Pressure differentials are created when a sealed enclosure is exposed to rapid, extreme or recurring changes in ambient conditions. Extreme changes in temperature (a sudden thunderstorm on a hot, sunny day), altitude changes (in transit), or the constant heating and cooling cycle of an internal heat source – any of these can cause a pressure differential within a sealed enclosure. A rapid decrease in temperature, for example, can create a vacuum inside the enclosure. If this pressure differential is not dissipated, the vacuum places continual stress on the housing seals. Over time, this can cause seals to fail, allowing moisture and other contaminants such as dust, dirt and other particulates to be drawn inside.

In addition to failed seals, moisture can get inside enclosures by diffusion – the process by which vapor moves by natural molecular motion through the walls of a polymer enclosure. When water vapor enters a sealed enclosure and has no way to escape, it will condense to a liquid state. If this condensation remains inside the sealed enclosure, it can compromise lens function and corrode sensitive electronics and battery contacts.