192 



GROWTH OF PLANTS 



are exposed to 30 to 100 ppm at times. The disaster of the Meuse Valley 

 in 1930 during six days' fog when 63 people died has been attributed to 

 sulphur dioxide, which was estimated to reach concentrations as high as 

 38 ppm. It has since been questioned whether these deaths were due to 

 SO2 and whether it reached anywhere near such a concentration. There 

 are many other statements in the literature concerning the effect of SO2 in 

 the air on health, most of them not well backed by facts. We seem long 

 on assumptions and short on measurements of the effect of SO 2 in various 

 concentrations and for various durations upon animals. The earlier meas- 

 urements of the effects of SO2 on animals were on the dosage basis, that is, 



180 360 540 720 



TIME TILL 50 PER CENT MORTALITY (HRS.) 



900 



Figure 73. Time till 50 per cent mortality versus concentration toxicity curves for 

 mouse and guinea pig in various SO2 concentrations. 



the animal was enclosed in a case and a single dose added, generally a large 

 one, and the effect observed. Sulphur dioxide is absorbed readily by the 

 walls of the case and more readily by the animal. Some later experiments 

 were performed by the continuous air flow method but the concentration 

 was not regulated and recorded accurately. 



Weedon, Hartzell, and Setterstrom " used the accurate continuous flow 

 fumigation chambers at the Institute to study the effect of SO 2 on guinea 

 pigs, mice, grasshoppers, and cockroaches. The concentrations used were 

 10, 35, 65, 100, 150, 300, and 1000 ppm and the duration of exposures ran 

 up to more than 1000 hours in some cases. No significant mortality or 

 signs of distress occurred in healthy animals in 33 ppm, even with 400 or 



