There are two concepts involved here? N95 masks without exhalation valvesand N95 mask with exhalation valves.
Let take a look at general N95 mask. All N95 mask can filter out at least 95% of tiny (0.3 micron) airborne particles. So wearing N95 mask can effectively help to reduce the risk of infection.
N95 mask exhalation valves will protect the users against from virus. At the same time it can provide comfort to the users. The priciple is that: the valve design with an open and close mode. When the user inhales,membrane closes to against the support structure then opening is obstructed, preventing air flow from passing to the user, thereby protecting the wearer. During exhalation, when sufficient positive air pressure is reached, the membrane lifts and the wearer’s unfiltered breath can be expelled from the valve. But if the users already infected with COVID-19 or have no idea that he/she has been infected with COVID-19, wearing the N95 mask with exhalation valves cannot protect other people.
So given the vagaries of the epidemic and limited medical instrumentation system, wearing N95 mask without exhalation valves is the best choice. It can not only protect yourself, but other people who surround you.