SAN FRANCISCO, Calif. (KRON) — The CDC recommends that people wear masks and face coverings in public settings to help prevent the spread of COVID-19.

Masks with exhalation valves or vents should not be worn to prevent the spread of COVID-19, the CDC said Friday.

Wearing a face mask helps prevent respiratory droplets from traveling into the air and onto other people when the person wearing the masks coughs, sneezes, talks or raises their voice — also known as source control.

But masks with one-way valves or vents allow air to be exhaled through a hole in the material, which can result in expelled respiratory droplets that can reach others.

These type of masks don’t prevent the person wearing the mask from transmission COVID-19 to others.

Masks are most likely to reduce the spread of COVID-19 when they are widely used by people in public settings.