Figure 3.

Trends in FEV₁ and FVC over time in operators (a), others (b), and welders (c). On the basis of SPIROLA v3.0.2 software. The mean FEV₁ chart shows group means for observed, predicted, and Z-score (standard deviation units from the predicted quantity) values. The same applies for FVC chart. The predicted values are derived from prediction equations that take into account age, height, sex, and race/ethnic background and are based on nationally representative healthy never-smokers. Irregular deviations of observed mean values from predicted values may be due to changes in measurement procedures or due to effects of occupational exposure or interventions.