In two studies with plantation workers who had sprayed paraquat over longer periods of time it was concluded that the long term occupational use of paraquat is not associated with lung damage or adverse effects in exposed workers (Senanayake et al 1993), and that it was not possible to show any differences in the lung function between spray workers and general or factory workers (Howard et al 1981). Methods for medical examination in these studies (chest x-rays and spirometric tests of the lung function) were insensitive, except measurement of diffusion capacity of carbon monoxide. Two other studies with workers who had sprayed paraquat over a longer period concluded that working with paraquat under field conditions is associated with desaturation of arterial oxygen during maximum exercise in a dose dependent fashion (but independent of recent exposure and acute poisoning) (Dalvie et al 1999), and that the increased prevalence of respiratory symptoms in the exposed workers suggested an effect of long-term paraquat exposure on respiratory health (Castro-Gutiérrez et al 1997). The hypothesis presented was that subacute exposure to paraquat (to lower doses, possibly over a longer time) may lead to decreased diffusing capacity, often with normal forced vital capacity, and that lung fibrosis is not caused except in cases of an acute and substantial exposure (Castro-Gutiérrez et al 1997; and reference therein: Levin et al 1979). In more intensely exposed workers the relative risk for chronic bronchitis was twice as high (not statistically significant), while for episodic shortness of breath accompanied by wheezing it was 2.9 (95% confidence inter-val: 1.4-6.3) (Castro-Gutiérrez et al 1997). A third study with 338 workers from plantations in Costa Rica found that paraquat exposure was associated with small but statistically sig-nificant changes in gas exchange in the lung; small farms with fewer workers were not in-cluded in the study and levels of exposure could differ there (Schenker et al 2004).