BACTERIA Moving air does not normally detach bacterial cells from the surface of a colony, at least when this is slimy, and in the absence of an active discharge mechanism natural processes capable of producing an aerosol of single bacterial cells are unknown. Mechanical disturbance of dust, clothing, surgical dressings, etc., however, carries into the air contamin ated particles of substratum acting as `rafts' and bearing clumps of bac teria (Bourdillon & Colebrook, 1946). Rafts of soil or dust particles are raised by wind, by `dust-devils' when the ground is heated by solar radiation, and by animal and human activity such as cultivation of bare ground. Rain splash, breakers, and sea spray continuously throw minute, potentially bacteria-laden, droplets into the atmosphere. Droplets expelled by coughing and sneezing arc important indoors (and see p. 158), yet processes which put bacteria into the air are still not satisfactorily known. This is also true of the yeasts whose frequent abundance in the air remains unexplained, except for the Sporobolomycetaceae which show the ballistospore discharge mechanism (pp.