 Re: Lamp vs Sun Ratio
Two criteria can properly be used to compare indoor vs. outdoor UV exposure. Those are "tanning" dose, of which Don Smith probably has the best bandwidth formulation, and MEDs, or erythema dose.
The simplest comparison is MEDs. 4 MEDs is 4 MEDS, whether it's indoors or outside. So depending on your location (latitude), and time of day of exposure, it may take anywhere from roughly 45 minutes to an hour to get 4 MEDs outside, up to never even reaching that dose if one were outdoors all day.
Besides that though, there's the body geometry problem. 4 MEDs on the top of a bald person's head isn't the same thing as on their stomach that is a vertical and sometimes shaded surface. So the time outdoors always needs to be increased to be comparing apples to apples.
But the short answer is for a typical mid summer, mid day, mid lattitude day, a full tanning session is roughly equal to 2 - 4 hours outdoors, considering average exposure over a person's total body.
Oh yea, I forgot to mention, the International standard for the erythema dose is different than the FDA's, so in reality, the 4 MED limit in a tanning bed is usually 3 CIE MEDs or less. But that's another story. |