Face coverings with valves or vents — largely worn in pre-pandemic times by construction workers — do little to protect a person infected with COVID-19 from passing it to others, the CDC said in a new guide to wearing masks.
“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,” the CDC said in an update, posted last week.
“Therefore, CDC does not recommend using masks for source control if they have an exhalation valve or vent.”
Instead, the agency urges people to wear cloth masks, noting that cotton prevents most of potentially infectious respiratory droplets from escaping.
The brand 3M, which makes masks used in hot or dusty construction sites, for example, is designed to release exhaled air through a valve.
A recent experiment conducted by Duke University tested 14 different types of face coverings to determine how effective each one was at stopping respiratory droplets. The study found that medical N95 masks were the most effective — followed by disposable surgical masks and polypropylene-and-cotton-mix masks rounding out the top three.