Many people say "it's useless to have a breathing valve", but in fact, it can be used for healthy people!
CDC (Centers for Disease Control and prevention) for SARS
Can I use a respirator with a breathing valve?
Generally, it is OK. The breathing valve can reduce the excessive humidity and heat of the mask when exhaling.
The valve opens to release the exhaled air and closes during the suction process to allow the inhaled air to pass through the filter. Unless the patient has a medical worker who will wear a surgical mask to protect the patient's medical condition (such as an open wound), the medical worker can wear a respirator with a breathing valve. Similarly, respirators with exhalation valves should not be placed on patients to prevent their droplets from splashing and prevent
The article also includes how to choose masks, how long they can be used, and how to protect us