**5. Current measures of ultraviolet radiation exposure and skin damage**

The current indicator of skin sun damage relies on the use of minimal erythema dose (MED), which refers to the amount of UVR that produces visible skin redness within 24 hours following exposure [86]. As an indicator of UV damage, MED is insensitive and inadequate because UV-induced molecular damage may occur at sub-MED UV doses [87, 88]. Other markers of UV exposure include clinical findings such as solar lentigines and solar elastosis. While these lesions are completely benign, they do have a positive association with NMSC, mainly due to the fact that they arise secondary to photodamage [89]. Although clinical findings of photodamage provide prognostic value, these lesions are neither sensitive nor specific as markers of skin cancer risk as many individuals with solar elastosis and lentigines will never develop skin cancer [89].

While the association between UVR and skin cancer is well established, quantitative assessment of skin UV exposure and its effect on skin cancer development remains unknown. In a small case control study of 58 patients with cutaneous SCC, the risk was greatest in patients who had more than 30,000 hours of cumulative lifetime sun exposure [90]. This is in contrast to BCCs where studies suggest that intense, intermittent sun exposure resulting in sunburns may be more important for the development of BCC [91]. Quantifying the amount of sun exposed hours necessary to induce NMSC is technically challenging and is not practical for implementation as a risk measure at the population level.
