Science & Technology: Melanoma & Tanning (Melanoma: Coincidental Correlation Does Not Prove Causation)

Does exposure to sunlight, UVR or a tanning device cause cutaneous malignant melanoma (CMM)?

The answer to this question is extremely important to the indoor tanning industry because, if UVR can be scientifically proven to cause CMM in humans, the survival of the industry is in jeopardy. On the other hand, if the scientific evidence shows that exposure to sunlight, UVR or a tanning device does not cause CMM, there will be no reason for health-conscious individuals to forgo the incontrovertible benefits of UVR exposure—such as maintaining an optimal vitamin D level year-round—in the carefully controlled environment of a professional indoor tanning salon. So, which answer is correct?

Coincidental vs. Causal Correlation

Research reveals that there is only a coincidental correlation—not a causal correlation—between UVR and CMM; therefore, neither sunlight nor a tanning device can be blamed for causing this disease. Think of this in terms of a post hoc ergo propter hoc relationship, in which it is incorrectly believed that one event comes before and, therefore, causes another when, in fact, the two events were examples of coincidental correlation—not cause and effect. Here are some examples of why coincidental correlation does not prove causation:

• Sleeping with one’s shoes on is coincidentally correlated with waking up with a headache. Therefore, going to sleep with your shoes on is the cause of headaches.
• Since the 1950’s, both the atmospheric CO2 levels and crime levels have increased sharply. Therefore, atmospheric CO2 causes crime.

However, the logical concept of post hoc ergo propter hoc requires that one event always occur before the other event in order to prove causation. For example, in order to prove exposure to UVR causes CMM, there must be a “molecular signature” of damage to the melanocyte genome each time there is UVR exposure and no such molecular signature has ever been demonstrated. Instead, the sunscreen industry, the dermatology community and vested-interest advocacy groups such as the Skin Cancer Foundation have used a coincidental correlation between sunlight/UVR/tanning devices and CMM to link them together. This is, at best, an example of the post hoc ergo propter hoc logical fallacy in action and, at worst, an example of scientific evidence being manipulated in order to promote a deceptive agenda (i.e. selling products and services, such as sunscreens containing UVA filters).

So, why is exposure to sunlight, UVR or a tanning device blamed for causing CMM given the fact that there is no valid scientific evidence (i.e., no molecular signature,) to support this premise? In order to understand the coincidental correlation that has led to this claim, you must understand how melanocytes function within an epidermal melanin unit (EMU).

Melanocyte Function

Each melanocyte cell produces melanin for approximately 36 keratinocytes; this combination is called an EMU. When the keratinocytes in the EMU sense that UVR is striking the skin, a signal is sent to the melanocyte telling it to produce more melanin. That melanin then will be oxidized by the longer UVR wavelengths to protect the keratinocytes’ DNA. This normal and natural response is the genesis of the coincidental correlation that has led to the erroneous belief that UVR causes CMM. However, UVR doesn’t damage the melanocyte (which is why there is no “molecular signature” proving causation) but, rather, it sets in motion a coincidental chain of events that can result in CMM. This chain of events begins when the melanocyte is unable to keep up with the demand for melanin by the keratinocytes in its EMU, and another signal is sent that tells the melanocyte (i.e., the “mother” cell) to:

• Detach from the keratinocytes in its EMU;
• Disengage from the basement membrane (and become temporarily free-floating and vulnerable); and
• Divide in order to form an identical “daughter” melanocyte cell.