Condensation occurs on the inside of your mask lens when diving, if not treated with a defog solution this condensation will create a mist like you see on the bathroom mirror after a shower. This layer of water makes looking through the mask lens impossible. Proper defogging of your dive mask lenses is vital to an enjoyable dive. Stream2Sea Mask Defog is a non-toxic and biodegradable, reef-friendly mask defog. Sea clearly with this eco-conscious mask & goggle defog, made with only the safest ingredients for our planet. Professional strength and comes in a 2 fl. oz. (59ml) travel size bottle.