Shady Doings

Summary and Comment |
November 9, 2012

Shady Doings

  1. Mark V. Dahl, MD

Exposure to ultraviolet radiation is multidirectional — seeking shade is not enough.

  1. Mark V. Dahl, MD

Avoiding sunlight protects skin from developing cancers. Most sun-protective behavior focuses on direct exposure. These researchers evaluated the effects of direct, diffuse, and reflected ultraviolet light on cancer development.

Sun exposure patterns were predicted using a model that forecasts total dose and anatomical distribution of exposure to ultraviolet radiation based on ground irradiance and body morphology. Researchers computed direct, diffuse, and reflected (from ground) radiation at 45 anatomic body areas of a virtual manikin with 4000 mesh data points, taking into account shading from other body parts. In addition to direct, diffuse, and ground irradiance, the computer also analyzed effects of sun position and posture. In this analysis, the virtual mannequin was standing with arms down — the usual posture of outdoor construction workers.

Reflected radiation was negligible, unless the ground was covered with snow. Direct radiation accounted for more than 50% of the total dose of midday summer sunlight, but only 24% of the yearly dose, because irradiance was strongly reduced by shade cast by other body parts, decreases in sun angle, changes in season, and cloudiness. Diffuse irradiation accounted for close to 80% of the total annual dose.


Shadows on the moon are black, but those on earth are not, because the atmosphere refracts, reflects, and diffuses light (especially the blue wavelengths) and ultraviolet radiation. Avoiding exposure is tricky, because we see the light and feel the heat of sunlight (infrared radiation), but we neither see nor feel ultraviolet radiation. “Seek shade” is good advice, but seeking it does not prevent additional photo-induced sun damage from diffuse and reflected ultraviolet radiation.


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