I know everyone is probably sick of hearing about sunscreen by now, so I promise this is the last post about it. There were just so many different aspects to cover-- many more than I had originally thought of.
The more I'm reading, the more I'm learning that skin cancer really is a concern, and I ought not to brush it off as something that doesn't affect me. Melanoma rates are rising every year in the United States, and overexposure to sun, especially in youth, appears to be linked to an increased risk of developing cancer later in life (1).
If the the melanoma rates really are related to sun exposure, I'm curious what has changed. For example, common sense would tell me that, in general, the population spends less time in the sun today than 100 years ago, not more. But yet, the population certainly did not liberally apply commercial sunscreen 100 years ago (2).
I found a few interesting speculations for why sun exposure might be more dangerous today then it was in past decades, and I think this could contribute to the rising rates of skin cancer. In The Five Paradoxes of Vitamin D (2), Chesney states that between 1600 and 1960 the skies in general where simply darker because of the amount of coal in the air. Coal and coke were the primary fuel used to heat both homes and industry, and this contributed greatly to hazy skies, especially over cities. However, between 1950-65 the types of fuels the world used diversified--populations starting using nuclear energy, hydroelectricity, and other non-coal hydrocarbons (2). In the last several decades there has also been aggressive campaigns to reduce many kinds of emissions from industrialized countries and to promote "clean air".
Another environmental factor that might contribute to the change of intensity from the sun is atmospheric dimming due to volcanic ash. Four catastrophic volcanoes erupted between 1815 and 1902: Tambora, Krakatoa, Santa Maria, and Mount Pelée. Tambora, which is in Indonisia, erupted in 1815 and was the most explosive eruption in 1300 years (170 times stronger than the Mt. St. Helens eruption in 1980). The atmospheric dimming was so noticeable that the following year, 1816 ,is known as "the year without a summer". Apparently, volcanoes in the 20th century have been less intense and the ash has been more localized which could be one reason dimming has fallen since 1990 (2).
One last environmental factor to consider when thinking about environmental changes is the presence of CFC's in the stratosphere. When CFC's reach the stratosphere certain chemical reactions cause the depletion of the ozone layer. With the depletion of the ozone layer, a higher number of UVB rays hit the earth (2) and this could contribute to an increased need for protection from the sun.
To be honest, I'm nowhere near an expert at environmental science. One question I have about atmospheric dimming is that if it didn't start declining until 1990 (and I believe it's been a pretty gradual decline), then would that really contribute to the increase in skin cancer we see today? As Mr. L pointed out, that would depend on how long it takes for skin cancer to develop. Are the effects that noticeable within a 23 year window? I just don't know enough about the etiology of skin cancer to answer this question.
So, to conclude this series on sunscreen and vitamin D, it appears that it probably is important to protect ourselves from the sun, but this doesn't diminish the importance of vitamin D. Some studies have associated a deficiency in vitamin D with autoimmune disease, fractures, cancer, and cardiovascular disease (1). A prior NHANES suggested that low vitamin D levels were even associated with all-cause mortality (1). When thinking about how I should ensure that my family receives enough vitamin D this is a big tension for me, since I prefer not to rely on supplements for essential parts of our diet, but I also don't want to significantly increase our risk of skin cancer (yes, there are dietary sources of vitamin D, but I think it's unlikely that we would get enough from our diet).
I do plan on being more consistent with sunscreen (since at least in one large study it didn't appear to be associated with vitamin D deficiency), but until I'm more convinced that moderate, non-sunburning sun exposure is a significant risk factor for developing skin cancer, I'm still not planning on slathering it on every day.
(1) Dart H, Wolin KY, Colditz GA. Commentary: eight ways to prevent cancer: a framework for effective prevention messages for the public. Cancer Causes Control. 2012 Apr;23(4):601-8. doi: 10.1007/s10552-012-9924-y. Epub 2012 Feb 26. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3685578/?report=reader. Accessed 7/11/2013.
(2)Chesney, RW. The five paradoxes of vitamin D and the importance of sunscreen protection. Clin Pediatr (Phila). 2012 Sep;51(9):819-27. doi: 10.1177/0009922811431161. Epub 2011 Dec 12. http://www.ncbi.nlm.nih.gov/pubmed/22166748. Accessed on 6/3/13.
(3)Linos E., et al.. Sun protective behaviors and vitamin D levels in the US population: NHANES 2003-2006. Cancer Causes Control. 2012 Jan;23(1):133-40. doi: 10.1007/s10552-011-9862-0. Epub 2011 Nov 2.http://www.ncbi.nlm.nih.gov/pubmed/22045154. Accessed on 7/9/13.