Table 4

Mean inhalable (Cinh), thoracic (Cthor), and respirable (Cres) concentrations and their 95% confidence intervals (values in parenthesis) for workers of the three selected exposure groups (mg/m3)

Exposures Exposure group

Forming Threading Heat treatment

Cinh 1。58 (1。47–1。71) 2。11 (1。89–2。40) 0。0801 (0。0546–0。174)

Cthor 1。47 (1。36–1。59) 1。62 (1。45–1。84) 0。0642 (0。0438–0。139)

Cres 1。34 (1。25–1。45) 1。40 (1。25–1。59) 0。0519 (0。0354–0。113)

Table 5

Estimated mean oil mist exposure concentrations and their 95% confidence intervals (values in parenthesis) at the head (Chead), tracheobronchial (Ctb) and alveolar (Calv) regions for workers of the three selected exposure groups (mg/m3)

Exposures Exposure group

Forming Threading Heat treatment

Chead 0。111 (0。102–0。119) 0。489 (0。439–0。557) 0。0159 (0。0108–0。0345)

Ctb 0。130 (0。121–0。141) 0。222 (0。197–0。250) 0。0123 (0。00838–0。0267)

Calv 1。34 (1。25–1。45) 1。40 (1。25–1。59) 0。0519 (0。0354–0。113)

oil droplets with greater particle sizes。 The above inference was consistent with the observation of a study conducted by Thorn- burg et al。 [34]。 In their study, they found that MWFs used in metal shearing machine with lower viscosity would result in the generation of oil droplets with MMAD (=21。9 µm) greater than those with higher viscosity (=6。10 µm)。 In addition, we also found that the impaction force involved in the forming process was much greater than that in the threading process。 Theoreti- cally, a greater impaction force might result in the generation of oil mists with less MMADs。 Obviously, the above inference was contradictory to the results obtained from this study。 Therefore, in this study, it might be reasonable to conclude that the magnitude of MMADs in oil droplets could be mainly affected by the viscosity of the involved MWFs rather than the impaction forces。 Finally, the smallest MMADc was found in the heat treatment workplace might worth further discussion。 Based on our filed observation, we found that oil droplets were generated at the center of the MWF tank (i。e。, the location where fasteners dropped into the MWF tank)。 Because of this, oil droplets with large particle sizes might not be able to escape from the MWF tank due to the gravitational effect and the large surface area of the MWF tank。

For MMADf, it can be found that threading (=0。501 µm) > forming (=0。499 µm) > heat treatment (=0。309 µm)。 Theo- retically, fine oil mists were generatedthrough evaporation andcondensationofMWFsduringthemanufacturing pro-