( ( ( ( ( ( ( The mass ) ( of ( ( dust ( in ( the air ) ) ) ( -LRB- ( ( usually ( milligrams ( of ( dust ( per ( ( cubic meter ) ( of air ) ) ) ) ) ) ) -RRB- ) ) ) ) ) ( may ( be measured ) ) ) , ) or ) ( ( ( the number ) ( of ( ( dust particles ) ( in ( ( the air ) ( -LRB- ( ( ( ( particles ( per ( cubic foot ) ) ) or ) ( ( per milliliter ) ( of air ) ) ) -RRB- ) ) ) ) ) ) ) ( can ( be measured ) ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (NN mass)) (PP (IN of) (NP (NP (NN dust)) (PP (IN in) (NP (DT the) (NN air))) (PRN (-LRB- -LRB-) (VP (ADVP (RB usually)) (VBN milligrams) (PP (IN of) (NP (NP (NN dust)) (PP (IN per) (NP (NP (JJ cubic) (NN meter)) (PP (IN of) (NP (NN air)))))))) (-RRB- -RRB-))))) (VP (MD may) (VP (VB be) (VP (VBN measured))))) (, ,) (CC or) (S (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NN dust) (NNS particles)) (PP (IN in) (NP (NP (DT the) (NN air)) (PRN (-LRB- -LRB-) (NP (NP (NP (NNS particles)) (PP (IN per) (NP (JJ cubic) (NN foot)))) (CC or) (NP (PP (IN per) (NP (NN milliliter))) (PP (IN of) (NP (NN air))))) (-RRB- -RRB-))))))) (VP (MD can) (VP (VB be) (VP (VBN measured))))) (. .)))
The mass of dust in  the air (usually milligrams of dust per cubic meter of air) may be measured, or the number of dust particles in the air (particles per cubic foot or per milliliter of air) can be measured.
(ROOT (S (NP (JJ Meter) (JJ much) (NN distance)) (VP (MD can) (VP (VB beta) (NP (JJ particles) (NN travel)) (PP (IN through) (NP (NN air))))) (. .)))