Double masking has emerged as a response to concerns about the direction of the pandemic and the transmissibility of new COVID-19 variants.
The protective function of a mask in terms of the percentage of small particles it filters out is called the filtration efficiency. Every extra layer worn increases the overall filtration efficiency of the mask, though not in a simple way. This was first shown in a laboratory setting in 1919 and confirmed in studies conducted in aerosol labs in Chicago and the U.S. National Institute of Standards and Technology during the current pandemic.
Nurses working in the 1919 influenza pandemic double masked, wearing two three-layer gauze masks. During the first SARS outbreak in 2003, healthcare workers in China made 12 to 16-layer masks when they ran out of certified personal protective equipment. Studies that included people wearing these masks suggested they did provide protection for the wearer.