Why Masks Matter
A comprehensive and scholarly summary of evidence on masks and mask effectiveness, published May 2024, by Greenhalgh and colleagues, covers evidence from aerosol science through biology, observational studies, trials, and social science. Each discipline has its own methods and makes its own contribution to a body of work that shows that masks work. Respirators work better. Wear the best mask available.
Masks and respirators for prevention of respiratory infections: a state of the science review
Authors: Trisha Greenhalgh; C Raina MacIntyre; Michael G Baker; Shovon Bhattacharjee; Abrar A Chughtai; David Fisman; Mohana Kunasekaran; Amanda Kvalsvig; Deborah Lupton; Matt Oliver; Essage Tawfiq; Mark Ungrin and Joe Vipond.
https://journals.asm.org/doi/10.1128/cmr.00124-23
Plain language piece by the same authors:
https://theconversation.com/masks-work-our-comprehensive-review-has-found-229658
Responding to a pandemic requires attention to multiple interventions. Masks, including better masks, ventilation, vaccination, distancing, rapid testing, PCR testing and contact tracing are each important.
Wearing a mask
Multiple studies consistently showed decreases in community transmission when mask mandates were enacted: many of the masks used at that time would have been cloth masks. We encourage people to wear the best mask available, ideally a respirator (N95). However, cloth masks also reduce contamination of the environment and reduce aerosol particles reaching the wearer.
Studies
Here are key references on the effectiveness of masks:
Abaluck RCT Bangladesh (working paper)
Danmask RCT (did not identify a difference)
Expert reaction to Danmask RCT (explains why Danmask did not identify a difference)
Leffler Between-country comparisons
Karaivanov Canada mask mandates
Talic 2022 meta-analysis, public health measures
Czypionka 2021 multi-faceted review
Andrejko 2022 CDC study, California; masks protect the wearer:
Cowger 2022 Mask mandates in schools
Level of exposure
How many people? The more, the higher the risk.
Will the other people be wearing masks?
How close will other people be?
How careful are the other people in their daily lives?
Will the other people be wearing masks?
What is the duration of the activity? The longer, the higher the risk.
Will people be talking loudly, shouting, singing, exercising, dancing or eating? These activities create many more aerosol particles than quiet talking.
Is the activity indoors or outdoors? Outdoor activities are lower risk, by a factor of about one twentieth, according to this non-peer-reviewed preprint, but not no risk.
Is the space well-ventilated or using air filtration (eg, MERV 13 or HEPA filtration)? (This can be hard to find out.)
Level of activity
People who are exercising may prefer a more breathable mask; however, exercising indoors with others is a high-risk activity so attention should also be paid to materials and fit. People exercising outside who will be passing others should wear a mask.
This summary chart, from a peer-reviewed modelling study, illustrates how masks, duration, activity, occupancy, indoor vs outdoor and ventilation affect the risk of transmission. The numbers in this example are from a community prevalence of 0.1%. The absolute numbers would be very much higher in many communities, for example, during the Omicron wave it was estimated at one point that 6-9% of the population in Hamilton, Ontario was likely currently infected.
Level of personal risk
The benefit of a mask, and the benefit of improving the quality of the mask, will be greatest in people who would be expected to experience more severe outcomes if infected (eg, older, with other health problems) and in people who are both more likely to become infected and more likely to experience severe disease if infected (ie, those who do not have the full benefit of vaccinations: people who are immune-suppressed, children below the age recommended for vaccination, people who are vaccine-hesitant and the children of vaccine-hesitant parents).