[kstewskis]

re: the humidity vs. skin cancer incidence issue:

Hang 'em, I took physics in college, your explanation was interesting, but not entirely correct. The amount of water vapor in the air has NO bearing on the amount of UVA, UVB or UVC rays of the sun beating down on the earth's surface.

Refraction of light, yes, will "appear" like more of a thermal/heat burn, but has no bearing from the harmful amount of burning ultraviolet rays of the sun. It is the chemical reactions from the UVA, UVB, ect...that occur within the skin cells that we believe cause skin cancer.

I wrote the American Cancer Society about this, and this was the reply I received this morning:

Thank you for contacting your American Cancer Society.

Your message indicates that you would like to know why skin cancer is far greater in certain parts of the United States than others. It seems you are interested in knowing if humidity is a significant factor in developing skin cancer in these areas.

The vast majority of skin cancers are due to unprotected ultraviolet radiation (UV) exposure. Most of this radiation comes from sunlight. People who live in areas with year-round, bright sunlight have a higher risk. For example, the risk of nonmelanoma skin cancer is twice as high in Arizona compared to Minnesota. This is because of the length of time the sun is out for longer periods of the day, and the summer season is typically longer in areas like Arizona. Spending a lot of time outdoors for work or recreation without protective clothing and sunscreen increases your risk. The risk is also increased for people who live at high altitudes.

We hope this information adequately addresses your question.

Hope this helps. Be safe this summer everyone.

[hang 'em]

To find the effects of atmospheric water vapor on UV here:

http://rain.atmos.colostate.edu/AT622_section10.pdf

and look at the upper right hand figure on the first page. It is a plot of the absorption coefficients for several atmospheric components (e.g. CO, CO2, ozone, NO2, H2O, etc.) The horizontal axis is wavelength in nm (nanometers). UA, UB are roughly in the range 2 to 4 nm. Look at the curve labeled H20. You will see that water vapor (H2O) has an absorption of nearly 100% in at some parts of the UV spectrum. Therefore water vapor can have a significant effect on UV.

Now as to the response from the Cancer Institute, realize that they want people to know that cloud cover is not perfect protection from UV. Of course it's not... BUT IT HELPS! Also, my comparison was meant to be between the SW (e.g. Arizona) and the SE (e.g. Alabama, NOT Michigan), two states at the same latitude, but with vastly different levels of average UV.

Here's a common sense answer to the issue: If you have to be outside naked for 3 hours in the mid day, would you rather it be a cloudy day?