According to some studies, UVC is the most effective type of UV light for killing powdery mildew spores. UVC can destroy up to 99% of spores within seconds, depending on the intensity and duration of exposure. However, UVC is also very dangerous for humans and plants, as it can cause severe burns, eye damage, mutations, and death. Therefore, should only be used with extreme caution and protection, and only when there are no humans or plants in the room.