"The purpose of a facemask, when worn by a patient suspected or confirmed with an illness such as influenza or tuberculosis, is to reduce the amount of large infectious particles released as the patient talks, sneezes, or coughs; this limits their concentration in the room air and reduces the infection risk to others who are present. However, facemasks by design do not seal tightly to the wearer’s face. Therefore, they allow unfiltered air to easily flow around the sides of the facemask into the breathing zone and respiratory tract of the wearer. In addition, the materials used for facemasks are not regulated for their ability to filter particles and are known to vary greatly between models. This makes it possible for small particles to pass through or around the facemask and be inhaled by the wearer. This is why they are not considered respiratory protection— facemasks do NOT provide the wearer with a reliable level of protection from inhaling smaller particles, including those emitted into the room air by a patient who is exhaling or coughing, or generated during certain medical procedures. The purpose of a respirator when worn by healthcare personnel, for example a N95 filtering facepiece respirator, is typically to protect the wearer by reducing the concentration of infectious particles in the air inhaled by the wearer. These particles may come from infectious patients who are exhaling, talking, sneezing, or coughing in the rooms in which healthcare personnel are working; from medical procedures performed on infectious patients (e.g., using bone saws or performing bronchoscopies); or from laboratory procedures (e.g., operating centrifuges, blenders, or aspiration equipment) that may aerosolize pathogens. Respirators are designed and regulated to provide a known level of protection when used within the context of a comprehensive and effective respiratory protection program (see the “Types of Respiratory Protection” section on page 15). For example, filtering facepiece respirators are designed to seal tightly to the face when the proper model and size is selected for the individual by using a fit test procedure. The wearer can then be assured that inhaled air is forced through the filtering material, which allows contaminants to be captured and reduces exposure to both large droplets and small infectious particles."

Website the data was collected from https://www.osha.gov/SLTC/respiratoryprotection/index.html

See: NIOSH/OSHA/CDC Toolkit

Direct link to pdf cited above  https://www.osha.gov/Publications/OSHA3767.pdf

I was going to include a study that showed when nurses wear masks, patients and nurses wear masks vs neither wearing masks there was no difference in flu transmission. Cant find the study now, I think it got memoryholed