DURING the past 25 years, six independent groups of workers have measured the decrease in concentration of particles in exhaled air during steady breathing, while their subjects inhaled various aerosols, and have expressed their findings as a relationship between particle size and lung deposition. These results fall into two ranges, three of the groups of workers obtaining much higher values for deposition than the other three for particles between 0.5 and 6µ diameter. The difference cannot be explained by the nature of the particles or the method of breathing1.