World Health Organization Interim Guidance – June 2020.

At present, there is no direct evidence (from studies on COVID-19 and in healthy people in the community) on the effectiveness of universal masking of healthy people in the community to prevent infection with respiratory viruses, including COVID-19.  https://www.who.int/emergencies/diseases/novel-coronavirus-2019/advice-for-public/when-and-how-to-use-masks

Healthy people should wear masks only when 'taking care of' coronavirus patients.  Louis Casiano, May 29, 2020.  https://www.msn.com/en-us/health/health-news/who-guidance-healthy-people-should-wear-masks-only-when-taking-care-of-coronavirus-patients/ar-BB14JBI4?li=BBnb7Kz

In a February 2020 meta-analysis of six randomized controlled trials involving 9,171 patients, there were no statistically significant differences in preventing influenza or viral infections using N95 respirators and surgical masks.  Effectiveness of N95 respirators versus surgical masks against influenza:  A systematic review and meta-analysis.  J Evid Based Med.2020;13:93-101.  https://doi.org/10.1111/jebm.12381

A May 2020 study involving 159 healthcare workers aged 21 to 35 years of age found that 81% developed headaches from wearing a face mask.  Headaches Associated With Personal Protective Equipment - A Cross-Sectional Study Among Frontline Healthcare Workers During COVID-19.  American Headache Society - 2020.  U.S. National Library of Medicine.  National Center for Biotechnology Information.  National Institutes of Health.  https://pubmed.ncbi.nlm.nih.gov/32232837/   https://headachejournal.onlinelibrary.wiley.com/doi/epdf/10.1111/head.13811 

A 2015 study indicated that hypoxia inhibits T-lymphocytes (the main immune cells used to fight viral infections) by increasing the level of a compound called hypoxia inducible factor-1 (HIF-1). In other words, wearing a mask, which has been shown to cause hypoxia, may actually set the stage for contracting COVID-19 and make the consequences much worse.  Cutting Edge:  Hypoxia-Inducible Factor 1 Negatively Regulates Th1 Function.  Laboratoire d'Immunobiologie, Université Libre de Bruxelles, 6041 Gosselies, Belgium.   The Journal of Immunology.  August 15, 2015.  https://pubmed.ncbi.nlm.nih.gov/26179900/  https://www.jimmunol.org/content/jimmunol/195/4/1372.full.pdf

In a 2008 study of surgical masks worn by 53 surgeons, researchers found that the mask reduced the blood oxygen levels significantly, creating a condition known as “hypoxia.” The decrease was more prominent in the surgeons aged over 35.  Preliminary Report on Surgical Mask Induced Deoxygenation During Major Surgery.  ScienceDirect article.  Ba?kent University. Faculty of Health Sciences. Department of Nursing and Health Services.  Available online May 23, 2012. https://www.sciencedirect.com/science/article/abs/pii/S1130147308702355?via%3Dihub  https://pubmed.ncbi.nlm.nih.gov/18500410/

Etc…