III. Contributing and causative factors
A.
Ultraviolet radiation (UV radiation)
Much of
the sun's intense ultraviolet light is absorbed by the ozone layer of
the earth's upper atmosphere. In the 1970's, scientists detected a gradual
thinning of the ozone layer and recognized that there was a resultant
increase in UV radiation exposure. [ 14
] For every 1% reduction in the ozone layer, there is thought to be
a 2% increase in UV radiation intensity. The effect has been more damage
to the skin, and a concomitant higher incidence of BCC, SCC and MM worldwide.
UV radiation is recognized as a known cause of cancer, [
15 ] and the incidence rate for MM in the United States has more than
doubled in the past 26 years from 6 to 13 per 100,000 persons. [
1 ]
Ultraviolet light
from the sun contains both UVB and UVA rays:
| • | UVB rays cause blistering sunburns that damage the skin directly and impair the functioning of the immune system. |
| • | UVA rays cause premature aging, eye damage, and impairment of the immune system. They penetrate the skin more deeply than UVB rays, damaging cells in both the epidermis and the underlying dermis. Tanning beds utilize UVA radiation at an intensity considered equivalent to the current ozone layer decreasing by 10%. [ 16 ] Long-term users of tanning beds have been found to have an almost eightfold greater chance of developing MM later in life. [ 17 ] |
Both UVB and UVA rays
are known to greatly increase the likelihood of developing MM. [
17 ] Intense intermittent, exposure to sunlight during childhood is
related to a higher risk of developing melanoma in adulthood, and approximately
80% of lifetime sun exposure occurs before the age of 18 years. [
18 ]
Physical factors that affect levels of UV radiation include:
| • | Latitude - The distance to the sun is shortest at the equator, (0°), and therefore UV radiation at this latitude is the most intense; intensity gradually decreases as the latitude designation increases. |
| • | Surface reflection - Reflection from snow, water, or sand increases UV radiation exposure. |
| • | Altitude - The higher the altitude, the thinner the atmosphere, and the greater the intensity of UV radiation. For every 1,000 feet above sea level, UV radiation increases 4-5%. |
| B. Risk factors [ 12 ] | |
| 1. Basal cell carcinoma and squamous cell carcinoma | |
| Genetic
risk factors include fair skin, light-colored eyes, and light hair.
Men are twice as likely as women to have BCC, and are three times
more likely to have SCC. Chemical exposure is a risk for both types of non-melanoma skin cancers. Implicated chemicals include arsenic, industrial tar, coal, paraffin, and certain types of oil. Skin damage from radiation treatments, inflammation, or burns increases risk. Psoralen and ultraviolet light treatment (PUVA), used as a treatment for psoriasis, is thought to increase the risk for BCC and SCC, and to increase the risk for MM six-fold. |
|
| 2. Malignant melanoma | |
| Genetic
factors include skin and hair coloration: individuals with red
or blond hair and fair skin that freckles or burns are at increased
risk. The risk of MM is about 20 times higher for Caucasians than
for African Americans. Family history is an additional genetic risk factor: the risk for MM increases by eight times if a family member, including mother, father, brother, or sister has developed melanoma. Moles: dysplastic nevi are a type of mole that may develop both in areas that are usually covered, and those that are exposed to the sun. The presence of these moles increases risk for MM by 6-10%. A family history (see above) of MM combined with the presence of dysplastic nevi, increases risk for MM 400 %. Immune suppression: individuals who are taking immune system suppressants, due to organ transplant or auto-immune diseases, are at increased risk for MM. |
|
| C. Contributing factors in Texas [ 19 ] | |
| • | Texas is situated between Latitude 26° N and 37° N; according to the World Health Organization, the months in the year when sun protection is necessary in these latitudes range from "February to November" to "all year." |
| • | Altitudes vary from sea level, at the coastline, to elevations higher than 8000 feet. The highest point in Texas, Guadalupe Peak, has an altitude of 8,749 feet above sea level. West Texas, with its mountain ranges and expanses of prairie and semi-arid landscapes, receives intense UV radiation through much of the year |
| • | Texas is 267,339 square miles in size, and represents 7.4% of the nation's total area. |
| • | The Texas economy is built largely on agriculture and industry, with more land farmed in Texas than in any other state. |
| • | Texas contains 4,790 square miles of inland water, and a 367 mile-long coastline, supporting recreation and thriving industries in tourism, fishing, and oil and gas exploration, all of which involve exposure to UV radiation. |
| • | Based on 1980 ‹ 1990 immigration and growth patterns, the year 2000 population projection for Texas is 20,344,798. Of this, the Caucasian population, at the highest genetic risk for skin cancer, is projected to be 11,100,275; the next highest risk population, Hispanic, is projected to be 6,302,361; followed by the African-American population at 2,314,852 [ 20 ] |