March 4, 2021

CDC Assesses Techniques to Improve Fit of Cloth and Medical Procedure Masks

In CDC’s Feb. 19 Morbidity and Mortality Weekly Report (MMWR), the agency evaluates common methods used to improve the fits of cloth and medical procedure masks. These types of masks fit more loosely than respirator facepieces, and the effectiveness of these masks at protecting wearers from airborne viral particles may be improved by modifying them to fit more closely to the wearer’s face. Two common modifications made to medical procedure masks with the intention of improving fit are known as “double masking” and “knotting and tucking.”

The “double masking” method is the practice of wearing a cloth mask over a medical procedure mask. The “knotting and tucking” method requires tying off the ear loops of a medical procedure mask near the point where the loops connect to the mask and then tucking in the excess mask material so that the mask is worn flat against the face with minimal side gaps. CDC’s experiments examined these methods’ effectiveness in improving mask fit and therefore filtration compared to unmodified use of single cloth and medical procedure masks. The experiments used pliable elastomeric head forms to represent mask wearers, outfitted either with mouthpieces that emitted potassium chloride aerosols (to simulate a cough) or with ventilators (to simulate breathing).

The first CDC experiment tested how effectively that double masking and knotting and tucking contained the aerosol particles emitted by a wearer during a simulated cough. The results demonstrated that the use of a single unmodified medical procedure mask blocked 56.1 percent of aerosol particles from the cough. The use of a single cloth mask blocked 51.4 percent, while the use of the double mask technique blocked 85.4 percent of aerosol particles and use of the knotted and tucked medical procedural mask blocked 77.0 percent.

The second CDC experiment assessed the effectiveness of the two techniques in reducing wearers’ exposures to aerosols emitted throughout a 15-minute period of simulated breathing. This experiment used two elastomeric head forms acting as the aerosol source and receiver. The results showed that double masking the source reduced aerosol exposure to the unmasked receiver by 82.2 percent. A configuration with a knotted and tucked mask-wearing source reduced another unmasked receiver’s exposure by 62.9 percent. When the source was unmasked and the receiver was double masked or knotted and tucked, the receiver’s exposure was reduced by 83 percent and 64.5 percent, respectively. When the source and receiver both were double masked or knotted and tucked, the receiver’s exposure was reduced by 96.4 and 95.9 percent, respectively.

The MMWR report concludes that these experiments highlight the necessity of a good fit for effective mask performance and that modifications to improve fit can improve mask filtration efficiency. The report cautions that the experiments were conducted using only one type of cloth and medical procedure mask among many commercially available choices and did not assess all possible mask combinations. In addition, the findings cannot be generalized to children or men with facial hair, and double masking may obstruct breathing or peripheral vision for some wearers while knotting and tucking may distort the mask’s shape so that it no longer protects wearers with larger faces. For a full discussion of the experiments and their findings, refer to the report itself.