< Masks

Do I have to wear a mask inside / outside?

Text updated on 2020-09-03

To protect others and protect yourself, the mask is strongly recommended for indoor use but not outdoor use.

Indoors, indoors: essential. Arguments:

  1. In the room where a sick person lives, there are viruses in the indoor air.
  2. It's hard to stay more than 2 meters away from each other to talk. But the mask blocks at least the large droplets, and breaks the flow of air coming out of the mouth.
  3. Many cases of contamination have been described in closed places: workrooms, restaurants, public transportation, etc. In some cases, the infected persons were more than 2 meters away from the contagious person.

It, therefore, is important to wear a mask indoors, whether in a shop, train, bus, open-space office, bathrooms, or any other room shared with other people.

Outside, outside: It depends... Arguments:

  1. Viruses are not found in outdoor air except in those rare crowded places.
  2. To date, no cases of open-air cluster contamination have been described. Nevertheless, it should be noted that the first spread of the epidemic occurred during winter, when people naturally spent more time in enclosed spaces than outdoors, and that subsequently many countries introduced confinement, which increased the time spent indoors.

It, thus, may be unnecessary to wear a mask in "green" areas (few cases), when walking in the open air in a less  frequented place, if you are sure that you will not to start a discussion with someone, if you avoid close encounters, and if you stay more than two meters away to greet each other.

On the other hand, in "red" areas (many cases) and a very busy place, or if you plan to visit a public space (building corridor or staircase, supermarket, etc.), it is recommended to put on a mask before leaving home.

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In Singapore (NCID Hospital), in the room of a non-intubated, coughing patient who is a strong carrier of the SARS-CoV-2 virus, no virus was found in the room air, the antechamber, or on the mask and gown of the caregivers, and no viral RNA was found after cleaning. Only a few RNAs were detected on surfaces (especially on one spot on the floor, near the bed).

Ong, S. W. X., Tan, Y. K., Chia, P. Y., Lee, T. H., Ng, O. T., Wong, M. S. Y., & Marimuthu, K. (2020). Air, surface environmental, and personal protective equipment contamination by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from a symptomatic patient. Jama, 323(16), 1610-1612.

In Wuhan, in Renmin, a well-equipped permanent hospital, no contamination was detected in the air, but only on surfaces of the intensive care units. In Fangcang, the Wuhan sports center converted into a temporary hospital that was neither partitioned nor ventilated, viruses were found everywhere (air and surfaces), including in the restrooms of the medical staff. The most contaminated air was observed in the bathrooms, narrow and unventilated rooms. Disinfection and ventilation and a decrease in the number of sick people (200 to 100) eliminated these Fangcang contaminations. Finally, in 11 public places in Wuhan, no virus was found in the air, except in 2 overcrowded places (these were in the crowded entrances of a hospital and department store).

Liu, Y., Ning, Z., Chen, Y., Guo, M., Liu, Y., Gali, N. K., ... & Liu, X. (2020). Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature, 1-6.

In Omaha (UNCM Hospital, USA), viruses were found in the air in the rooms and in the nearby hallway, as well as on almost all surfaces of the rooms, suggesting an aerosol. But the number of viruses was small, except on a ventilation grill. In the air in the ICU patient rooms, 20-50 viral RNAs were found per litre of air. In the air in the corridor next door, the virus was found in 2/3 of the samples, each litre of air containing 2-8 viral RNAs (on average 2.6/L, so 1.3 RNAs were breathed in for every 0.5L of air breathed). Note that none of these samples were able to infect cells in culture, and no (masked) caregiver at this hospital was infected.

Santarpia, J. L., Rivera, D. N., Herrera, V., Morwitzer, M. J., Creager, H., Santarpia, G. W., ... & Lawler, J. V. (2020). Potential transmission of SARS-CoV-2 in viral shedding observed at the University of Nebraska Medical Center. MedRxIV.

The analysis of 318 cases in which one person contaminated at least two other people in China (for a total of 1,245 people affected by COVID-19) reveals that the majority of the contaminations took place in a closed space, mainly in the home and during transport, but also in restaurants, and that only one contamination took place following an open-air discussion with a person returning from Wuhan.

Qian, H., Miao, T., Li, L. I. U., Zheng, X., Luo, D., & Li, Y. (2020). Indoor transmission of SARS-CoV-2. medRxiv.

In Japan, many cases of "cluster" contamination have occurred in closed places: a sports hall, restaurant boat, hospital, festival where there were tents with minimal ventilation for eating.

Nishiura, H., Oshitani, H., Kobayashi, T., Saito, T., Sunagawa, T., Matsui, T., ... & Suzuki, M. (2020). Closed environments facilitate secondary transmission of coronavirus disease 2019 (COVID-19). medRxiv.

Compilation of 135 cases of COVID-19 contamination: analysis of the table provided by the authors indicates that more than 95% of the cases are indoor transmissions. For the remaining 5%, it is not clear whether transmission occurred outdoors. For example, contamination on construction sites may have occurred in construction huts.

Leclerc, Q. J., Fuller, N. M., Knight, L. E., Funk, S., Knight, G. M., & CMMID COVID-19 Working Group. (2020). What settings have been linked to SARS-CoV-2 transmission clusters? Wellcome Open Research, 5(83), 83.

60 members of a choir gathered in a room to sing for 2.5 hours on March 10, 2020 in Washington state, USA. They kept a safe distance, used hydro-alcoholic solutions, but did not put on masks. Three weeks later, 45 of them tested positive for COVID-19.

Los Angeles Times article of March 29, 2020.

Scientific article analyzing the cases of contamination during the choir rehearsal of March 10, 2020.

Hamner, L., Dubbel, P., Capron, I., et al (2020). High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice - Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep 2020;69:606-610.

Aerosols, micro-droplets of less than 5 µm that remain in the air for a long time, are produced mainly during invasive procedures in the intensive care unit (intubation, ventilation, bronchoscopy).

Odor, P. M., Neun, M., Bampoe, S., Clark, S., Heaton, D., Hoogenboom, E. M., ... & Kamming, D. (2020). Anaesthesia and COVID-19: infection control. British Journal of Anaesthesia.

The mask blocks large droplets and breaks the airflow out of the mouth.

Leung, N.H.L., Chu, D.K.W., Shiu, E.Y.C. et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat Med 26, 676-680 (2020). https://doi.org/10.1038/s41591-020-0843-2

Study of two groups participating in an event, each group being in a different bus for one and a half hours with a single asymptomatic COVID case in one of the buses. The probability of being infected was 11.4 times higher in the bus with the infected person, regardless of the distance to the infected person, demonstrating aerosol contamination in an enclosed space for an extended period of time.

Shen Y, Li C, Dong H, et al (2020) Community Outbreak Investigation of SARS-CoV-2 Transmission Among Bus Riders in Eastern China. JAMA Intern Med. Published online September 01, 2020. doi:10.1001/jamainternmed.2020.5225

The SARS-CoV-2 coronavirus is stable in aerosols for the duration of handling (3 hours). Reduction from 103.5 to 102.7 TCID50 per litre of air (50% tissue-culture infectious dose).

Van Doremalen et al, N Engl J Med 2020; 382:1564-1567. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1.

Further reading

Do I have to wear a mask if I don't have any symptoms?

Where and when should I put on a mask?

Do I have to wear a mask if I already had COVID-19?

Can a mask be reused?

Why is it recommended not to wear a surgical mask for more than 4 hours?

Is there any point in wearing a transparent face shield?