In the field, tear gas can be used as a test substance to obtain a rough measure of the protective ability of a mask. Even in a modern mask with a good sealing edge, the fit will start to deteriorate already when the soldier has a 24 h-old beard stubble. The protection will deteriorate with the length of the stubble. Whole beards usually result in very poor protection. Two special terms are used with regard to protective clothing, penetration and permeation.
Penetration implies that gases or liquids pass through seams or holes, etc.