32。6 ± 0。538 ◦C, respectively)。 Therefore, it is expected that

forming workers might be exposed to higher oil mist concen-

trations than threading workers by considering the emission of oil droplets caused by the impaction force, and the generation of oil mists due to the evaporation and condensation processes。 The above inconsistency might result from one or more of the following facts: (1) threading process contained more emission sources (i。e。, 15 threading machines) than forming process (i。e。, 13 forming machines); (2) the workplace area of the forming pro- cess (=734。4 m2) was much bigger than that of threading process (=194。7 m2); and (3) more enclosure was found in each forming machine (opening = 0。60–1。82 m2) than that in each threading machines (opening = 3。96 m2)。

Finally, we found the heat treatment process had the low- est concentration among the three selected industrial processes (p < 0。005), which warrants the need for further discussion。 In fact, we did find that the temperatures measured from those

MWF tanks used in the heat treatment process (=97。1 ± 2。34 ◦C)

were much higher than the other two process temperatures。 How- ever, we also found that all MWF tanks were heated only ∼2h per day to meet the heat treatment purpose。 The above scenario might explain why the lowest oil mist levels were found in the heat treatment workers。

3。2。Particle size distribution of oil mists in the workplace atmosphere

Fig。 1 shows particle size distributions of oil mists in the atmosphere of the three selected workplaces。 Table 3shows the MMADs and GSDs for both coarse mode (i。e。,MMADc,

GSDcfor dae ≥ 3。5 µm) and fine mode (i。e。,MMADf,

GSDf fordae < 3。5 µm)forsizedistributionsofoil mists

obtained from this study。 For MMADc, it can be seen that forming (=13。0 µm) > threading (=9。20 µm) > heat treatment (=8。16 µm)。 The above results were quite consistent with the results obtained from a clutch manufacturing plant (>8 µm) [33]。 It is known that the coarse mode oil droplets were mainly generated by the mechanical force。 Therefore, the magnitude of MMADc could be affected mainly by both the magnitude of the involved impaction force and viscosity of the involvedMWF。

For both forming and threading processes, the viscosity of the involved MWF for the former (115。6 cSt at 40 ◦C, 12。2 cSt at 100 ◦C) was lower than that used in the latter (183。7 cSt at 40 ◦C, 17。2 cSt at 100 ◦C)。 Based on our results, it suggests that MWF with a lower viscosity could result in the generation of

Table 3

Particle size distribution of oil mists collected from workplaces of the three selected industrial processes in the fastener manufacturing industry

Industrial process Fine mode Coarse mode

MMADf (µm) GSDf (µm) Fraction (%) MMADc (µm) GSDc (µm) Fraction (%)

Forming (n = 4) 0。499 2。02 73。5 13。0 1。34 26。5

Threading (n = 4) 0。501 1。65 62。3 9。20 1。57 37。7

Heat treatment (n = 4)