COVID-19 Indoor Safety Guideline
Martin Z. Bazant
aimed at the general public (in Basic Mode) and
translated into many languages, now with safe CO2 levels (in Advanced Mode).
Spreadsheet, updated with occupancy, time and CO2 calculations.
First posted September 4, 2020
A growing chorus of scientists is
sounding the alarm that COVID-19 is
mainly spreading in homes or
other enclosed spaces
whenever people spend extended periods breathing tiny aerosol droplets
suspended in air
infected by the virus.
Public health advice has been
slow to catch up with the rapidly advancing science,
and official guidelines still only set a
minimum social distance (6 feet in the U.S.)
or maximum occupancy (25 persons in Massachusetts)
for indoor spaces.
Although the need for
to control indoor air quality (IAQ) has been emphasized,
no quantitative guideline has been proposed, specific to COVID-19.
To protect against airborne transmission,
it is common sense
that the exposure time, room size, ventilation and human activity must also be considered:
- Standing 6 feet apart is safe for a few seconds, but maybe not for a few hours;
- 25 people are safer in a large gymnasium than in a crowded bar;
- 6-foot separation is safer in a ventilated hospital than inside a sealed tent;
- At any distance, remaining quiet and calm is safer than singing or exercising;
- Social distance can be safely reduced if facemasks are worn.
Using mathematical models from chemical engineering and
epidemiology, I have derived a safety guideline for well-mixed indoor spaces,
in collaboration with John Bush,
which combines all the key variables above in a bound on "cumulative exposure time". The guideline
is intuitive and quantitative,
calibrated against the latest data
for COVID-19 indoor spreading
and respiratory aerosol emissions,
and easy to apply using a spreadsheet or app (linked above).
Airborne transmission risk, as quantified by the guideline,
is always present indoors and critical to consider when devising policies, such as:
The guideline thus provides specific recommendations on how to limit COVID-19 transmission through well-mixed indoor air,
but one should also consider various caveats emphasized in the
including the possibility of short-range aerosol transmission in
Such effects, which can lead to fluctuations in droplet concentrations around their mean values,
are only partially
addressed by choosing a sufficiently small tolerance in the well-mixed guideline and will depend on the
details of airflow and human behavior in a specific indoor space. Especially when masks are worn, however,
rising thermal plumes around each person
disrupt short-range transmission and promote mixing of indoor air.
- Contact tracing.
Compared to the existing definition of a "close contact"
individual within 6 feet of an infected person for at least 15 minutes), the guideline predicts when
all persons in a room should be considered close contacts for testing or quarantine.
Official guidance emphasizes
the isolation of infected persons, but it is also important to separate infected indoor air. Whenever this is not possible, as in most cases of home quarantine with healthy family members,
the guideline provides specific recommendations for ventilation, filtration, and/or facemask use
given typical exposure times and room characteristics.
- Re-opening schools and businesses.
Running the numbers suggests that many
classrooms and stores could safely accomodate normal occupancy by controling ventilation and requiring
facemask use, while recommending - but not strictly enforcing -
social distancing beyond that of natural human behavior (around 1m or 3 feet), especially in light of
other negative impacts on children.
- Home safety. The same measures also apply to homes, where people spend the most time together indoors,
and thus are most
likely to transmit the virus. Especially
among the elderly in nursing homes and those with
pre-existing medical conditions,
which account for the majority of deaths in the U.S. and worldwide from COVID-19,
more aggressive control of
IAQ and cumulative exposure time could save more lives than any measures
imposed on less vulnerable populations.
A method to assess Covid-19 transmission risks in indoor settings, MIT News, April 15, 2021.
Monitoring CO2 to assess risk of indoor airborne SARS-CoV-2 transmission, Medical News, April 13, 2021.
MIT App Helps Businesses Quantify COVID-19 Risk, Safety, IHRSA, Jan. 22, 2021.
MIT Professors Launch Website to Estimate Risks of Contracting COVID Indoors,
Newsweek, Dec. 2, 2020.
One person in the room with you has COVID-19. Here's how long it takes to get infected,
Fast Company, Nov. 30, 2020.
Aerodynamics of Infectious Disease: Airflow Studies Reveal Strategies to Reduce Indoor Transmission of COVID-19, SciTechDaily, Nov. 22, 2020.
Covid-19's wintry mix: As we move indoors, dry air will help the coronavirus spread, STAT News (Boston Globe), October 21, 2020.
How to Keep the Coronavirus at Bay Indoors,
New York Times, September 27, 2020.
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