I don't know why that myth got started. Airlines won't allow you to fly if you have an exhalation valve, despite the fact that far more exhaled air and droplets escape from the side openings and nose bridge of paper masks. But that myth is still all over the internet, including official sites. The CDC actually has 2 answers, 1 for the public and one for medical professionals.
An N95 mask with an exhalation valve still provides the same electrostatic protection when exhaling as when inhaling. Most exhalation valves in N95 masks are filtered, which is required for NIOSH certification.
But, when answering questions from medical professionals the CDC:
"Is it okay to wear a N95 face mask with an exhalation valve to protect me and others from COVID-19?"
"Yes, an N95 filtering facepiece respirator will protect you and provide source control to protect others. ["Source control" means you as the source, spreading it to others.] A NIOSH-approved N95 filtering facepiece respirator with an exhalation valve offers the same protection to the wearer as one that does not have a valve. As source control, findings from NIOSH research suggest that, even without covering the valve, N95 respirators with exhalation valves provide the same or better source control than surgical masks, procedure masks, cloth masks, or fabric coverings."
