Amid a growing consensus on the effectiveness of face masks at preventing the spread of coronavirus, the Centers for Disease Control and Prevention has issued new guidelines that cautions against wearing one certain type of mask.

Known as valved or vented N95 masks, the masks contain openings through which air is exhaled and are designed to filter out particles. They are frequently used on construction sites.

In recommendations released last week, the CDC warned that the masks can be counterproductive, resulting in the user expelling respiratory droplets into the air.

"This type of mask does not prevent the person wearing the mask from transmitting COVID-19 to others," the agency wrote. "Therefore, CDC does not recommend using masks for source control if they have an exhalation valve or vent."

The warning was consistent with a Duke University study that came out on Friday that assessed the performance of 14 different types of face masks. By measuring the amount of droplets ejected through a mask when the user is talking, researchers concluded that valved N95 masks fared worse than the surgical mask, a fitted N95, as well as certain cotton masks.

"The performance of the valved N95 mask is likely affected by the exhalation valve, which opens for strong outwards airflow," the study said. "While the valve does not compromise the protection of the wearer, it can decrease protection of persons surrounding the wearer."

The experiment also determined that neck fleece or gaiters often worn by runners dispersed the largest droplets of potential virus into small droplets, making it worse than wearing no mask at all, and dispelling the assertion that any face covering is useful in preventing virus transmission.

While surgical masks, also known as medical masks, were found to be the most effective in limiting virus spread, the CDC does not recommend them for the public, saying that they should be reserved for healthcare workers and other frontline workers.

Despite their growing popularity, the CDC's stance on face shields remains unchanged. The agency said it did not currently recommend plastic face shields, which are sometimes used by frontline healthcare workers.

"At this time, it is not known what level of protection a face shield provides to people nearby from the spray of respiratory droplets from the wearer. There is currently not enough evidence to support the effectiveness of face shields for source control," the agency wrote.

But similar to the debate over face masks earlier in the pandemic, some doctors have disagreed. Those in support of using face shields have pointed to their protection for the eyes and their relative ease of use. Some schools districts that are reopening are incorporating face shields to facilitate interaction for younger students who rely on visual cues.

In a recent article in the Journal of the American Medical Association, Eli Perencevich, an epidemiologist at the University of Iowa, wrote, “While medical masks have limited durability and little potential for reprocessing, face shields can be reused indefinitely and are easily cleaned with soap and water, or common household disinfectants. People wearing medical masks often have to remove them to communicate with others around them; this is not necessary with face."

Studies have been mixed, and some experts have argued that the shields do not offer protection against aerosol transmission of the virus.

In a concession, however, the CDC acknowledged that wearing a mask may not be feasible for everyone, such as for people who are deaf or hard of hearing and rely on lip reading as a form of communication.

In those cases, the agency recommends face shields that wrap around the sides of the wearer’s face and extend below the chin, as well as hooded versions.