The definitive study of masks and the 1918 Influenza http://ajph.aphapublications.org/doi/pdf/10.2105/AJPH.10.1.34 "Masks have not been proved efficient enough to warrant compulsory application for the checking of epidemics, according to Dr. Kellogg, who has conducted a painstaking investigation with gauzes. This investigation is scientific in character, omitting no one of the necessary factors. It ought to settle the much argued question of masks for the public."
Wearing a mask while outdoors is not completely necessary. If you are outdoors, wear a mask if it is difficult to maintain the 6-foot social distancing rule from other people (such as going to the grocery store or pharmacy or walking on a busy street or in a crowded neighborhood), or if it is required to by law. But you do not need to wear a mask if you’re in your backyard or on personal property and it is very unlikely that you’ll end up within 6 feet of someone you don't know. Another small couple of examples are if you are in any other secluded location where the likelihood of running into someone else is very low, or if you are eating and/or drinking outdoors.
People should wear nonsurgical cloth face masks when they go out in public during the COVID-19 pandemic in the U.S. Up to 25% of people with COVID-19 may not show symptoms, the CDC found out. Moreover, a new small study found that COVID-19 may be most infectious when symptoms are mildest and least noticeable, meaning that people may be spreading the virus before realizing they have it. "This means that the virus can spread between people interacting in close proximity — for example, speaking, coughing or sneezing — even if those people are not exhibiting symptoms," the CDC said in a news statement.