Lung function decline in 4-monthly repeated spirometric measurements: due to silt aerosol exposure or decreasing effort?

BACKGROUND Workers on dredgers and lighters on rivers are exposed to the inhalation of aerosols and dusts. OBJECTIVE The aim of this study was to investigate effects of river silt aerosol and dust exposure on the respiratory health of dredging employees. METHODS Six examinations were performed over a period of 2 years at 4-monthly intervals in 54 seamen with higher silt aerosol exposure and 36 controls of the same employer. RESULTS No significant differences could be observed between the groups at any time of the study but there was an unexpected significant decrease in the age-corrected expiratory vital capacity (FVC), forced expiratory volume in 1 s (FEV(1)) and midexpiratory flow rate (MMEF(25/75)) over the six series in both groups. This may indicate a loss of effort of the participants in re-examinations since biological and technical influences were highly unlikely to be the cause of these findings. CONCLUSIONS Ignoring this possible decline of effort in frequently repeated measurements may result in overestimating potential effects of occupational exposure.