Wednesday, July 16, 2008

Sunscreen and Melanoma

Melanoma is the most deadly type of skin cancer, accounting for most skin cancer deaths in the US. As Ross pointed out in the comments section of the last post, there is an association between severe sunburn at a young age and later development of melanoma. Darker-skinned people are also more resistant to melanoma. The association isn't complete, however, since melanoma sometimes occurs on the soles of the feet and even in the intestine. This may be due to the fact that there are several types of melanoma, potentially with different causes.

Another thing that associates with melanoma is the use of sunscreen above a latitude of 40 degrees from the equator. In the Northern hemisphere, 40 degrees draws a line between New York city and Beijing. A recent
meta-analysis found consistently that sunscreen users above 40 degrees are at a higher risk of melanoma than people who don't use sunscreen, even when differences in skin color are taken into account. Wearing sunscreen decreased melanoma risk in studies closer to the equator. It sounds confusing, but it makes sense once you know a little bit more about UV rays, sunscreen and the biology of melanoma.

The UV light that reaches the Earth's surface is composed of UVA (longer) and UVB (shorter) wavelengths. UVB causes sunburn, while they both cause tanning. Sunscreen blocks UVB, preventing burns, but most brands only weakly block UVA. Sunscreen allows a person to spend more time in the sun than they would otherwise, and attenuates tanning. Tanning is a protective response (among several) by the skin that protects it against both UVA and UVB. Burning is a protective response that tells you to get out of the sun. The result of diminishing both is that sunblock tends to increase a person's exposure to UVA rays.

It turns out that UVA rays are more
closely associated with melanoma than UVB rays, and typical sunscreen fails to prevent melanoma in laboratory animals. It's also worth mentioning that sunscreen does prevent more common (and less lethal) types of skin cancer.

Modern tanning beds produce a lot of UVA and not much UVB, in an attempt to deliver the maximum tan without causing a burn. Putting on sunscreen essentially does the same thing: gives you a large dose of UVA without much UVB.

The authors of the meta-analysis suggest an explanation for the fact that the association changes at 40 degrees of latitude: populations further from the equator tend to have lighter skin. Melanin blocks UVA very effectively, and the pre-tan melanin of someone with olive skin is enough to block most of the UVA that sunscreen lets through. The fair-skinned among us don't have that luxury, so our melanocytes get bombarded by UVA, leading to melanoma. This may explain the incredible rise in melanoma incidence in the US in the last 35 years, as people have also increased the use of sunscreen. It may also have to do with tanning beds, since melanoma incidence has risen particularly in women.

In my opinion, the best way to treat your skin is to tan gradually, without burning. Use clothing and a wide-brimmed hat if you think you'll be in the sun past your burn threshold. If you want to use sunscreen, make sure it blocks UVA effectively. Don't rely on the manufacturer's word; look at the ingredients list. It should contain at least one of the following: titanium dioxide, zinc oxide, avobenzone (Parsol 1789), Mexoryl SX (Tinosorb). It's best if it's also paraben-free.

Fortunately, as an external cancer, melanoma is easy to diagnose. If caught early, it can be removed without any trouble. If caught a bit later, surgeons may have to remove lymph nodes, which makes your face look like John McCain's. Later than that and you're probably a goner. If you have any questions about a growth, especially one with irregular borders that's getting larger, ask your doctor about it immediately!


Ross said...

Thanks for the info on sunblock ingredients, I will refer back to this before I leave for Mexico!

Stephan Guyenet said...

My pleasure, glad I could help.

Ross said...

I noticed that Neutrogena has stopped using parabens in their "UltraSheer dry-touch sunblock." I'm looking at an old tube that I bought 6 months ago and it contains propylparaben and ethylparaben. The new one contains neither. There is no mention of it, they are just not there in the ingredients list. I wonder if this is an example of a company responding to shifting changes in consumer demand?

Stephan Guyenet said...


I bet it is; there has been a lot of fuss over parabens lately. Thanks for pointing that out.

Adolfo David said...

Hello Stephan, there is a little mistake in this article. Mexoryl SX (Ecamsule) is NOT Tinosorb. Tinosorb can be Tinosorb M (Bisoctrizole) or S (Bemotrizinol) but they are only approved in Europe, where I live, NOT in USA. ("Cosmetic Dermatology", Leslie Baumann MD, 2009). Also in Europe we enjoy Mexoryl XL, which is better than Mexoryl SX, the only one Mexoryl approved in USA.

Regards from Spain

Unknown said...

great article. not many people know about this. they all feel safe using sunscreen...

you did not mention the beneficial effects of UVB, though, which could even prevent cancer!

I suggest (against all "rules" of the health institutes) sunbathing for a short time (about 60% of the time that burns you) at NOON and WITHOUT sunscreen to get enough UVB and then wear a t-shirt and a hat.

best greetings,