The steadily widening recommendations for everyone to wear a mask is less to protect me from you, but to protect you from me.

The current estimate is that perhaps up to 40% of all new infections were transmitted from an infected person who was not having any symptoms, just simply breathing out the virus, in either respiratory droplets (currently thought to be most common means of transmitting or acquiring the infection), or the far smaller aerosolized viral particles, which can stay in the air for some dozens of feet (thought to be a less common means of transmission, although this is being challenged this week).

The PM 2.5 masks I've seen have expiratory valves, as do many of the N95, N99, N100 masks. If the wearer of a mask with an expiratory valve is asymptomatic or presymptomatic, yet infected, then every breath out through the valve is shedding virus. Wearing this type of mask, with an expiratory valve, puts others at risk.