Why masks with valves do not slow the spread (1 of 2)

This video, created using a schlieren imaging system, shows airflow dynamics when wearing an N95 mask with an exhalation valve (left) and without an exhalation valve. The valve is designed to allow air to escape unfiltered. Masks with valves do not slow the spread of COVID and should not be worn for that purpose, according to the CDC. Read more about this video: Credit: Matthew Staymates/NIST.