“We know that wearing a mask outside health care facilities offers little, if any, protection from infection.” New England Journal of Medicine. Here is the technical journal article.

https://www.nejm.org/doi/full/10.1056/NEJMp2006372

In another paper (link below), less technically worded:

Summary / Abstract Masks

and respirators do not work. There have been extensive randomized controlled trial (RCT) studies, and meta-analysis reviews of RCT studies, which all show that masks and respirators do not work to prevent respiratory influenza-like illnesses, or respiratory illnesses believed to be transmitted by droplets and aerosol particles.

Furthermore, the relevant known physics and biology, which I review, are such that masks and respirators should not work. It would be a paradox if masks and respirators worked, given what we know about viral respiratory diseases: The main transmission path is long-residence-time aerosol particles (< 2.5 μm), which are too fine to be blocked, and the minimum-infective-dose is smaller than one aerosol particle.

The present paper about masks illustrates the degree to which governments, the mainstream media, and institutional propagandists can decide to operate in a science vacuum, or select only incomplete science that serves their interests. Such recklessness is also certainly the case with the current global lockdown of over 1 billion people, an unprecedented experiment in medical and political history.

(1) (PDF) Masks Don’t Work: A review of science relevant to COVID-19 social policy. Available from: https://www.researchgate.net/publication/340570735_Masks_Don’t_Work_A_review_of_science_relevant_to_COVID-19_social_policy [accessed May 30 2020].