The new guidance puts the agency more in line with many countries worldwide that have already recommended masking the public, including the US Centers for Disease Control and Prevention which made the recommendation in early April.

Last, the WHO emphasized that its updated guidance is based on fresh data and offers detailed technical advice on homemade mask construction.

Specifically, WHO now recommends that healthy members of the public wear homemade or commercially-available fabric masks in places where the new coronavirus is circulating widely and where physical distancing (staying 6-feet apart, etc.) is not possible or is difficult.

The WHO noted that breathability—the difference in pressure across the mask as you breathe, reported in millibars (mbar) or Pascals (Pa)—should be below 49 Pa/cm2 for a medical mask. But, for non-medical masks, an acceptable pressure difference should be below 100 Pa..

Last, the WHO calculated the filter quality factor, dubbed “Q,” which is a function of filtration and breathability, with higher values indicating better overall efficiency. The bare minimum for homemade fabric mask should be Q of 3, according to WHO expert consensus..

The WHO determined that a minimum of three layers is required for fabric masks. But, masks may need more, depending on the fabric used. For instance, folding cotton handkerchiefs into four layers still only led to maximum filtration efficiency of 13 percent, the WHO noted. Notably, the homemade masks recommended by the US CDC only have two or three layers of cotton..

With the lower standards even for the best homemade masks, the WHO stressed that these masks are for source control only, not personal protection—that is, they can help prevent the person wearing the mask from spreading the virus, but they will not necessarily protect the wearer from becoming infected. As such, it’s important that mask wearing should always be accompanied by frequent hand washing and physical distancing..

The ideal combination of material for non-medical masks should include three layers as follows: 1) an innermost layer of a hydrophilic material (e.g. cotton or cotton blends); 2), an outermost layer made of hydrophobic material (e.g., polypropylene, polyester, or their blends) which may limit external contamination from penetration through to the wearer’s nose and mouth; 3) a middle hydrophobic layer of synthetic non-woven material such as polypropylene or a cotton layer which may enhance filtration or retain droplets.

In addition to the fabric masks for public use, the WHO is also now recommending that in areas where COVID-19 is spreading, all healthcare worker always wear medical masks—even if they are not treating COVID-19 patients specifically.

And, in areas with COVID-19 spread, all people aged 60 years or over and anyone with an underlying health condition should wear a medical mask when physical distancing is not possible