IN a recent communication, Davies and Muir1 pointed out that the experimental results from six independent studies of the retention of particles in the human respiratory system fall into two groups, the results of one group indicating higher values for the deposition than the other. They have attempted to explain this difference by suggesting that in two of these investigations2,3 errors were introduced by the experimental method, whereby the exhaled air was sampled continuously, including during the inhalation period. They said that there was an inaccuracy because the last part of the exhaled air, which had penetrated deeper into the lungs, contained fewer particles than the average. This may be so, but Davies and Muir give a false estimate of the magnitude of the error and do not account for the difference between the two groups of results.