( ( ( ( ( ( ( ( This ( is ( a ( ( very low ) concentration ) ) ) ) , ) ( ( ( ( ( roughly one ) ( radon atom ) ) ( in ( a ( billion-billion ( air atoms ) ) ) ) ) , ) however ) ) , ) ( ( the ( radon atoms ) ) ( are radioactive ) ) ) and ) ( that ( makes ( the difference ) ) ) ) . )
(ROOT (S (S (NP (DT This)) (VP (VBZ is) (NP (DT a) (ADJP (RB very) (JJ low)) (NN concentration)))) (, ,) (S (NP (QP (RB roughly) (CD one)) (NN radon) (NN atom)) (PP (IN in) (NP (DT a) (JJ billion-billion) (NN air) (NNS atoms))) (, ,) (ADVP (RB however))) (, ,) (S (NP (DT the) (NN radon) (NNS atoms)) (VP (VBP are) (ADJP (JJ radioactive)))) (CC and) (S (NP (DT that)) (VP (VBZ makes) (NP (DT the) (NN difference)))) (. .)))
This is a very low concentration, roughly one radon atom in a billion-billion air atoms, however, the radon atoms are radioactive and that makes the difference.
(ROOT (S (NP (NNP Air)) (VP (VBZ is) (VP (VBN made) (PP (IN of) (NP (NNS atoms))))) (. .)))