Final answer:
About 99% of the solar UV radiation reaching Earth's surface is UV-A, while UV-B makes up roughly 1%, with UV-A rays being less harmful but more prevalent than the more damaging UV-B rays.
Step-by-step explanation:
The portion of UV radiation that makes up sunlight consists of UV-A and UV-B rays. The UV-A rays account for about 95% of the solar UV radiation that reaches the Earth's surface. These rays have longer wavelengths, ranging from 320 to 400 nm, penetrate the skin deeply, and can contribute to skin cancer through indirect DNA damage. UV-B rays, with wavelengths between 290 and 320 nm, make up a smaller portion of sunlight, but they are more harmful due to their shorter wavelengths and higher energy.
Although much of the UV-B and all UV-C radiation is absorbed by the ozone layer in the upper atmosphere, the UV-B rays that do reach the Earth's surface are responsible for direct DNA damage and a higher risk of skin cancer. It's estimated that 99% of the solar UV radiation that reaches the Earth's surface is UV-A, which implies that UV-B rays comprise the remaining 1%.