…WHO’s June 5, 2020, guidance memo1 on face mask use states there’s no direct evidence that universal masking of healthy people is an effective intervention against respiratory illnesses.
A policy review paper published in Emerging Infectious Diseases in May 2020 — the Center for Disease Control and Prevention’s own journal — has also reviewed “the evidence base on the effectiveness of nonpharmaceutical personal protective measures … in non-healthcare settings,” and they too found no evidence of benefit: “Although mechanistic studies support the potential effect of hand hygiene or face masks, evidence from 14 randomized controlled trials of these measures did not support a substantial effect on transmission of laboratory-confirmed influenza.”
If and when you do use a mask, you have to follow proper protocols, or else you can nullify what little protection they do offer against potentially infectious respiratory droplets. As detailed in WHO’s guidance memo, you need to make sure your medical mask is:11
Changed when wet, soiled or damaged
Untouched. Do not adjust or displace it from your face for any reason. “If this happens, the mask should be safely removed and replaced; and hand hygiene performed”
Discarded and changed after caring for any patient on contact/droplet precautions for other pathogens