I have been worried to see some people - all, I think, without scientific training - proclaiming that there is no need to protect ourselves against the sun because there is no evidence that sulight causes any health problems.
Ultraviolet radiation (UVR) from the sun has been part of the environment since the first cells began to form. When we discuss the effects of UVR on human health and the environment, the range of UV wavelengths is often subdivided into:
- UVA (400–315 nm), also called Long Wave or "blacklight"
- UVB (315–280 nm), also called Medium Wave
- UVC (< 280 nm) also called Short Wave or "germicidal”
The key questions are these:
Can sunlight cause health problems?
Do the benefits of sunlight outweigh their risks?”
UVB is required for the conversion of 7-deoxycholesterol to vitamin D, (the sunshine vitamin!) which is critically important in the maintenance of healthy bones, although there may also be another mechanism by which vitamin D is generated in the body. As we have seen research is making clear that vitamin D has other potential roles in the maintenance of human health. Low levels of vitamin D have been linked to:
- Maintaining the integrity of cell membranes
- Type 2 diabetes mellitus
- Multiple sclerosis
- Pre-eclampsia (hypertension and accompanying problems during the late stages of pregnancy)
- Some types of cancer
- Fibromyalgia-like pains
- Immune deficiency: Africa Americans do not generate enough of a protein needed to ward off tuberculosis. Why? Because the protein needs vitamin D to be activated, and dark skin is inefficient at absorbing and converting UVR. It may also be that we see epidemics of colds and flu in the winter because that is when we have low levels of vitamin D, which allows the viruses to overwhelm our immune defenses.
This does not necessarily mean that taking extra vitamin D will ward off all of these problems.
In the days before the Industrial Revolution, unless we lived in the frozen North, we had no trouble in getting the amount of vitamin D that we needed. In most of the United States, during the summer months, 10-15 minutes outdoors at midday will generate around 10,000 international units (IU’s) of vitamin D in an average fair-skinned person. This is far in excess of the government’s dietary recommendations of 200 IU’s/day in people up to age 50, 400 IU’s up to age 70 and 600 IU’s in people over 70. Not surprisingly many experts – me included – believe and have provided evidence that these daily requirements are much too low. (Have a look at the comments here.)
Of course many of us do not spend much time outside and don’t take in as much in the way of vitamin D containing foods - such as milk and salmon – as we should. I’ve seen evidence to suggest that we in Atlanta are probably at the Northernmost point in the United States were we could hope to get enough sunshine and therefore vitamin D from modest winter exposure to the sun.
Recent data has suggested that if you spend no time at all in the sun, then you may need as much as 4,000 IU’s of vitamin D/day, though that figure has not yet been widely accepted.
Exposure to UVR, whether of solar or artificial origin, also carries potential risks to human health. UVR is a known carcinogen and excessive exposure, at least to the solar radiation in sunlight, increases the risk of cancer of the lip, basal cell, and squamous cell carcinoma of the skin and melanoma, particularly in fair-skinned populations. There is also evidence that solar UVR increases risk of several diseases of the eye, including cortical cataract, some conjunctival neoplasms, and perhaps also melanoma of the eye.
We have good data for the existence of a threshold amount of UV-B exposure that may lead to the formation of cataracts. The amount needed to cause cataracts depends in part on the amount of pigment in the eye, so albino rats get cataracts with much lower exposures to UV-B.
So what to do?
Sunlight has a definite benefit in preventing or treating many clinical problems and it is no surprise that after 3000 millennia we are adapted to make use of the sun’s largesse. What is less easy to understand is why an excess of sunlight can cause so many problems, unless it is our hairlessness and environmental change that has lead to a loss of the ozone layer.
Some years ago it was suggested that sunscreens may themselves cause skin cancer, but the data has shown that to be false. Indeed modern sunscreens almost certainly reduce melanoma risk.
So how do we balance the positive and negative effects of sunlight? A recent review precisely reflects my own thinking:
- We need some sunlight
- Depending on where you live, you need only a few minutes each day
- Sunscreens confer protection on the skin without blocking all the health benefits
- If you have a medical reason for avoiding sunlight, then your health care provider should measure your vitamin D status.