On Feb. 10, the Centers for Disease Control and Prevention released new mask guidelines based on a study of how mask fit affects the wearer’s exposure to airborne particles. With more transmissible COVID-19 strains emerging around the world, it is important to properly use the right kinds of face masks. No matter how good a mask’s material is, it won’t work well if it doesn’t fit well. A lot of filtration efficiency is lost from leakage around the face mask, such as around the bridge of the nose or at the gaps between any facial hair and the mask. A well-fitting mask is one that seals tightly onto your face, ensuring that almost all the air you breathe goes through the filtering material rather than through gaps in the sides.