Relation between the Airborne Diameters of Respiratory Droplets and the Diameter of the Stains left after Recovery

Abstract

SEVERAL workers have investigated the numbers and sizes of droplets expelled during expiratory manoeuvres such as coughing or sneezing. When droplets are recovered by settling, by impaction or by some form of air sampler, it is necessary to calculate the airborne diameter of the original droplet from the diameter of the stain left after recovery. Duguid¹ repeated the experiments of Strauss² in which large droplets of saliva (1–3 mm in diameter) were measured with a micrometer eyepiece, first while they hung from fine glass capillaries and then again after they had fallen, flattened and evaporated on a slide. When a glass slide was used, the diameters of the original droplets were about half those of the stain marks. Buckland and Tyrrell³, using glass slides, calculated the airborne diameter of droplets as 1/2.5 of the diameter of the stain marks left after impaction. May⁴ described the use of glass slides coated with magnesium oxide in estimating the airborne diameter of droplets; droplets landing on this surface will penetrate the magnesium oxide crust like stones falling into snow, and the diameter of the entry-hole is related to the airborne diameter of the droplet. He investigated this relationship for droplets with airborne diameters ranging from 20µ to 250µ, determining the airborne diameter by an “absolute method” which involved trapping the droplet in oil, and measuring it microscopically⁵. He concluded that the airborne diameter averaged 0.858 times the magnesium oxide entry-hole diameter, and that this relationship was independent of the speed of impaction and, within the range investigated, of the diameter of the droplet involved.