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The Centers for Disease Control and Prevention (CDC) issued recommendations recently that the general public wears a cloth face mask in public where other social distancing measures are difficult such as at grocery stores and pharmacies. This recommendation was initiated because recent studies have shown that a significant portion of individuals infected with COVID-19 lack symptoms (asymptomatic) or are in the early stages of the infection (pre-symptomatic). These individuals unknowingly can transmit the virus either through droplets or aerosols during speaking, coughing, or sneezing. The wearing of cloth masks is intended to reduce the risk of the transmission (broadcast) of the virus, but these cloth masks, old t-shirts or bandanas have very significant limitations when it comes to preventing the inhalation of the bioaerosols and subsequent infection. Hence CDC continues to insist that masks are not a substitute for following CDC guidelines, including social distancing.
The two most critical factors in any device in preventing the inhalation of an infectious agent like an aerosolized virus is the filtration efficiency of the media and the seal or fit between the face and the filtering material. Filtration is best achieved through impaction, interception, and diffusion. Surgical mask meeting ASTM specifications, and respirators like the filtering facepiece N95 are designed and manufactured to rigid filter media specifications. In the case of the N95, the filter media and design have been tested and certified by the National Institute for Occupational Safety and Health (NIOSH). The N95 filtering facepiece respirator, if worn correctly, is at least 95% efficient in filtering out select particles and bioaerosols.
While there is merit in wearing a homemade cloth mask to prevent the expulsion of bioaerosols from an infected person, they generally have a very low filter efficiency for preventing the inhalation of bioaerosols. Studies have shown that the best filtration efficiency of cloth materials is 38%, and bandana or handkerchief range from 2% (single layer) to 13% (four layers). Generally speaking, the denser the material, the more impaction, interception, and diffusion is achieved, which in turn usually means higher resistance in airflow.
The second factor associated with the efficacy of a mask or respirator is its fit or the seal between the face and the filtering device. During inhalation, air drawn in from the nose or mouth will follow the path of least resistance. Filtering media by design restricts airflow, and if the filter media or respirator is not tight around the nose and mouth, incoming air with the bioaerosols, will bypass the filter and enter the nose or mouth. Where respiratory protection is needed to prevent the inhalation of hazardous agents, NIOSH approved respirators, and fit testing protocols are required to assure the filtering facepiece or respirator properly fits the person relying on the device for protection.
Bottom line, do not relax compliance with social distancing because you are wearing a cloth mask. There is merit in wearing a cloth mask to prevent the transmission of COVID-19 when worn by infected persons shedding the virus, but they haven’t been shown to be effective in preventing the inhalation of the virus.
The best way to avoid the inhalation of the virus is to follow CDC guidelines and exercise social distancing. If you choose to wear a cloth mask, wear it tightly and cover the nose and mouth. Avoid readjusting the mask once on and avoid touching your face and eyes. After every use, consider it to be contaminated and wash and disinfect after every use.
John L. Henshaw, MPH, CIH
Occupational Safety and Health Professional
Former Administrator of the Occupational Safety and Health Administration (OSHA)