Ozone-loss-related skin cancer to peak mid-century

"We expect the damage to the ozone layer to peak about now and therefore we expect the peak in [ultraviolet] exposure to be around today," Arjan van Dijk of the RIVM's Laboratory for Radiation Research (LSO) told environmentalresearchweb. "So children who live now will receive more damage relevant for developing skin cancer than children born in a few decades' time, assuming that there is no change in behaviour."

It's the ultraviolet dose integrated over a person's lifetime that affects their likelihood of getting skin cancer. "Every year you collect some damage," explained Van Dijk. "From that moment on, this damage will add to your yearly probability of developing skin cancer. As a result, any increased damage taken by today's population will lead to increased incidence rates during the upcoming remaining lifespans. There's a delay of several decades between receiving your dose and having the highest risk."

But Van Dijk and colleagues' scenario analysis shows that without the Montreal Protocol and later amendments, which limited emissions of ozone-harming chemicals, the number of people developing skin cancer at the end of the century would have been far higher. The ozone layer absorbs ultraviolet radiation.

"If there had been no agreement on emissions, squamous cell carcinoma would have prevailed above all other types," said Van Dijk. "It's less dangerous than melanoma but the number of people getting it would have been much higher without the agreement. The gain that we have now that we've made the agreement is enormous."

For squamous cell carcinoma, the number of people getting cancer is proportional to the dose to the power of 2.5. For melanoma, on the other hand, the relationship is more like a square root, so that the incidence of the cancer is less sensitive to dose. The third type of skin cancer, basal cell carcinoma is "hardly lethal".

Even without the depletion of the ozone layer, however, increased life expectancy would be expected to double the number of cases of skin cancer by the end of the century.

Using data from cancer registry bureaus from 250–300 countries from around the world, Van Dijk assigned different types of skin a protection factor to ultraviolet light similar to the sun protection factor used in sunscreens. For example, a typical person from Amsterdam has a skin protection factor of 1, somebody from Ireland or Iceland has a protection factor of 0.3 (receiving three times the ultraviolet dose of the typical Amsterdam dweller), while someone from Africa might have a skin protection rating of 30. These ratings enabled the researchers to apply their model on a global scale.

Perhaps not surprisingly, the model showed that people with higher skin protection factors tend to get skin cancer less often than people with paler skins. For squamous cell carcinoma, natural variability in skin protection factor can make a difference in skin cancer incidence of up to five orders of magnitude while for melanoma the difference can be tenfold.

Van Dijk stresses that his numbers are not yet to be taken literally, but are a guide to where the model needs improvement. "It's always easy to assign the weakest link in a chain but in order to be able to do that you first have to make a chain," he said. "This is our first attempt to make a global chain. It will take time to get quantitative solutions, but now we can see the sensitivities."

Now the researchers hope to improve the model by coupling it to advanced climate models, an attempt that is currently being undertaken in the context of the SCOUT-O3 project. Other work in progress involves the use of aerosol maps from satellite data, and improving understanding of the relationship between ultraviolet light sensitivity and skin complexion (of which colour is only one aspect – thickness may be equally important). Some aspects of skin cancer are not related to ultraviolet exposure and the team would also like to separate these from the model.

Van Dijk says his approach could also apply to crop yield modelling since crops that receive more than a threshold level of ultraviolet exposure tend to grow less well.

Van Dijk reported the progress of LSO-RIVM at the European Geosciences Union meeting in Vienna, Austria.