The size distribution of airborne particles carrying micro-organisms

The ability of a particle to remasn airborne, its ability to pass through filters, the site at which it may be deposited in the respiratory tract and the rate at which it will be removed from the air by sedimentation are all dependent on the size and density of the particle. In the course of a variety of investigations we have determined the size distribution of particles carrying various species of bacteria and fungi using the size-grading slit-sampler described by Lidwell (1959). A few of the results obtained have already been quoted in part, but the majority have not been published previously. The air sampler used separates the airborne particles into four size ranges, each of which is deposited on the surface of the agar medium contained in one of four 6 in. Petri dishes. This apparatus is constructed so that the air sample entering through a slit 7 mm. wide, impinges on to the surface of the first Petri dish at such a velocity that only the larger particles, i.e. those having an equivalent particle diametert greater than about 18,un are deposited. The air stream carrying the smaller particles is then ca.used to impinge in turn on to the surface of the three remaining Petri dishes, each time at an increased velocity and through a narrower slit, so that the minimum particle size for 50 °/0 deposition is about 10/6 for the second dish, 4,u for the third and less than l,u for the fourth and last. When the plates have been incubated the colonies found will be derived from organisms which entered the sampler carried on airborne particles corresponding approximately to the four size ranges, greater than 18ju, between 18 and lO,u, between 10 and 4ju and less than 4,sb. These size limits correspond to the value of equivalent particle diameter for 50 °/0 deposition so that there is, in fact, a considerable size overlap between the fractions. In spite of this however, reasonably good estimations of the particle size distribution within the sample and hence of the median equivalent particle diameter and of the dispersion, expressed either as an interquartile range or, if appropriate, as a standard deviation, caUn be made by plotting on probability paper the cumulative fraction oversize against the 50 °X0 collection limits, namely 18@2, 9@6 and 4 2,s6. As there are internal losses in the instrument it is necessary to correct the numbers of colonies counted in the later stages in order to arrive at a good estimate of the size distribution in the original sample. The numbers found on