Many gyms and health clubs seem to be filling up again with
people eager to return to their old routines and communities or get in shape
for summer — at the same time that new omicron variants are pushing
COVID
infections up. So how safe is it to go back to the gym?
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Put another way, how many microscopic aerosol particles are the
other cyclists in your spin class breathing out into the room? How many is the
runner on the nearby treadmill spewing forth? A small study about respiration
and exercise published Monday in the Proceedings of the National Academy of
Sciences provides some rather startling answers.
The study looked at the number of aerosol particles 16 people
exhaled at rest and during workouts. These tiny bits of airborne matter —
measuring barely a few hundred micrometers in diameter, or about the width of a
strand of hair, and suspended in mist from our lungs — can transmit the
coronavirus if someone is infected, ferrying the virus lightly through the air
from one pair of lungs to another.
The study found that, at rest, the men and women breathed out
about 500 particles per minute. But when they exercised, that total soared
132-fold, topping out above 76,000 ppm, on average, during the most strenuous
exertion.
These findings help explain why several notable COVID
superspreader events since 2020 have occurred at indoor gym classes. They also
could renew some people’s concerns about indoor gym programs as COVID-19 cases
increase again in much of the nation and raise questions about how to best
reduce risks of exposure when we work out.
In general, packing hard-breathing bodies into enclosed spaces
is a bad way to avoid transmission of COVID-19 or other respiratory diseases.
In 2020, 54 South Koreans developed COVID after Zumba classes with infected
instructors and then passed it to family members and acquaintances. Later that
year, all 10 members of a spin class in Hawaii taught by an infected instructor
tested positive afterward, as did another 11 who came into close contact with
one of the class members, a personal trainer and kickboxing instructor.
Scientists investigating these and similar outbreaks speculated
that inadequate ventilation and high respiration rates among the exercisers
contributed to the wildfirelike spread of COVID at the affected gyms. But the
scientists could only guess about the extent to which exercise had increased
the levels of aerosol particles in the gym areas. Accurately measuring the rise
in floating particles during exercise is difficult.
So for the new study, a group of exercise scientists and fluid
dynamics researchers in Germany devised a novel way to measure aerosol
emission, using a single stationary bicycle and rider inside an airtight tent.
The cyclists wore silicone masks that captured their exhaled breaths, sending
the air through tubes to a machine that counted each particle as it passed.
The researchers first measured people’s particle production as
they sat still and then as they rode at an increasingly punishing pace until
they were too exhausted to continue. Particles were counted constantly.
The scientists expected the exercisers’ aerosol output to grow
as intensity ramped up. We all breathe more deeply and swiftly as we work out
harder. But the extent of the increase “surprised us,” said Henning Wackerhage,
a professor of exercise biology at the Technical University of Munich and a
senior author of the new study.
The rise in aerosol emissions began moderately as riders warmed
up and started pedaling harder. But as they reached a threshold at which their
exercise became noticeably more strenuous — about when a jog becomes a run or a
spin class switches into intervals — the rise in emissions became exponential.
The riders started huffing out about 10 times as much air per minute as at
rest, while the numbers of particles per minute soared more than 100-fold as
riders approached exhaustion (with considerable variation from person to
person).
In a room filled with spin-bike riders, treadmill runners or
boot campers, “the aerosol particle concentration would increase a lot,” said
Benedikt Mutsch, a graduate student at the Institute for Fluid Mechanics and
Aerodynamics at the University of the German Armed Forces in Munich and study
co-author. The more particles, the more possibility of COVID-19 infection if
any exercisers are infected.
“The study provides mechanistic data to back up the assumption
that exercising indoors is a higher-risk activity when it comes to transmission
of COVID-19” than taking your exercise outside, said Linsey Marr, a professor
of civil and environmental engineering at Virginia Tech and expert on airborne
transmission of viruses.
But these risks can be mitigated. “Good ventilation and air
exchange are a great way to reduce transmission risk,” said Chris Cappa, a
professor of civil and environmental engineering at the University of
California, Davis, and expert in airflow dynamics.
Also, stay well away from other exercisers. “Social distancing
of 6 feet or more is always important,” Mutsch said. But it may not be enough
during strenuous, indoor exercise classes. The new study did not track where
cyclists’ aerosol particles flowed, but it is likely they stream well beyond 6
feet, he said. So keep at least 8-10 feet apart during strenuous workouts,
which requires large rooms and small classes.
The classes themselves should likewise be well spaced. “If there
are back-to-back exercise classes, some of the air from that first class will
carry over to the second,” Cappa said. Be sure there are breaks of at least 15
and preferably 30 minutes between sessions to allow the air to clear.
Mask up as well. “Respiratory face masks reduce aerosol
emissions,” Wackerhage said.
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