( ( Free radicals ) ( ( are ( ( formed ( during ( a ( chain reaction ) ) ) ) ( usually ( when ( oxygen ( reacts ( with ( ( certain substances ) ( in ( the body ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Free) (NNS radicals)) (VP (VBP are) (VP (VBN formed) (PP (IN during) (NP (DT a) (NN chain) (NN reaction))) (SBAR (ADVP (RB usually)) (WHADVP (WRB when)) (S (NP (NN oxygen)) (VP (VBZ reacts) (PP (IN with) (NP (NP (JJ certain) (NNS substances)) (PP (IN in) (NP (DT the) (NN body)))))))))) (. .)))
Free radicals are formed during a chain reaction usually when oxygen reacts with certain substances in the body.
(ROOT (S (NP (NNP Combustion) (NN reaction)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (NN substance)) (VP (VBZ reacts) (ADVP (RB quickly)) (PP (IN with) (NP (NN oxygen))))))) (. .)))