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.
