Early on, the March and April lock downs were taken seriously and the nation stifled the spread, largely without masks. By June, wide scale mask usage was mandated, but large summer surges in infections and deaths occurred throughout the south and western USA. This recent article in Nature magazine provides a comprehensive discussion on the questionable effectiveness of masks: https://www.nature.com/articles/d41586-020-02801-8
Certainly, in the context of protecting vulnerable people in confined indoor areas, home, business and public transit, mask wearing makes sense. The most disciplined examples of mask usage are among those in the medical community who routinely use masks to protect both providers and patients. In close quarters, disciplined mask usage makes sense.
The broad mandates for masks have are limited in effectiveness for several reasons:
Limitation 1: Inconsistent Usage
For the general public, mask usage is not consistent. Variations in mask materials and wearing habits likely render masks only partially effective. Most people wear masks loosely, some covering their mouths only, others with large gaps. If you have worn a mask with glasses, and you may notice your glasses fog; this is an indication that it is not doing it’s job. The mask is catching some droplets, but also redirecting particulates. You may not be affecting the person in front of you, but you are likely affecting the person to your side. If masks were to be truly effective the general public would need to adopt the discipline of trained medical professionals utilizing properly worn N95 masks technology. This level of discipline would be impossible to effectively put into practice.
Limitation 2: Masks Filter Droplets, But Not Aerosol Components
Droplets (5 to 10 micron) we breath redistribute into finer aerosols (1 to 2.5 micron). Masks will filter larger particles on exhale, but finer aerosol particles will not be filtered by common surgical or cloth masks (only a properly worn N95 mask is effective). So a mask will help cut back on exhaled droplets, but on the intake, they are not likely to effectively filter viral aerosol particles. Here is physicist’s analysis of mask effectiveness: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7297694/
Limitation 3: A False Sense of Security
The usage of masks may impart a false sense of security. Let us assume mask usage on average reduces risk of transmission by 50%. Let us also assume that because of the feeling of safety the mask imparts, close contact is increased by a factor of 2 and the time spent between parties is increased by a factor of 2. In this case overall risk would be increased by a factor of 2. This effect may explain why mask usage by the general public has shown little correlation with reduction in the disease spread. Mask usage may also embolden sick individuals to go out where they would otherwise be staying home. In addition, poor mask hygiene can contribute to the spread of disease. Analysis here: https://rationalground.com/interactive-tool-masks-covid-19/
Where are masks likely to be effective?
To determine where masks are likely to be effective we first look at the factors that govern transmission risk. The CDC indicates that the dominant mode of transmission is person to person contact (typically close contact airborne particles), followed by persistent airborne modes, and less likely through contaminated surfaces. Masks clearly don’t help in situations of person to person contact or transmission through contaminated surfaces. CDC summary of COVID-19 transmission modes and risks: https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-covid-spreads.html
For close contact and airborne COVID-19 transmission we consider the following factors:
- Exposure dwell, how long people remain in close proximity with others.
- Density, how close people are to one another.
- Airflow and how often the air is refreshed, affects viral particle density.
- Additional consideration for mask usage: susceptibility to the virus. Children have high resistance to COVID-19. Elderly and vulnerable populations have high susceptibility. Young and middle age adults are susceptible to the infection but have very low infection rates.
Considering the risk factors we can can determine where mask usage makes sense:
- Masks provide little benefit in low density environments with good airflow and short term exposure: outdoors, short contact, sparse crowds.
- Masks provide little benefit for low risk environments dominated by non vulnerable populations (children and schools with young and middle age adult teachers).
- Masks may be helpful in medium density, medium airflow, short to medium exposure indoor scenarios (offices, stores, schools with elderly teachers).
- Masks may be helpful for those subjected to higher exposure levels or risks in medium airflow indoor situations (clerks, teachers).
- Masks can help mitigate risk for in high density poor airflow environments with significant exposure duration (homes , indoor close quarter social events, long term care facilities, crowded public transit).
The risk of transmission goes up with exposure duration. Longer exposure increases risk. Overall risk may increase if people wearing masks increase their exposure duration if mask material or usage is sloppy. The CDC suggest that 15 minutes exposure is a high risk threshold: https://www.cdc.gov/coronavirus/2019-ncov/php/public-health-recommendations.html
Separation Effects: Inverse Square Law
If an aerosol distributes in 3 dimensions, it will be dispersed according to the inverse square law. If the particulate density is 100% at 3 inches away from the mouth, 3 feet away the density is 0.7% (a reduction of 142X) and 6 feet away the density will be reduced to 0.17% (a factor 600X). Keeping your distance makes sense. A mask will cut down on the initial emitted particle density, depending on the material and how well it is worn. Beyond 3 feet, the mask is not likely to make a big difference except in the case of coughing, sneezing, loud talking or singing. Beyond 6 feet, mask usage will have little effect.
