This is easily determined by the height ratio of any object to the size of its shadow (if the height is measured vertical to the earths gravitational field, the projected shadow is ideally measured on a flat, level surface; furthermore, for objects wider than skulls or poles, the height and length are best measured relative to the same occluding edge). The greatest risk is at solar noon, when shadows are at their minimum and the suns radiation passes most directly through the atmosphere. Regardless of ones latitude (assuming no other variables), equal shadow lengths mean equal amounts of UV radiation. The skin and eyes are most sensitive to damage by UV at 265–275 nm wavelength, which is in the lower UVC band that is almost never encountered except from artificial sources like welding arcs. Most sunburn is caused by longer wavelengths, simply because those are more prevalent in sunlight at ground level. Ozone depletion
In recent decades, the incidence and severity of sunburn have increased worldwide, partly because of chemical damage to the atmospheres ozone layer. Between the 1970s and the 2000s, average stratospheric ozone decreased by approximately 4%, contributing an approximate 4% increase to the average UV intensity at the earths surface. Ozone depletion and the seasonal "ozone hole" have led to much larger changes in some locations, especially in the southern hemisphere. Tanning
Suntans, which naturally develop in some individuals as a protective mechanism against the sun, are viewed by most in the Western world as desirable.
This has led to an overall increase in exposure to UV radiation from both the natural sun and tanning lamps. Suntans can provide a modest sun protection factor (SPF) of 3, meaning that tanned skin would tolerate up to three times the UV exposure as pale skin.Sunburns associated with indoor tanning can be severe.The World Health Organization, American Academy of Dermatology, and the Skin Cancer Foundation recommend avoiding artificial UV sources such as tanning beds, and do not recommend suntans as a form of sun protection. Diagnosis
Differential diagnosis
The differential diagnosis of sunburn includes other skin pathology induced by UV radiation including photoallergic reactions, phototoxic reactions to topical or systemic medications, and other dermatologic disorders that are aggravated by exposure to sunlight. Considerations for diagnosis include duration and intensity of UV exposure, use of topical or systemic medications, history of dermatologic disease, and nutritional status. Phototoxic reactions: Non-immunological response to sunlight interacting with certain drugs and chemicals in the skin which resembles an exaggerated sunburn. Common drugs that may cause a phototoxic reaction include amiodarone, dacarbazine, fluoroquinolones, 5-fluorouracil, furosemide, nalidixic acid, phenothiazines, psoralens, retinoids, sulfonamides, sulfonylureas, tetracyclines, thiazides, and vinblastine. Photoallergic reactions: Uncommon immunological response to sunlight interacting with certain drugs and chemicals in the skin. When in excited state by UVR, these drugs and chemicals form free radicals that react to form functional antigens and induce a Type IV hypersensitivity reaction. These drugs include 6-methylcoumarin, aminobenzoic acid and esters, chlorpromazine, promethazine, diclofenac, sulfonamides, and sulfonylureas.