There are news sources that quote the CDC on not using a mask with a valve. Some people here post they are using them, because it is easier to wear and be comfortable breathing.

As the CDC explains: "respirators with exhalation valves should not be used in situations where a sterile field must be maintained (e.g., during an invasive procedure in an operating or procedure room) because the exhalation valve allows unfiltered exhaled air to escape into the sterile field."

And as one points out: "As the saying goes (for COVID-19), “your mask protects me, my mask protects you,” so please use a valve-less mask to do your part in reducing transmission."

It is now well known that 50% or more infected people have no symptoms.