'Double wearing of a mask significantly enhances virus protection,' US health officials said

The Centers for Disease Control and Prevention (CDC), which leads the control of the new coronavirus infection (COVID-19) in the United States, has announced that 'double wearing of a mask significantly enhances the protection from the virus.'

Maximizing Fit for Cloth and Medical Procedure Masks to Improve Performance and Reduce SARS-CoV-2 Transmission and Exposure, 2021 | MMWR
https://www.cdc.gov/mmwr/volumes/70/wr/mm7007e1.htm

CDC study backs mask doubling with exposure to risky particles down 83% | Ars Technica
https://arstechnica.com/science/2021/02/double-masking-more-than-doubled-cough-particle-blocking-in-cdc-study/

An experiment conducted by Dr. John Brooks and colleagues at the CDC is to compare a so-called 'double mask', in which a cloth mask is worn over a medical mask, with a normal mask. Dr. Brooks and colleagues wore a mask on a rubber doll and investigated the effect of reducing aerosols , which are droplet particles that can contain the new coronavirus.

As a result of this experiment, it was found that the aerosol reduction effect was 42% for the medical mask alone, but reached 92.5% for the double mask of the medical mask and the cloth mask.

It was thought that this difference was caused by the 'opening' that was created when wearing a medical mask. As shown in the image (A) below, a general medical mask has a problem that an opening is created on the side surface of the mask when it is worn, and droplets diffuse from this opening. On the other hand, in the case of a double mask as shown in image (B), this opening is filled, so the diffusion of droplets can be further prevented.

Therefore, Dr. Brooks et al. Also conducted an experiment on a simple method of closing the opening, which is to tie the mouth strap of a medical mask (hereinafter referred to as a knot mask) as shown in the image below.

When we investigated the probability that the non-infected person would receive the aerosol released by the infected person, with one of the two dolls as the 'infected person' and the other as the 'non-infected person', only the infected person would wear a mask. The aerosol reduction effect is 82.2% for double masks and 62.9% for knot masks. If only the non-infected person is wearing a mask, 83% have a double mask and 64.5% have a knot mask. When both the infected and non-infected people were wearing masks, the results were 96.4% for double masks and 95.9% for knot masks.

Dr. Brooks et al. Conducted this research only in the laboratory, because they used dolls instead of humans, and there are various types of medical masks, even if it is called a 'double mask'. After acknowledging that the effect differs depending on what kind of mask is combined, he concludes that 'proper wearing is important to maximize the effect of the mask.'