Modern silicon-based detectors for high-energy physics operate in an experimental environment with sub-zero temperatures. At those temperatures, even low traces of humidity will produce vapor condensation with damages to the detectors. Monitoring relative humidity is then a requirement for efficient detector operation. However, the performance of any relative humidity sensor operating in high-energy physics detectors is highly degraded in time due to high radiation doses. Therefore, this paper provides a comprehensive review of all the humidity sensors that have been tested so far for use in high-energy physics applications. Different working-principles of the tested humidity sensors are introduced as well as the description of the radiation effects on the sensing materials. Actually, there are only few publications in literature about humidity sensors as related to this novel and challenging application, while it becomes increasingly important to keep under control the performance-degradation of any silicon-based detector in most advanced high-energy physics.