The principal messaging from health authorities in the COVID-19 outbreak to the general public has been that masks protect others from the wearer. There are two important concepts in this simple message that need to be unpacked, and I want to put down my thoughts on them. I believe the underlying miscommunication of concepts (and failure to update with new understandings, in some cases of old knowledge) has led to a great deal of the political debate about mask wearing (caveat — I am not a political scientist).
As a purely technical point, masks behave as filters. Masks, especially those which are in use by the general public are not anisotropic, at least in the direction of airflow. The key meaning of this is that particle removals are the same whether from the wearer to the surroundings (on talking, coughing, sneezing or exhalation), or from the surroundings to the wearer (on inhalation). The following table, excerpted from a recent paper on testing cloth mask materials with simulated gaps showed that cloth masks of various types could achieve upwards of 54 % removal of small particles and 44% of large particles — and noteworthy was that with or without simulated gaps, most fabric masks performed as well or better than surgical masks.
|Material||<300 nm average ± error||>300 nm average ± error|
|N95 (no gap)||85 ± 15||99.9 ± 0.1|
|N95 (with gap)||34 ± 15||12 ± 3|
|surgical mask (no gap)||76 ± 22||99.6 ± 0.1|
|surgical mask (with gap)||50 ± 7||44 ± 3|
|cotton quilt||96 ± 2||96.1 ± 0.3|
|flannel||57 ± 8||44 ± 2|
|cotton (600 TPI), 1 layer||79 ± 23||98.4 ± 0.2|
|cotton (600 TPI), 2 layers||82 ± 19||99.5 ± 0.1|
|natural silk, 1 layer||54 ± 8||56 ± 2|
|natural silk, 2 layers||65 ± 10||65 ± 2|
In communicating about mask use, there is frequent use of the word “protect”. The first definition of this word is: “to cover or shield from exposure, injury, damage, or destruction”. To the lay public this is construed as “eliminate”, There are two problems with use of this word in communicating about masks. First is that no risk can be eliminated, but only reduced (perhaps to a level where the residual risk is regarded as acceptable). Second, masks must be considered as part of a multiple barrier strategy (possibly a subject of a future post). It is becoming clear with COVID19 that transmission may occur not only via large particles (which the medical community has historically called droplets), but small particles.
Total protection by any one intervention is not necessary. In fact, concepts from industrial and occupational health recognize that the use of PPE such as masks are a last resort after interventions such as engineering design, administrative controls, etc. are employed. The combination of all such interventions are what yields a needed risk reduction.
Complete elimination by controls, while an admiral goal, is not necessary. In the current pandemic, the key desire is to reduce the reproduction number of cases below 1 so that there are diminishing numbers of cases and the case incidence rate is sufficiently low to permit testing, contact tracing and isolation as a final means of driving illness levels down. Hence, for example, if the uncontrolled reproduction number (R0) is 4, then only an overall 75% reduction of risk is needed.
From the tests on homemade masks, it is clear that most materials can provide risk reductions to levels that would be desired, especially in conjunction with other measures such as improvement of indoor ventilation, social distancing, and prudence in gathering of large groups.
If I were designing a risk communication message about masks, I would phrase it along the lines that “wearing a mask reduces risk to you and those you interact with”. Hopefully such changes will start to come about.