination of moderate air temperatures, low wind speed, and weak turbulence all increased the amount of time the virus can be airborne before it is dispersed.
"There is a lot of confusion in the community, stating that if we go outside, there is no way that the virus can affect us," Bhaganagar said in an interview. "It depends a lot on the local conditions."
Bourouiba said that wind is part of the reason that the outdoors is not as dangerous, in terms of viral particle accumulation, since strong wind dilutes and can more rapidly disperse the viral particles.
Still, if a wind current is blowing these particles directly into someone's face downstream, outdoors may not be safe either. Masks are necessary when multiple people are in proximity, outdoors or in well ventilated spaces, to reduce the range of the exhalation concentrated cloud, Bourouiba said.
While it's clear that outdoors provides better protection against infection, Bourouiba added that until a greater percentage of the population is vaccinated, keeping up with safety restrictions around others is critical.
"It's not the time to lower our guard," she said.