Masks with Exhalation Valves or Vents
The purpose of masks is to keep respiratory droplets from reaching others to aid with source control. Masks with one-way valves or vents allow exhaled air to be expelled out through holes in the material. This can allow exhaled respiratory droplets to reach others and potentially spread the COVID-19 virus.
CDC does not recommend using masks if they have an exhalation valve or vent.
Duke University also did a study that compared a variety of commonly available mask types and observed that some mask types approach the performance of standard surgical masks, while some mask alternatives, such as neck fleece or bandanas, offer very little protection. Bandanas and neck fleeces such as balaclavas didn’t block the droplets much at all.