Skin Cancer and Other Cancers
Overall, cancer rates of the major death causing cancers are higher in sun deprived areas of the world and decrease significantly as one gets closer to the equator. In the USA, the Northern states which receive more limited amounts of sunlight have the highest rates of cancers of the breast, colon, prostate, and ovarian cancer.
Of the sun-promoted cancers, about 1 to 3% of actinic keratoses (sunburns) progress into cancer in 10 to 20 years. These generally are not aggressive cancers and must be left untreated for long periods of time to result in death. About 600,000 non-melanoma skin cancers occur in the USA per year resulting in about 2,200 deaths.
Malignant melanoma skin cancer is most common in people who do not receive regular sun exposure and most frequently develops on parts of the body that are not regularly exposed to the sunlight. It is the rarest form of skin cancer but is the most deadly. It affects the cells which produce melanin and seems to be more prevalent among city dwellers than among people who work outdoors such as farmers. In 1991 cancer experts estimated that there would be about 32,000 cases in the USA during the year of which 6,500 would be fatal.
Does Sunlight Exposure Increase or Decrease Cancer Rates?
For decades, many vocal dermatologists, have held, somewhat as a quasi-religious belief, that sunlight increases the death rate from cancer. These assertions are usually supported by rather small, brief, and superficial studies (a typical dermatology study runs from one week to two months followed by a press conference). On the other hand, cancer research scientists have often come to the opposite conclusion. (It should be emphasized that most dermatologists are much more cautious and careful.)
Does Sunlight Reduce Breast, Colon, Ovarian & Prostate Cancer?
Esther John, an epidemiologist from Northern California Cancer Center, reported in 1997 that her study shows that women who get enough sun to make good levels of vitamin D can lower the risk of breast cancer by 40 percent and perhaps even more.
In a study sponsored by the Department of Defense, John studied 4,881 white women, including 133 who had breast cancer. She found that women in the South have a 41 percent lower risk of dying of breast cancer, compared to cold-belt states like Michigan. Women with the lowest risk lived 20 or more years in areas with high exposure to sunlight. But as little as one hour a day of sunlight might be sufficient to improve breast cancer outcomes, she said.
The study was part of a national research project that has been following the health of about 8,000 women since 1970. The research monitors the development of cancer and other health events, and compares the effects of such things as diet and exercise of women who get disease and those who don't.
John said the study confirmed earlier findings that women who live in the southern tier of states below Kansas tend to get breast cancer significantly less than those who live in the North. The difference, she said, is sunlight. Southern states receive more year-round sunlight than Northern states do. As a result, people in the South get more natural ultraviolet ray exposure. John said winter sunlight is so weak in the North that people living at or above 40 degrees north latitude do not get enough sun from November to February to make the required levels of vitamin D. Boston is at 42 degrees latitude.
John said the study did not determine just how much sun exposure is needed to protect against breast cancer, but she said it probably is less than the amount that would cause skin damage. "It is possible that all it takes is 10 or 15 minutes outside in bright sunlight to get a benefit,'' she said. "But we don't really know that yet. There needs to be more study."
In other studies, researchers at the University of California in San Diego found a close relationship between breast cancer death rates in 87 regions around the U.S. and the amount of solar radiation calculated to be striking the ground in those cities - the higher the solar radiation, the lower the breast cancer. Gorham et al. found increased mortality rates associated with Canadian cities with the most acid haze air pollution which block UV rays. Garland et al. found the highest incidence in countries located at high latitudes. Furst et al. reported that the there is an increased incidence associated with decreased total sunlight levels.
A 1994 study showed that Northern women have five times greater risk of ovarian cancer than southern women who receive more direct sun exposure. Colon cancer rates increase colon in areas with the least natural light, e.g., geographical location at high latitude1. Prostate cancer mortality rates are associated with decreased UV light.
Most of these studies attribute the anti-cancer effect of sunlight to increased vitamin D synthesis in the body. Sunlight is the body's only natural and reliable source of vitamin D. Studies suggest that vitamin D is linked to the prevention of breast, colon, prostate and ovarian cancers- cannot be reliably supplemented in our diet.
Skin melanoma does not appear to be a sunlight caused cancer. A study on 4,000,000 members of the U.S. Navy followed sailors who developed skin melanoma. The results revealed a slightly higher melanoma rate, 9.5 per 100,000 as compared to the rest of the U.S. population of the same age group, 9.2 per 100,000. Sailors who worked under deck (indoor workers) showed the highest incidence of skin cancer. Sailors who worked outdoors were closest to the U.S. average. Other studies have suggested that a lack of sunlight may increase the risk of developing melanoma, while limited sun exposure may actually decrease the risk.
Dr. Gordon Ainsleigh in California believes that the use of sunscreens causes more cancer deaths than it prevents. He estimates that the 17% increase in breast cancer observed between 1991 and 1992 may be the result of the pervasive use of sunscreens over the past decade3. Recent studies have also shown a higher rate of melanoma among men who regularly use sunscreens and a higher rate of basal cell carcinoma among women using sunscreens.
Dr. Ainsleigh estimates that 30,000 cancer deaths in the United States could be prevented each year if people would adopt a regimen of regular, but moderate sun exposure5. Ainsleigh considers cancers (a - e) in table below as sunlight-inhibited cancer while cancer (f) is sunlight-promoted.
Sunlight-Promoted vs. Sunlight-Inhibited Cancers
Source: Cancer Statistics 1991, by the American Cancer Society.
||U.S. New Cases
||U.S. Deaths Yearly
||Deaths as % of cases
|Sunlight Inhibited Cancers
|(b) Female breast
|(c) Non-Hodgkin's lymphoma
|(d) Granulocytic leukemia
|Totals for Sunlight-inhibited cancers (total, lines a-e)
|Sunlight Promoted Cancers
|(f) All Nonmelanoma skin cancers (total, line f)
Skin Cancer Increase Not Due to Ozone Depletion
But what about ozone depletion and skin cancer? Professor Johan Moan of the Norwegian Cancer Institute found that the yearly incidence of melanoma in Norway had increased by 350% for men and by 440% for women during the period 1957 to 1984. He also determined that there had been no change in the ozone layer over this period of time. He concludes his report in the British Journal of Cancer by stating "Ozone depletion is not the cause of the increase in skin cancers."