The damage is recognized by the body, which then triggers several defense mechanisms, including DNA repair to revert the damage, apoptosis and peeling to remove irreparably damaged skin cells, and increased melanin production to prevent future damage. Melanin readily absorbs UV wavelength light, acting as a photoprotectant. By preventing UV photons from disrupting chemical bonds, melanin inhibits both the direct alteration of DNA and the generation of free radicals, thus indirect DNA damage. However, human melanocytes contain over 2,000 genomic sites that are highly sensitive to UV, and such sites can be up to 170-fold more sensitive to UV induction of cyclobutane pyrimidine dimers than the average site  These sensitive sites often occur at biologically significant locations near genes. Sunburn causes an inflammation process, including production of prostanoids and bradykinin. These chemical compounds increase sensitivity to heat by reducing the threshold of heat receptor (TRPV1) activation from 109 °F (43 °C) to 85 °F (29 °C). The pain may be caused by overproduction of a protein called CXCL5, which activates nerve fibers.Skin type determines the ease of sunburn. In general, people with lighter skin tone and limited capacity to develop a tan after UV radiation exposure have a greater risk of sunburn. The Fitzpatricks Skin phototypes classification describes the normal variations of skin responses to UV radiation. Persons with type I skin have the greatest capacity to sunburn and type VI have the least capacity to burn.
Medical uses
Hydrochlorothiazide is used for the treatment of hypertension, congestive heart failure, symptomatic edema, diabetes insipidus, renal tubular acidosis. It is also used for the prevention of kidney stones in those who have high levels of calcium in their urine.Multiple studies suggest hydrochlorothiazide could be used as initial monotherapy in people with primary hypertension; however, the decision should be weighed against the consequence of long-term adverse metabolic abnormalities. A review of randomised trials showed varying efficacy in cardiovascular outcomes on age, ethnicity and existing cardiovascular risks. A systematic, multinational, large-scale analysis by Suchard et al. supported equivalence between drug classes for initiating monotherapy in hypertension, however thiazide or thiazide-like diuretics showed better primary effectiveness and safety profiles than angiotensin-converting enzyme inhibitors and non-dihydropyridine calcium channel blockers.Hydrochlorothiazide is less potent but not necessarily less effective than chlorthalidone in reducing blood pressure. More robust studies are required to confirm which drug is superior in reducing cardiovascular events. Side effect profile for both drugs appear similar and are dose dependant.Hydrochlorothiazide is also sometimes used to prevent osteopenia and for treatment of hypoparathyroidism, hypercalciuria, Dents disease, and Ménières disease.