Which UV wavelength is considered the longest and least harmful?

The longest and least harmful UV wavelength is UVA. This category of ultraviolet radiation has wavelengths ranging from 320 to 400 nanometers, making it less energetic when compared to UVB and UVC. The lower energy level means that UVA is less capable of causing immediate effects such as sunburn, which is primarily associated with UVB radiation.

UVA rays penetrate deeper into the skin and are associated with long-term effects such as skin aging and the formation of wrinkles, but they are still considered less harmful than UVB and UVC rays in terms of direct damage to DNA and immediate skin reactions. UVB rays, while having a shorter wavelength (290 to 320 nanometers), are more energetic and can cause sunburn as well as contribute to skin cancer risk. UVC, on the other hand, has the shortest wavelength (100 to 280 nanometers) and is the most harmful type of UV radiation, but it is largely absorbed by the Earth's ozone layer and does not reach the surface under normal conditions. D is not a standard classification of ultraviolet radiation.

Consequently, UVA is recognized as the safest type of UV radiation relative to its counterparts, highlighting its classification as the longest and least harmful wavelength.