Sun injury - Questions and Answers - Part 1

The following is material that I presented at our recent Marine and Diving Medicine course in Curaçao. Sun is an important topic since sunburn can ruin an otherwise excellent vacation and UV is an important contributor to skin cancer.


Solar Injury - Part One

1) Is solar radiation all bad?
Clearly no. Without infrared radiation - we would freeze to death. Without light, we would all starve, by viture of having no plants to eat. finally, without UV radiation causing Vitamin D production we would get rickets and increased levels of cancers.


2)What are the three relevant kinds of Ultraviolet radiation (UVR) of importance to humans?
UVR comes in 3 flavors. The first UVC, has the shortest wavelengths, and therefore the greatest energy, measuring 200 - 290 nanometers. UVC is of importance only where the protective ozone layer has been destroyed, e.g. arctic regions and extreme southern Chile and Argentina. Elsewhere, UVC is entirely absorbed in the stratosphere by ozone and oxygen.

UVB measures (290-320nm) and is responsible for Vitamin D production from 7-dehydroxycholesterol, via the skin, liver and kidney. UVB is also responsible for tanning, burning and skin cancers, though not melanoma.

UVA has longer wavelengths (320 - 400), and contributes to photoaging, tanning, burning, and some skin cancers. UVA also triggers phototoxicity of certain pharmaceuticals.

3) What environmental features most affect the amount of UVA and UVB that reaches the skin?

Time of day: 65% between 10a and 2p, but with daylight savings all bets are off!
Season is important. June has 100x more UVR than December
Latitude: 3% decrease with every degree of latitude from equator. Albuquerque 35 degrees N, Taos is 36.
UVA varies less with daytime and latitude changes - scatters less than shorter wavelength UVB.

Surface features: water reflects small amount of UVR. Snow reflects much more. (85%).

Clouds - attenuate UV radiation by 20-80% - generally 40%. Better absorber of IR radiation (heat).

Altitude: 10% increase with every 1000 feet elevation. Wind exacerbates solar damage - erythema- of skin. Moisture increases dermal absorption of UVR.

Pollution increases absorption of UVR through particulates and ozone.

4) Which goes through auto glass? Answer UVA, not UVB

5) What is albedo?
Reflection of solar radiation from white objects, e.g. and especially snow!
Snow reflects 85% of UVR.

6) What is a sunburn?
Erythema - cutaneous inflammatory response. Accompanied by DNA damage, cytotoxicity. Microscopy may reveal  edema, vasodilation, endothelial cell swelling. “Sunburn cells” appear late - enlarged nuclei and vacuolated cytoplasm.Histamine is released.  Mast cells degranulate. Melanocytes have vacuolization.

7) For a given dermal dose of UVR, can anything mitigate sunburn?
Antioxidants given BEFORE but not after, can decrease erythema. Topical indomethacin (an NSAID) will reduce erythema for 24 hours post exposure. Antihistamines do not work. Topical steroids can blanch some of the erythema. Evidence for oral steroids is lacking.

8) What is the MED
MED stands for mean erythema dose, and is the smallest amount of UVR, generally UVB, that can cause discernable dermal erythema. MED is often measured in time, eg. the MED may be  20 minutes of exposure in Florida in a fair skinned person. DNA absorption of UVB correlates with extent of erythema. Pyrimidine dimers correlate with erythema

9) What is a tan? Immediate pigment darkening occurs from melanin precursors that are preformed. Delayed pigment darkening occurs from UVB and UVA (mostly UVA) when melanocytes proliferate and melanin synthesis occurs.

10) Can you get a sunburn on the palm of your hand.
Probably not because the stratum corneum absorbs and scatters 95% of UVR depending on thickness.

11) What causes rednecks to have red necks?
Photoaging is accompanied by connective tissue damage and decreased elasticity.

12) Should I wear sunglasses when I ski (or snowboard)?
Yes you should wear sunglasses to prevent ocular overexposure of UVB. Snowblindness, or photokeratitis of the cornea, is characterized by corneal edema, corneal surface defects, and blurred vision. Remember - you can always improvise a pair of sunglasses from duct tape!

13) What causes basal cell carcinomas?
Intermittent sun exposure in freckly fair skinned people (e.g the author of this review).

14) Is the age of solar exposure important in developing skin cancer?
Yes. Australians who arrived from Britain younger than 18 have high rates of Basal Carcinomas. If they arrived older than 18, immigrants to Australia had skin cancer rates similar to their British counterparts.

15) Is cumulative lifetime exposure to UVR asociated with Squamous cell carcinomas?
Answer: No! Intermittent exposure, fair skin and red hair are risk factors, cumulative lifetime exposure is not. E.g. the leatherneck construction worker may not be at risk for SCCA.

16) Does high cumulative lifetime exposure to UVR cause melanomas?
No, increased occupational exposure over a lifetime decreases the risk of melanoma! Lower latitude and intermittent high doses of UVR, especially at young ages seems to correlate with melanoma risk. Melanoma is associated with increased number of nevi. Nevi numbers increase with childhood UVR doses.

References: Wilderness Medicine, Ed. Paul Auerbach