to compare in - s and in - e among the three groups , the values were log - normally transformed to an approximately normal distribution before analysis , the geometric means ( gm ) and the geometric standard deviations ( gsd ) were calculated , and the steel rank sum test was applied . a single regression model and a single correlation analysis were used to evaluate the relationship between in - e and in - s . the mean age of the subjects was 38.5 years ( range 2063 ) , 87.5% were male , and 47.5% were current smokers . the mean duration of indium exposure was 8.2 years ( range 0.734.7 ) . in - e ranged from 0.004 to 24.0 g / m , and in - s ranged from 0.1 to 8.5 g / l . the gm ( gsd ) of in - e was 0.97 ( 8.68 ) in the smelting workers , 1.22 ( 2.19 ) in the ito workers and , 0.10 ( 6.49 ) in the other workers , respectively . the in - e values of the smelting workers and the ito workers were marginally ( p=0.0709 ) and significantly ( p=0.0069 ) higher than the in - e values of the other workers . the gm ( gsd ) of in - s was 0.93 ( 4.50 ) in the smelting workers , 0.58 ( 3.26 ) in the ito workers , and 0.12 ( 1.62 ) in the other workers , respectively . the in - s values of the smelting workers and the ito workers were significantly ( p=0.011 and 0.022 , respectively ) higher than those of the other workers . figure 1a shows a scattergram of log(in - e ) and log(in - s ) . the simple regression equation was log(in - s)=0.322log(in - e)0.443 , the regression coefficient was statistically significant ( p=0.0002 ) , and the simple correlation coefficient was 0.555 ( 95%ci 0.2900.741).fig . 1 . scattergram of log(in - e ) and log(in - s ) classified by job types.the x - axis represents log(in - e ) and the y - axis represents log(in - s).r : simple correlation coefficient . * : p<0.05 . scattergram of log(in - e ) and log(in - s ) classified by job types . the x - axis represents log(in - e ) and the y - axis represents log(in - s ) . because the distribution of the in - e and in - s values was different among the three groups , scattergrams were displayed for each group ( figs . the simple correlation coefficients were 0.489 ( 95%ci 0.4170.908 ) in the smelting workers ( fig . due to the small number of subjects , the distributions of the in - e and in - s values in the smelting and ito workers were not statistically significant , but they seemed to be linear ( fig . as far as we know , this is the first paper concerning the relationship between in - e and in - s . our research team has been interested in the relationship between in - e and in - s for a long time . however , we believed that it would be difficult to clarify this relationship because we believed that in - s may strongly reflect the amount of indium load in the lungs and , thus , the current in - s would not correlate to the current in - e . the reasons are supposed as follows : ( 1 ) due to the hardly soluble characteristics of indium compounds , the clearance rate of accumulated indium compounds in the lungs is very small7 ; ( 2 ) the in - s levels of formerly indium - exposed workers do not readily decrease4 and the biological half - life of in - s in formerly exposed workers who perfectly removed from indium exposure is around 8 years8 ; and ( 3 ) many indium processing plants have instituted labor hygiene management systems , and in - e and indium inhalation concentration may not be parallel due to the wearing of effective protective devices6 . in this study , we had a chance to measure in - s and in - e in 11 plants that had not yet taken action to improve the work environment . workers in these plants wore no or ineffective respiratory protective devices , and the in - e levels were expected to be low , so the indium load in the lungs of the workers could be expected to be small . however , it should be noted that extrapolation of this study s results to other indium - exposed populations requires careful consideration of whether the indium lung load in the population is actually negligible and whether the population wears appropriate respiratory protective devices . after the log transformation of in - s and in - e , this suggests that in - s may increase in - e - concentration - dependently in the workers in this study . figures 1b , 1c and 1d seem to suggest that the relationship between in - e and in - s may vary due to the chemical form of the indium compounds . ito , indium trioxide , indium metal , or indium alloy was the major chemical form in the ito workers , the smelting workers9 , and the other workers . though the number of workers was very small , in - e was correlated to in - s in the ito workers and in the smelting workers , but not in the other workers . we have no information about the difference in the kinetics of each chemical form in the lung , but indium metal or indium alloy may show different kinetics unlike ito or indium trioxide . first , the number of study subjects was not sufficiently large , and information of relatively high in - e levels was lacking . this may make it difficult to smoothly interpret the results , and indicates a need to expand the study . secondary , day - to - day variations of in - s are considered to be negligible , but day - to - day variations of in - e can not be ignored . we may need to measure in - e on multiple days . in conclusion , in - e and in - s seem to be positively correlated , but more data are needed to draw final conclusions . studies of the kinetics in the lungs of each indium chemical form are also necessary to elucidate the relationship between in - e and in - s . this study was supported by grants - in - aid for scientific research ( project no . 24590758 and 26860443 ) from the ministry of education , culture , sports , science and technology of japan ( 2012 - 13 and 2014 - 2015 ) .
the aim of this study was to assess the relationship between indium exposure concentration in the respirable dust fraction ( in - e ) and indium in serum ( in - s ) in workers . 
 methods : a total of 39 workers were studied . 
 the study subjects were categorized into 3 groups , namely , smelting workers ( n=7 ) , ito workers ( n=6 ) in an ito grinding plant , and other workers ( n=26 ) . in - e and in - s ranged from 0.00424.0 g / m3 and 0.18.50 g / l , respectively . 
 the simple regression equation was log(in - s)=0.322log(in - e)0.443 . 
 the simple correlation coefficients for the smelting workers , ito workers and other workers were 0.489 , 0.812 and 0.163 , respectively . 
 the differences in the relationships among the three groups suggest that in - s may vary with the chemical form to which the workers were exposed . in - e and in - s 
 seem to be positively correlated . 
 the correlation coefficient was higher for both smelting and ito workers than for other workers .