ABSTRACT

Department of Therapeutics, Llandough Hospital, Penarth, Vale of Glamorgan, U.K.

EARLY STUDIES OF OUTDOOR AIR

Man has long believed that the air could bring disease to man and crops, but it was not until the invention of the microscope in the 17th century that it was possible to observe the array of particles that are carried in the air. With his lens, Antonie van Leeuwenhoek (Dobell, 1932) was just able to observe bacteria. It gradually became recognized that the air carried bacteria, yeasts, fungal spores, spores of mosses and ferns, algae, pollen grains and even protozoa. Initial studies were concerned with the controversy surrounding spontaneous generation of organisms and it was Pasteur (1861) who, by drawing air through gun cotton and after dissolving the gun cotton examining the deposit under a microscope, discovered that the air contained a variety of different particles. However, he did not pursue these studies and the realisation that the air contained a variety of microbes resulted in a concerted effort by medical men to discover the microbes that caused disease (Bulloch, 1938). The original work of Miquel (1899) in Paris into airborne bacteria stands as one of the most sustained series of volumetric measurements of the microbial population of the air ever attempted. Samples were collected over a 16-year period in plugs of gun cotton and after this was dissolved the filtrate was cultured in flasks of filtered saline beef extract. From his studies he discovered that in a park 5 km from the centre of Paris bacteria were nearly three times as numerous in summer as in winter; in the centre of Paris counts were twice as many as in the park, but with a similar seasonal fluctuation. He also sampled a narrow unhygienic street and the main sewer of Paris, which proved to be no more contaminated than the air of the streets outside. In the park there were on average 290 bacteria m−3 air, in the centre of Paris 7480 bacteria m−3, in the unhygienic street 5550 m−3 and in the main sewer 3835 m−3. He also noted a steady annual decline which he attributed to improved street cleaning and washing to lay dust. Miquel came to the conclusion that the source of most outdoor airborne bacteria is the surface of the ground. He also studied the variations during the day and attributed increases during the course of the day to mechanical causes such as road sweeping and traffic. Miquel lost interest in fungal spores and developed media which selectively discouraged mould growth. Fungal spores in the air were then largely ignored until investigated by Cadham (1924), who confirmed spores of cereal rust fungi as a cause of asthma and re-kindled an interest in airborne fungal spores driven by allergists.