“The vast majority of SARS-CoV-2 transmission occurs indoors, most of it from the inhalation of airborne particles that contain the coronavirus. The best way to prevent the virus from spreading in a home or business would be to simply keep infected people away. But this is hard to do when an estimated 40% of cases are asymptomatic and asymptomatic people can still spread the coronavirus to others.
“Masks
do a decent job at keeping the virus from spreading into the
environment, but if an infected person is inside a building, inevitably some
virus will escape into the air.
“I
am a professor of mechanical engineering at
the University of Colorado Boulder. Much of my work has focused on how to
control the transmission of airborne infectious diseases indoors, and I’ve been asked
by my own university, my kids’ schools and even
the Alaska State Legislature for advice on how to make indoor spaces safe
during this pandemic.
“Once
the virus escapes into the air inside a building, you have two options: bring
in fresh air from outside or remove the virus from the air inside the building.
It’s
all about fresh, outside air
“The
safest indoor space is one that constantly has lots of outside air replacing the stale air
inside. In commercial buildings, outside
air is usually pumped in through heating, ventilating and
air-conditioning (HVAC) systems. In homes, outside air gets in through open
windows and doors, in addition to seeping in through various nooks and
crannies.
“Simply
put, the more fresh, outside air inside a building, the better. Bringing in
this air dilutes any contaminant in a building, whether a virus or a something
else, and reduces the exposure of anyone inside.
Environmental engineers like me quantify how much outside air is getting into a
building using a measure called the air
exchange rate. This number quantifies the number of times the air
inside a building gets replaced with air from outside in an hour.
“While
the exact rate depends on the number of people and size of the room, most
experts consider roughly six air changes an hour to be good for a
10-foot-by-10-foot room with three to four people in it. In a pandemic this
should be higher, with one study from 2016 suggesting that an exchange rate of
nine times per hour reduced the spread of SARS, MERS and H1N1 in
a Hong Kong hospital.
“Many
buildings in the U.S., especially schools, do not meet recommended
ventilation rates. Thankfully, it can be pretty easy to get more outside air
into a building. Keeping windows
and doors open is a good start. Putting a box fan in a window
blowing out can greatly increase air exchange too. In buildings that don’t have
operable windows, you can change the mechanical ventilation system to increase
how much air it is pumping. But in any room, the more people inside, the faster
the air should be replaced.
Using CO2 to measure air circulation
“So
how do you know if the room you’re in has enough air exchange? It’s actually a
pretty hard number to calculate. But there’s an easy-to-measure proxy that can
help. Every time you exhale, you release
CO2 into the air. Since the coronavirus is most often spread by
breathing, coughing or talking, you can use CO2 levels to see if the room is filling
up with potentially infectious exhalations. The CO2 level lets you estimate if
enough fresh outside air is getting in.
“Outdoors,
CO2 levels are just above 400 parts per million (ppm). A well ventilated room
will have around 800 ppm of CO2. Any higher than that and it is
a sign the room might need more ventilation.
“Last
year, researchers in Taiwan reported on the effect of
ventilation on a tuberculosis outbreak at Taipei University.
Many of the rooms in the school were under ventilated and had CO2 levels above
3,000 ppm. When engineers improved air circulation and got CO2 levels under 600
ppm, the
outbreak completely stopped. According to the research, the increase
in ventilation was responsible for 97% of the decrease in transmission.
“Since
the coronavirus is spread through the air, higher CO2 levels in a room likely
mean there is a higher chance of transmission if an
infected person is inside. Based on the study above, I recommend trying to keep
the CO2 levels below 600 ppm. You can buy good
CO2 meters for around $100 online; just make sure that they are
accurate to within 50 ppm.
Air cleaners
“If
you are in a room that can’t get enough outside air for dilution, consider an
air cleaner, also commonly called air purifiers. These machines remove
particles from the air, usually using a
filter made of tightly woven fibers. They can capture particles containing bacteria and viruses and
can help reduce disease transmission.
“The
U.S. Environmental Protection Agency says that air cleaners can do this for the coronavirus,
but not all air cleaners are equal. Before you go out and buy one, there are
few things to keep in mind.
“The
first thing to consider is how effective an air cleaner’s filter is. Your
best option is a cleaner that uses a high-efficiency particulate air (HEPA)
filter, as these remove more than 99.97% of
all particle sizes.
“The
second thing to consider is how powerful the cleaner is. The bigger the room –
or the more people in it – the more air needs to be cleaned. I worked with some
colleagues at Harvard to put together a tool to help teachers and schools
determine how powerful of an air cleaner you need for different
classroom sizes.
“The
last thing to consider is the validity of the claims made by the company
producing the air cleaner. The Association of Home Appliance Manufacturers
certifies air cleaners, so the AHAM Verifide seal is a good place to start.
Additionally, the California Air Resources Board has a list of air cleaners that are certified
as safe and effective, though not all of them use HEPA filters.
Keep air fresh or get outside
“Both
the World Health Organization and U.S. Centers for Disease Control and Prevention say
that poor ventilation increases the risk of transmitting the coronavirus.
“If
you are in control of your indoor environment, make sure you are getting enough
fresh air from outside circulating into the building. A CO2 monitor can help
give you a clue if there is enough ventilation, and if CO2 levels start going
up, open some windows and take a break outside. If you can’t get enough
fresh air into a room, an air cleaner might be a good idea. If you do get an
air cleaner, be aware that they don’t remove CO2, so even though the air might
be safer, CO2 levels could still be high in the room.
“If
you walk into a building and it feels hot, stuffy and crowded, chances are that
there is not enough ventilation. Turn around and leave. By paying attention to
air circulation and filtration, improving them where you can and staying away
from places where you can’t, you can add another powerful tool to your
anti-coronavirus toolkit” (The Conversation).
Shelly
Miller is a Professor of Mechanical Engineering, University
of Colorado Boulder.
Shelly Miller receives
funding from the National Science Foundation, Environmental Protection Agency,
Centers for Disease Control, National Institutes of Health, and additional
nonprofit organizations. She is affiliated with American Association of Aerosol
Research and the International Society of Indoor Air Quality and Climate.
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