In terms of health effects, which UV type is most harmful to humans?

The most harmful type of UV radiation to humans is UVB. This form of ultraviolet radiation is primarily responsible for causing sunburn and plays a significant role in the development of skin cancers, including malignant melanoma and basal cell carcinoma. UVB rays penetrate the outer layers of the skin, leading to cellular damage and an inflammatory response, which is characterized by redness and pain.

While UVA rays also contribute to skin damage and are associated with skin aging and some types of skin cancers, their impact is often indirect, as they penetrate deeper into the skin. UVC rays, on the other hand, are largely absorbed by the Earth's atmosphere and do not reach the surface in significant amounts, making their direct effects on human health negligible in everyday life.

D, which is an unconventional term and not widely recognized within the classification of ultraviolet radiation, does not correspond to any established category and thus can be disregarded in the context of UV radiation's impact on human health.