Round opacities with an ILO profusion score of ≥ 0/1 were observed in 387 cases (15%), those with an ILO profusion score of ≥ 1/0 were observed in 99 cases (3.8%), and those with an ILO profusion score of ≥ 1/1 were observed in 3 cases (0.15%). Type p opacities (ie, diameter, ≤ 1.5 mm) were predominant at the first check-up (82%), and type q opacities (ie, diameter, 1.5 to 3 mm) were predominant at the third check-up (59%). Type r opacities (ie, diameter, 3 to 10 mm of diameter) were observed in only one case. Category 1 opacities began to appear after a minimum exposure period of 16 years, with an apparently exponential relation to the period of exposure (Fig 1
). Irregular opacities with an ILO profusion score of 1/1 were found in two cases, and those with an ILO profusion score of ≥ 1/0 were found in 20 cases. We would point out that there is a discrepancy between the number of cases with opacities that appear in Tables 1 and 2
due to the frequent changes in the kind of work performed occurring among these workers. The information obtained from the third survey of work history was thought to be the most reliable. This means that data are missing about the type of work performed, while other data previously obtained appear in other variables (ie, opacities and accelerated FEV1 decline). The type of irregular opacities was s/t in 82% of cases and s/s in the rest (s denotes a width ≤ 1.5 mm, and t denotes a width of 1.5 to 3 mm). Round opacities are significantly related to work risk evaluated at three levels according to predicted exposure to silica, with a significant lineal trend (Table 1). The relation approaches significance in multivariable analysis including tobacco use (Fig 1). The presence of round opacities bear a significant relationship with tobacco use in the overall analysis, with a lineal trend, and also in multivariable analysis including type of work. The number of cases with category 1 round opacities was 9, 68, 0, and 11, respectively, in subgroups R0S0, R0S1, R1S0, and R1S1, and incidence for 105 person-years was 97.4, 244.4, < 120, and 473.3, respectively. The incidence was noticeably greater in the R1S1 group, in which the two risk factors (ie, rock work and tobacco use) were combined, indicating a possible interactive effect. There is a potential reliability issue. This may indicate synergy, however, it may not be prudent to reach this conclusion as the R1S0 group consisted of only 42 persons. With regard to irregular opacities, a relation with tobacco use is suggested rather than with work risk, although, due to the small number of events, a valid statistical correspondence cannot be established.