according to an evidence review published Jan. 11 in Annals of Family Medicine.
Nevertheless, cloth masks may provide some degree of protection, filtration studies indicate. If clinicians use cloth masks, they should take into account the fit, material, and number of layers, the review authors wrote.
And if cloth masks are used as a last resort, such as during shortages of personal protective equipment (PPE), additional measures may help, such as pairing cloth masks with plastic face shields.
“We recommend frequent cloth mask changes to reduce the risk of moisture retention and washing according to hospital laundry standards to decrease the risk of ineffective cleaning,” review author Ariel Kiyomi Daoud, a researcher at the University of Colorado at Denver, Aurora, and colleagues wrote.
The investigators identified and analyzed nine studies related to cloth masks’ ability to prevent respiratory viral infections among health care clinicians. The studies generally were not specific to SARS-CoV-2. They focused on four nonrandomized trials, three laboratory efficacy studies, one single-case experiment, and one randomized controlled trial.
Filtration and fit
“Seven publications addressed the filtration efficacy of commercial cloth masks or materials used to create homemade masks ... in a laboratory setting,” the researchers wrote. These studies found that cloth materials prevent some level of penetration, but generally have “lesser filtration efficiency and greater variability than medical masks” do.
One study found that the materials with the greatest filtration efficacy – vacuum bags and tea towels – had low airflow, which limits their use.
Two studies found that additional layers may increase the viral filtration efficacy of cloth masks.
Several studies that assessed mask fit and airflow found that cloth masks “have worse fit and a greater level of particle leakage, compared to medical masks,” the authors reported. Most studies did not examine cloth masks’ ability to protect wearers from respiratory droplets or contact, which the World Health Organization consider the primary means of SARS-CoV-2 spread, with aerosols playing a smaller role. “Thus, we must interpret these results with caution in the context of COVID-19,” the authors wrote. “For a primary care clinician without access to medical masks, our qualitative synthesis of the literature suggests that it is better to wear a cloth mask than no mask,” as long as other protective measures are considered along with cloth mask use.
Generally consistent guidance
Agencies and researchers have shared similar recommendations about the use of cloth masks in health care settings.
“Health care workers are at the frontline and they need to be protected,” said Abrar Ahmad Chughtai, MBBS, MPH, PhD, an epidemiologist at University of New South Wales, Sydney, in an interview. “Many studies show that respirators are more effective, compared to medical masks, and medical masks are more effective, compared to cloth masks. So ideally, all frontline health care workers should use respirators. If respirators are not available, then medical masks should be used. Cloth masks are not as effective as medical masks and ideally should not be used in health care settings.”
Dr. Chughtai has written about cloth masks for protection against SARS-CoV-2 and was an investigator for a 2015 randomized trial that compared medical masks and cloth masks in health care workers.
In that trial, which was considered in the review, greater rates of influenza-like illness occurred in the cloth mask arm, compared with the medical mask arm.
“Studies show that three or more layers of cloth may reduce the spread of droplets and aerosols from the wearers,” Dr. Chughtai said. “So, cloth masks may be used in community settings to prevent spread of infections from the sick, particularly asymptomatic, people.”
In addition, cloth masks “may be used by health care workers as a last resort, if no other option is available,” he said. In that case, they should have at least three layers, fit to the face, and be washed regularly.