Airflow Considerations: Outdoors versus Indoors
Airflow affects particle persistence and density. Higher airflow and faster refresh = less exposure and risk. In a residential house, the air refresh might be once every 3 hours (assuming windows closed). A good industrial HVAC system will refresh the air 7.5 times an hour. Outdoors, even minimal air movement (1 mile per hour) will refresh the air within a six-foot radius 8 times per minute.
1 mph wind = 5280 feet / hour = 88 feet / minute. For a 12-foot diameter circle, that would be new air every 7 seconds or >500 refreshes per hour.
The better the airflow the lower the risk. Relative to the risk of an indoor home environment, a typical office would pose 1/25 less risk and outdoors the risk would be >1/1000 less.
- Outdoor activities pose almost no risk and masks are of little use.
- Restaurants with good table spacing and ventilation are low risk.
- A well-ventilated office with good airflow (top to bottom) poses little risk.
- A big box store with good airflow and short dwells poses little risk.
- A close quarter service business would be a situation where a mask would be helpful (hair salon, doctor, dentist etc.).
- Close quarter high exposure indoor bar areas would be high risk, as well as close quarter in home social events. Masks would be helpful, but in practice, would typically not be worn.
- Churches with separation and good airflow would pose little risk, but masks may be helpful if people are singing.
Mask usage by those who are at little risk of COVID-19 (either risk of contracting the disease or of severe consequences) makes sense in the context of protecting those who are vulnerable. So in an indoor environment where there are vulnerable people, mask usage would be beneficial. In an environment where all are low risk, like school or social events, the use of masks have little benefit practical benefit.
Conclusion: Sometimes Masks Make Sense, Sometimes They don’t.
Masks are certainly one tool to keep contagious people from infecting others and for protecting vulnerable people. Blanket masks mandates for all interactions are not necessary and provide little value. Focused recommendations for mask usage where appropriate are much more likely to be effective, primarily to protect vulnerable populations. In particular, mask usage in close quarter indoor situations would be a clear case where mask usage would be of benefit to protect vulnerable people. Vulnerable service professionals, teachers and store clerks with high exposure would benefit from consistent mask usage.
Where Masks Make Sense
- Dense business settings where the interaction is more prolonged and intimate, masks add value (doctors offices, dentists, hair salon) … generally the staff will be protecting themselves here from exposure, but in some cases, the client or patron should also where a mask. Transit situations (planes, trains, subways and bus) or ride share services, would also be good situations where masks can help. Checkers or servers, who have more longer-term exposure to a variety of threats, would benefit from masks.
- Home situations or long -term care facilities are where vulnerable people can do little to protect themselves other than to limit access (care about who comes into the home or facility). In these cases, the use of masks or PPE can be effective at protecting the vulnerable folks and should be combined with other measures (like screening and distancing).
Where Masks Add No Value
- Outdoors, with basic separation or short-term interactions, masks add little value. A sporting event with a spread -out crowd or a fair with a moving crowd would benefit little from mask usage. Masks might help in a tightly packed static crowd, but with any separation at all or constant movement, they would not. The best remedy would be to spread people out or have people keep moving.
- Indoor large businesses with good airflow and moving customers also would see little benefit from mask usage.
- Office environments with spacing and barriers and good ventilation are would also not benefit from mask usage. If separation cannot be maintained, masks can be helpful.
Blanket mask mandates have had little effect on the spread of the virus. With diligent use, they are certainly helpful in individual situations. However, in general, the compliance with mask mandates has been spotty and ineffective at stopping the spread of the COVID-19. Mask usage likely gives folks a false sense of security in close quarters, and could be actually raising the risk of transmission if contact rates are increased.
For those who are not vulnerable, wearing a mask is of little utility. For those who are vulnerable, mask wearing can help provide protection. For those situations where vulnerable and non-susceptible people must interact indoors, mask wearing by both parties makes sense. In these cases mask usage needs to be consistent and complimented with distancing to be effective.
Additional articles of interest:
Danish mask study showing limited effectiveness of masks mandates: Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask Wearers
CDC article suggesting masks do help inhibit viral spread: Scientific Brief: Community Use of Cloth Masks to Control the Spread of SARS-CoV-2
An assessments of the CDC brief:
A look at community masking effectiveness in Arizona:
American Institute for Economic Research article examining the question of mask usage and effectiveness: The Year in Disguises