tl;dr: The UV index is primarily concerned with the level of UV-A that can cause sunburn and, potentially, skin cancer. The actual UV-A flux at Mars is not dramatically different from that at Earth (less because of distance, more because of no attenuation). *But the big difference is the unattenuated UV-C and UV-B!
The total solar constant at the Earth is 1366 watts/square meter with about 8% in the UV. Much of the UV portion of the radiation is absorbed by ozone in Earth's upper atmosphere. (1) Importantly, almost all of the shortest wavelengths (UV-C 200-290 nm) and most of the mid wavelengths (UV-B 290-320 nm) are absorbed. (2) So the roughly half of the UV that reaches Earth's surface is mostly the less harmful UV-A (320-400 nm) radiation. (1,2). Since the solar flux per unit area decreases as the square of the distance and the distances from the Sun of the Earth and Mars are 149.6 (10)6 km and 229.9 (10)6 km respectively (3), the solar constant at Mars is about 588 watts per square meter. So the solar UV portion is reduced from 110 watts/square meter to about 47 watts/square meter. However, the mean atmospheric pressure on the Martian surface is less than 0.007 Bar. (4) Also, there is no ozone in the Martian atmosphere. So there is virtually no atmospheric absorption of UV at Mars. This means that the most harmful UV-C and UV-B radiation is unattenuated, so much greater than that at the Earth's surface.
Bottom Line: a human will need serious UV protection to survive on Mars.
The increased UV radiation is due to the fact that Mars does not have a thick atmosphere with UV blocking gases, especially ozone. However, there are other sources of radiation harmful to humans where the lack of a significant Martian global magnetic field also plays a role. A NASA site on Space Radiation includes a note that "Research studies of exposure in various doses and strengths of radiation provide strong evidence that cancer and degenerative diseases are to be expected from exposures to galactic cosmic rays (GCR) or solar particle events (SPE)". (5)
Although the flux of galactic cosmic rays are relatively constant, the most important SPEs are the rather rare SEP (Solar Energetic Particle) events. These SEP events can be harmful to humans, especially the particles with energies of 40 MeV or greater. (6) They can be produced by energetic solar flares or shock waves associated with coronal mass ejections. Although they can travel at a significant fraction of the speed of light, there would be a few minutes warning. In a modular Martian colony, each module could be designed so, in the event of an SEP warning, the inhabitants could huddle under the module's water supply, which would provide some protection. If the roofs of the modules were transparent silica glass domes, incorporation of a little iron oxide (the material that makes Martian sand red) in the glass would provide protection from the UV, especially the most dangerous UV-C and UV-B. Incorporating some high Z (high atomic number) metal salts would add some protection from the galactic cosmic radiation.
Although the protections discussed above may be helpful, they should not be interpreted as meaning that protection, especially from the constant bombardment of GCR, is straightforward--it is not. See the answers and comments to the question in the link below for more information. (7)
7 Why can't we use the same radiation shielding in Mars that we used when going to the moon?