It may take a fairly long time to remove the mask to switch to breathing from the atmosphere. To avoid this, some full-face masks have a valve to switch between breathing from the set and breathing from atmosphere. The 'dive/surface valve', or 'snorkel valve', should be easily operated by the diver, even if wearing thick diving gloves, but not projecting so far that it can be easily knocked against things in low visibility water or in the dark, or in hand-to-hand combat underwater.