( Doctors ( ( ( have long ) ( known ( that ( ( exposure ( to ( ( ultraviolet radiation ) ( from ( the sun ) ) ) ) ) ( increases ( ( the risk ) ( for ( skin cancer ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Doctors)) (VP (VBP have) (ADVP (RB long)) (VP (VBN known) (SBAR (IN that) (S (NP (NP (NN exposure)) (PP (TO to) (NP (NP (JJ ultraviolet) (NN radiation)) (PP (IN from) (NP (DT the) (NN sun)))))) (VP (VBZ increases) (NP (NP (DT the) (NN risk)) (PP (IN for) (NP (NN skin) (NN cancer))))))))) (. .)))
Doctors have long known that exposure to ultraviolet radiation from the sun increases the risk for skin cancer.
(ROOT (S (NP (NP (NNP Exposure)) (PP (TO to) (NP (JJ ultraviolet) (NN radiation)))) (VP (MD can) (VP (VP (VB increase) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NN pigment)))) (PP (IN in) (NP (DT the) (NN skin)))) (CC and) (VP (VB make) (S (NP (PRP it)) (VP (VB appear) (NP (JJR darker))))))) (. .)))