This series of posts has been focusing on the environmental factors that have been strongly linked to an increased risk of cancer. It has covered smoking, infection and obesity – and today it will focus on excessive sun exposure. Sun exposure is a particularly complicated thing to understand because it is made up of two inextricable aspects: it exposes the skin to UV radiation, which is damaging and is what leads to skin cancer, and it is necessary to produce vitamin D, which is necessary for a healthy organism. However, vitamin D has been shown to still be produced at healthy levels by people who use the recommended amounts of sunscreen. This is why when scientists discuss UV radiation as a cause of skin cancer they refer to excessive sun exposure.
In particular, UV radiation is directly linked to melanoma, which is a type of skin cancer that arises from melanocytes, the cells that give skin its pigmentation (through a pigment known as melanin) and make up moles on healthy skin (if you are interested in finding out more on melanoma, you can check out my post on melanocytes here). When UV radiation hits the skin, it not only activates the pigment-producing activity of the skin, but it tells the melanocytes to start reproducing. This makes a lot of sense: if you are under a lot of sunlight, you are going to need more and more melanin (which in a way is the body’s own internal “sunscreen”) and therefore you are going to need more and more melanin-producing cells – melanocytes. As we have seen in most of the episodes in this series, forcing cells to replicate means that they have to duplicate their DNA over and over again. With increased DNA replication comes an increased risk of random mutations happening – which is the first and most crucial step towards cancer.
UV radiation exposure is increasing throughout the world, crucially for two reasons. First of all, the ozone layer that blocks a lot of the harmful UV radiation is getting thinner – which means more and more UV radiation is getting through. Secondly, tanning for cosmetic purposes is becoming increasingly common – especially through artificial UV radiation (that means, tanning beds or tanning booths). As one would expect, exposing the skin directly to high levels of UV radiation is incredibly harmful and has been associated with a much much higher risk of being diagnosed with melanoma. What is becoming increasingly clear to epidemiologists around the world is that intermittent exposure to UV radiation is what truly increases the risk of melanoma – as opposed of chronic exposure. This means that getting in the sunshine sporadically and for high doses (which is what most people do when they go on holiday once or twice a week) is much much more dangerous than being always exposed to the sun (which is what people like farmers who work outdoors do). The fact the pattern of sun exposure is key to melanoma risk makes sense when we look at the other aspect of UV radiation risk – sunburn. The number of times a single patient gets sunburned – and how early in their life these burns happens – is directly linked with the patient’s risk of being diagnosed with melanoma. Of course being exposed to sunlight intermittently means it’s much more likely an individual patient is going to get sunburned – which in turn means they are much more likely to become diagnosed with melanoma. On the other hand, chronic exposure to radiation means both a lower chance of getting sunburned (people who are always in the sunshine will have more melanocytes, which means they are much less likely to get burned) and at the same time a higher dose of vitamin D. While the link between vitamin D and melanoma is still under investigation, this might explain the slightly protective effect of chronic sunshine exposure when looking at melanoma risk.
What’s more, the risk of melanoma associated with UV exposure is highly dependent on the way you look. In other words, there are specific phenotypes that make patients more sensitive to sunlight – such as pale complexion, freckles, red hair and so forth. Once again, the data indicating these factors are important is backed by common-sense based on the risk of getting burned. Everybody knows those with a paler complexion tend to burn more easily in the sunshine – which means that having any of these at-risk phenotypes makes you more at-risk or sunburn and consequently for melanoma.
UV exposure is an exciting cancer risk to look at because it is something that the general population can easily control. Intermittent sun exposure can be minimized by wearing sunblock and wearing protective clothing – especially among those with highly at-risk phenotypes. Fortunately, the public is becoming increasingly aware of the risks of UV radiation and of what they can do to protect themselves.
What do you think? How do you protect yourself from excessive UV exposure?