IEC 61300-2-21:2009(E) provides a test to determine the resistance of a fibre optic device to the deteriorative effects of high temperature, humidity and cold conditions. It is intended to reveal defects in a device under test (DUT) caused by breathing as opposed to absorption of moisture. The test covers the effect of the freezing of trapped water in cracks and fissures as well as condensation. The changes with respect to the previous edition are: - to describe the apparatus and procedure in greater detail; - to defin with precision the number of 24 cycles in the severity.