Watertight, dry nitrogen-filled devices are not subject to mold build-up – at least on the inside. Mold can grow on the exterior lens elements if the device is stored in a moist, warm and dark environment for longer periods. Mold spores are practically everywhere and fingerprints (i.e. grease stains) provide an ideal breeding ground. In such cases or in the tropics, where high humidity is the rule of thumb it is recommended to store the devices in an airy, bright location. If they are warmed by the sun, any moisture on the surface vaporizes; without moisture there is no mold buildup. Mold often marks its presence with a fine, branching structure on the surface of the lens elements. The problem is that metabolic byproducts damage the surface permanently so that a simple cleaning may remove the mold but not its traces.