MadSci Network: Medicine

Re: How come there're less skin cancer in NewYork then Australia?

Date: Sat May 22 12:27:37 1999
Posted By: John Carlson, Medical student, MD/PhD (parasitology) , Tulane University, School of Medicine
Area of science: Medicine
ID: 925012971.Me

Hello Arisa,

You have asked a very interesting question. Your ideas linking ozone depletion to increased skin cancer rates are important for evaluation. First I will explain the difference between producing pollution and suffering from pollution. Then I will describe the pattern of ozone destruction. Finally, I'll give you an idea about why skin cancer rates are higher in Australia than New York that is independent of ozone destruction. Along the way I'll throw in some extra information just for fun!

Pollution Moves

Unfortunately, the people who make pollution aren't usually the ones that suffer from it. In general, pollution does not stay still. Pollution can be moved easily by water and air. For example, pollutants are often dumped into river water. The polluted water then floats downstream to another city where the people are affected by it. Canada is a country with many beautiful lakes. The life in some of the Canadian lakes has been killed off by acid rain. But the pollution that caused the acid rain did not come from Canada, it came from factories in the United States.

Because pollution moves around, international treaties are made to prevent one country's pollution from hurting another country's people. For example, the United States now has a Clean Air Act to help prevent air pollution from spreading to Canada.

About Ozone Destruction

UV light causes mutations in DNA, which can lead to the formation of skin cancers. The ozone layer helps keep UV light from getting down to the ground. Some kinds of pollution (in particular, CFCs) get up into the atmosphere and destroy ozone. But the ozone-destroying pollution doesn't go straight up. Instead the pollution is carried by wind currents to above the North and South Poles. Far more ends up over the South Pole than the North Pole. Because Australia is much closer to the South Pole than New York is, ozone depletion may be worse over Australia than it is over New York. This would cause rates of skin cancer to go up in Australia, while skin cancer rates in New York would remain the same.

The ozone layer has been a frequent topic written about by the MAD Scientists. I encourage you to read through some of these other essays:

Karla Wilson has posted a description of the hole in the ozone layer which includes a great map.

Linda Engebretson wrote an interesting essay about why the ozone holes are where they are.

John Christie wrote an easy to understand essay about changes in the ozone layer.

Skin Cancer in Australia

Places near the equator receive more of all kinds of light than areas away from the equator. Australia is nearer to the equator than New York is, so Australia has always received more UV light than New York.

If you'd like to read more about how UV light causes skin cancer, please read two great MAD Scientist essays already written:

James Cotton's essay describes the specifics of how UV light mutates DNA.

Scott Dietert's essay describes skin cancer and explains the link between ozone loss and increased skin cancer.

Many inhabitants of both Australia and New York are descendants of Europeans. Europeans tend to have very light skin. Light skin is very bad at blocking out large amounts of UV light. When Europeans moved Westward to New York, they did not move much closer to the equator, so they did not increase their exposure to UV light. When Europeans moved Southward to Australia however, they moved much closer to the equator. Their pale skin is not as good at protecting them from UV light as the darker native Australian skin is.

Although pollution may be making skin cancer worse in Australians by destroying atmospheric ozone, skin cancer would be much more common there anyway.

Exposure to different levels of UV light at different places on Earth has lead to the evolution of different skin colors in different people. Darker skin protects much better against UV light than light skin. That's why native people from near the equator usually have darker skin than natives farther from the equator.

Why do humans who evolved far from the equator have lighter skin? Although UV light can cause cancer, it is also used for a good purpose in the human body. We use UV energy to make vitamin D from cholesterol! So the lighter skin allows the smaller amount of UV light available in the North to enter the body to be used to make this important vitamin!

Thank you for your interesting question. If there's anything else you're curious about, please ask more questions!

John Carlson,
MAD Scientist

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