Sunbathing Calculator

The governing quantity is the minimal erythemal dose, or MED, defined as the ultraviolet dose that produces just-perceptible reddening of the skin about 24 hours after exposure. For Fitzpatrick type II skin one MED is roughly 250 joules per square meter of erythemally weighted UV. The UV Index is defined so that one index unit equals 25 milliwatts per square meter of erythema-weighted irradiance, which is why time to burn falls inversely with the index: the tool divides a per-skin-type baseline, from 67 minutes for type I up to 500 minutes for type VI at index 1, by the index you enter. Intensity rises about 4 percent per 300 meters of elevation, roughly 40 percent at 3,000 meters, because less atmosphere lies overhead. Fresh snow reflects up to 80 percent of incident UV, open water 10 to 20 percent and dry sand 15 to 25 percent. SPF multiplies the unprotected burn time, but the number is measured in a laboratory at an application density of 2 milligrams per square centimeter. Most people apply a quarter to a half of that, so realised protection is often nearer the square root of the label value. SPF measures erythema, which is driven mainly by UVB, so a high number without broad-spectrum labelling still permits substantial UVA exposure and photoaging. The 0.75 safety factor applied here is a convention, not a clinical threshold. Most importantly, reddening is a late signal: DNA damage begins well before skin visibly burns, so time to burn should never be read as a safe dose.

The skin classification underlying every one of these estimates was introduced in 1975 by Thomas B. Fitzpatrick, then chair of dermatology at Harvard Medical School. It began as four categories describing how a person burns and tans during a first 30-minute summer sun exposure, and its purpose was clinical dosing: physicians needed a way to set safe starting doses for psoralen plus UVA phototherapy. Types V and VI, covering brown and dark brown skin, were added in 1988. Because the scale was built to predict UV response rather than to describe ancestry, it remains a phototype measure rather than a racial or ethnic one. The UV Index has a separate origin. Environment Canada introduced the first public forecast index in 1992, in the period when Canadian atmospheric scientists were documenting stratospheric ozone depletion. The World Health Organization, World Meteorological Organization, United Nations Environment Programme and the International Commission on Non-Ionizing Radiation Protection subsequently harmonised national variants, publishing a common international format in the mid-1990s and a joint global practical guide in 2002 that fixed the low, moderate, high, very high and extreme bands used here. Sun Protection Factor is older than both: the Austrian chemist Franz Greiter, founder of the Piz Buin brand, popularised the SPF concept during the 1960s and 1970s, and the United States Food and Drug Administration formalised SPF labelling in its 1978 sunscreen monograph, adding broad-spectrum and water-resistance rules in 2011.