Sunlight has both positive and negative effects on the human body. For example, it is well known that sun exposure can cause burns to the skin and increase the risk of cancer over the long term; however, sun exposure is also required for synthesis of Vitamin D in the skin. This results in a need to find an optimal balance between healthy sun exposure and dangerous over-exposure.
Solar ultraviolet (UV) exposure has been linked with non-melanoma skin cancer since the late 19th century, and subsequent studies have shown that sunburns contribute to the pathogenesis of squamous cell carcinoma (SCC), basal cell carcinoma (BCC) and malignant melanoma (MM). SCC and BCC rarely metastasize, but MM often grows and metastasizes aggressively, with a high rate of mortality. Actinic keratosis is another type of lesion caused by sun exposure, and is now considered an early form of SCC.
Short-term intense UV-exposure, with or without burning, but particularly with burns in childhood, has shown to increase risk of MM, and the overall population risk of MM increases with proximity to the equator.
The spectrum of solar UV radiation has different wavelength bands with different properties. UV-A radiation is 315-400 nm, UV-B radiation is 280-315 nm, and UV-C radiation is below 280 nm. The earth’s ozone layer absorbs UV-C radiation, so UV-A and UV-B are the ranges that have effects on the body. Almost all UV-B radiation is absorbed by the topmost layer of the skin--the epidermis. UV-A radiation penetrates deeper into the dermis. The result is that effects of UV-B radiation, including skin cancer, are found mainly in the epidermis, whereas effects of UV-A such as aging of the skin, are found in the dermis. UV-B radiation causes mutagenic changes in the DNA, whereas the effects of UV-A are due to oxidative damage and other indirect effects.
Although short-term intense exposure to sun is linked to malignant melanoma and other cancers, chronic less intense sun exposure has not been found to increase the risk of melanoma, and some studies have even shown it is protective. Some of those protective effects may be due to the role of sunlight in vitamin D synthesis, as the human body requires exposure to UV-B radiation to synthesize vitamin D. Without sun exposure, oral vitamin D supplements are required to maintain health.
Rising awareness of the connection between sun exposure and skin cancer has led to the widespread use of sunscreens and protective clothing. While reducing the risk of cancer, these protective measures may also have led to lower levels of vitamin D. The US recommended daily allowance (RDA) of vitamin D is 600 IU, but some experts believe those recommendations are too low, and that most people need 1000 or more IU for protection against fractures, some cancers, and other illness.
Both positive and negative effects of sun must be taken into account when considering sun exposure. Protection from sunlight can reduce the risk of skin cancer; however, with strict sun protection practices, alternate sources of vitamin D must be included in the diet to compensate for the lack of beneficial UV-B rays to stimulate vitamin D synthesis in the skin.