The All Clear cap construction separates and insulates the bulb from direct water contact, thereby minimizing the impact of colder temperature water which can impact bulb performance. The All Clear has been tested and proven effective for treating water down to 39°F (4°C). The product has been tested in conditions with the ambient air temperature down to 32°F (0°C). Battery performance may be impacted and less efficient in very cold temperature conditions. In the event that you are in sub-freezing conditions, the All Clear unit should be warmed to 32°F (0°C) prior to use.