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COVID-19 Indoor Safety Guideline

Martin Z. Bazant

Online App

Basic Mode:
- Estimates the "safe" time in an indoor space after an infected person enters (given occupancy and risk tolerance)
- User-friendly menus for room specfications and human behavior (including masks)
- Translated into 15 languages using British and metric units
- Updated as new variants emerge (including Delta and Omicron)
Advanced Mode:
- Accounts for infection prevalence and immunity
- Estimates personal risk
- Computes safe time at given CO₂ level (if monitored)
- Enables control of all parameters
- Exports to and imports from Excel workbooks

Publications

A guideline to limit indoor airborne transmission of COVID-19, M. Z. Bazant and J. W. M. Bush, PNAS 118 (17), e2018995118 (2021). (MIT Press Release, Authors' Statement)
Monitoring carbon dioxide to quantify the risk of indoor airborne transmission of COVID-19, M. Z. Bazant, O. Kodio, A. E. Cohen, K. Khan, Z. Gu and J. W. M. Bush, Flow 1, E10 (2021). (Press Release)
Quantifying the tradeoff between energy consumption and the risk of airborne disease transmission for building HVAC systems, M. J. Risbeck, M. Z. Bazant, Z. Jiang, Y. M. Lee, K. H. Drees and J. D. Douglas, Science and Technology for the Built Environment, in press (2021). (medRxiv preprint)
Modeling and multiobjective optimization of indoor airborne disease transmission risk and associated energy consumption for building HVAC systems, M. J. Risbeck, M. Z. Bazant, Z. Jiang, Y. M. Lee, K. H. Drees and J. D. Douglas, Energy and Buildings 253, 111497 (2021). (medRxiv preprint)
Assessment of airborne disease transmission risk and energy impact of HVAC mitigation strategies, M. J. Risbeck, M. Z. Bazant, Z. Jiang, Y. M. Lee, K. H. Drees and J. D. Douglas, ASHRAE Journal (2021).

Teaching

Courses:
- 10.S95x Physics of COVID-19 Transmission, a free, self-paced massive, open, online course (MOOC), launched on edX in November 2020, returnedon MITx Online in November 2022.
- 10.S95 Physics of Covid-19 Transmission, MIT OpenCourseWare lecture videos.
Seminars:
- COVID-19 Symposium, Columbia School of Public Health (June 16, 2021) MIT PACT Seminar (May 14, 2021)
- School re-opening discussion, Q&A: 3 feet vs. 6 feet (Feb. 5, 2021, two weeks before updated CDC guidance)
- Mechanical Engineering Seminar, U. Illinois (Jan. 12, 2021)
Interviews:
- Clearing the air (The Economist, Babbage Podcast, June 2, 2021)
- How long can you be in a room with someone infected with COVID-19? (ABC News, Dec. 11, 2020)
- Modeling how COVID spreads (Startup to Storefront Podcast, Dec. 7, 2020)
- Free scientific debate (Cambridge, Oct. 20, 2020)

Press

Going Out and Worried About Covid Safety? There's a Calculator for That., NYT, Dec. 21., 2021.
Johnson Controls develops industry-first AI-driven digital solution ot manage clean air, energy, sustainability, comfort and cost in buildings, PRNewswire, Oct. 25, 2021.
Carbon dioxide monitors could track indoor COVID-19 risk in near real-time say researchers , Eureka Alert, Oct. 4, 2021.
Social distancing indoors may provide false sense of security, covid transmission study finds, Washington Post, April 27, 2021.
MIT study challenges indoor social distancing, highlights 'inadequacy' of 6-foot rule, Fox News, April 25, 2021.
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 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.
How to Keep the Coronavirus at Bay Indoors, New York Times, September 27, 2020.

Background

Originally 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 building engineering 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:

Contact tracing. Compared to the existing definition of a "close contact" (any 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.
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.

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 paper and other literature, including the possibility of short-range aerosol transmission in respiratory jets. 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.