 lung burden analysis was performed on 126 autopsy cases of persons who died in new york city from 1966 through 1968 . 
 of the 126 cases , 107 were probably non - occupationally exposed , judging by occupational history and asbestos body content of lung . 
 fifty - three of the 107 cases contained short chrysotile fibers / fibrils , < 5 microns in length , present in 3-fold greater amounts than were found in laboratory background controls . 
 the fiber concentrations ranged from 1.8 to 15.7 x 10(6 ) f / gm / dry lung tissue , and the proportion of fibers > or = 5 microns in length was only 0.34% of the total chrysotile population found . 
 other inorganic particles present included fragments of amphiboles . 
 in contrast to these data , the lung parenchyma of persons occupationally exposed to asbestos commonly showed the presence of other fiber types , especially amosite and crocidolite , at very much higher concentrations and greater fiber length . 
 any chrysotile present would usually be in fiber bundle form , with both fibers and fibrils > 5 microns in length . 
 comparison of the lung fiber content of occupationally exposed persons with that of the general population showed marked qualitative and quantitative differences . 
 fibers are durable , and are retained in a range of concentrations . 
 their length and dose , among other factors , which control their biological potential are different in the two populations ; the risk factors for chrysotile - induced disease are not the same.imagesfigure 1 . 
 afigure 1 . 
 bfigure 1 . 
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