Malignant melanoma is the most-feared type of skin cancer and has been discussed widely in the (non-medical) scientific literature dealing with the stratospheric ozone layer. It turns out, however, that melanoma may have nothing to do with the alleged depletion of the ozone layer or with the Antarctic Ozone Hole (AOH). Melanoma shows distinct differences from the more common and less dangerous forms of skin cancer, Basal-cell and Squamous-cell carcinomas. As a consequence, EPA regulations can do little to stem the growing incidence of this malignant skin cancereven though EPA regularly (mis)uses "lives saved from melanoma" in the benefit-cost data it presents to Congress.
First, there is no doubt that CFCs ("Freons") can deplete the stratospheric ozone layer. However, they are most effective at altitudes above 40 km, where there is little ozone; so that they have little influence on the total column amount of ozone and therefore on the amounts of cancer-causing solar ultra-violet radiation. At the 20 to 25 km level, where most of the stratospheric ozone is located, there are also other chemical species that deplete ozone, derived from water vapor (WV) and from nitrogen oxides [see Ravishankara et al in Science 2005].
Such depletions had been under discussion since about 1970, in connection with planning for a Supersonic Transport (SST) aircraft. Long before there was any mention of Freons, the National Academy was publishing depletion estimates that ranged all the way up to 70 percent! It came as a great shock, therefore, when published calculations showed that cattle raising and rice growing agriculture already generate large amounts of atmospheric methane that eventually lead to as much stratospheric WV as a fleet of 500 SSTs.
In fact, to become important for ozone destruction, the chlorine atoms released from CFCs require the presence of particles (for so-called heterogeneous reactions) and therefore are most effective following volcanic eruptionsor as is the case in the Antarctic Ozone Hole (AOH), when stratospheric temperatures become cold enough to form ice particles, called polar stratospheric clouds.
Chlorine is a natural constituent of the stratosphere, injected by volcanoes and likely also by the dissociation of salt particles convected upward from the ocean. Up until 1988, a year after the Montreal Protocol was ratified, the published data still showed no increasing trend of stratospheric chlorine compounds. This would indicate that the human contribution from CFCs was negligible. However, the data changed after 1988 and so did even skeptical scientific opinion about the contribution from CFCs.
We know from many lines of evidence that Basal-cell and Squamous-cell cancers are produced by UV-B, the region of the sun's ultraviolet spectrum between 280 and 320 nanometers. This is also the region in which ozone absorbs strongly. As a result, the incidence of those cancers increases greatly in going from the pole to the equatornot only because of changes in the ozone layer but because of the change in the solar zenith angle. As it becomes steeper, it traverses less stratospheric ozone, and therefore the UV-B intensity at the surface should be greatest around local noon at low latitudes.
It is interesting to note that there have been no observations in the last 40 years that would demonstrate an actually increasing trend in UV-B at the surface. This may be due to various atmospheric interferences or even the presence of ozone pollution in most areas. It has been hypothesized that if all of the smog over Los Angeles were to disappear, skin cancer rates there would approach those seen in El Paso, TX.
At US latitudes, the average UV-B is calculated to increase by 5% to 10% for every 60 miles (100 km) move to the southand so does the observed skin cancer rate. Note, however, that these skin cancers affect primarily Caucasians and other fair-skinned individuals and that the incidence may be increased by the fact that people dress more lightly in warmer climates.
Melanoma is a very different beast. First, laboratory studies by Dr. Richard Setlow at Brookhaven National Laboratory have shown that UV-A is much more important than UV-B in initiating melanoma. UV-A covers the region from 320 to 400 nanometers of the solar spectrum, where ozone does not absorb.
Medically, it has been observed, that melanoma can occur on many parts of the body that are not exposed to solar radiation at all. It does not show the strong preference for Caucasians of other forms of skin cancer. It seems that there is an association between melanoma and childhood exposure to solar UV causing sunburn; but this association is not certain.
The upshot: Melanoma has little to do with ozone or Freons. The Montreal Protocol will do nothing to stop the rise in melanoma cases. EPA's benefit-cost calculations, used to support the regulation of important chemicals, like methyl bromide (MeBr), are essentially fraudulent if they include avoidance of melanoma cases.
And some personal advice: Exposure to the sun without proper protection is unwise, particularly when the sun is near the zenith, around 2 to 4 PM. Sun creams can help but they should also screen against UV-A and not just UV-B. Remember Noel Coward's cabaret song "Mad dogs and Englishmen go out in the midday sun."