THE HYGIENISTS AND THEIR INVESTIGATION OF THE AIR While the causes of infectious diseases of man and animals were being unravelled in laboratories and clinics, a series of field investigations into the air-spora was in progress to find whether fluctuations in number and types of microbes present in the atmosphere were connected with out breaks of such diseases as cholera, typhoid, and malaria.
Salisbury (i866) investigated the air-spora in connexion with malaria in the Ohio and Mississippi Valleys, by exposing sheets of glass above marshy places during the night and examining them microscopically. He observed small, oblong, Palmella-like cells singly or in groups on the upper side of the glass sheets, but never in the droplets which formed on the underside. He believed that these cells were produced from a grey mould growing on the surface of prairie soil, and were in fact its spores which were liberated at night and rose some 3o to loo ft. in the air, none being present during the daytime. Their liberation could be prevented by covering the ground with a layer of quicklime or straw.
Some form of the `aeroconiscope', invented by Maddox (187o, 1871), was in favour with many investigators in this period. The model used by Cunningham (1873) consists of a conical funnel, with the mouth directed into the wind by a vane, ending in a nozzle behind which is placed a sticky microscope cover-glass on which were impacted dust particles driven into the cone by the wind (Fig. 2). Cunningham's studies were made in two Calcutta gaols where cholera and other fevers were rife, and where medical statistics were available. He sampled for 24-hour periods, and illustrations of representative catches of airborne organisms, mainly fungus spores and pollens, were published in a series of splendid colour plates. He found no correlation between these micro-organisms and the incidence of fevers in the gaols. Moist weather diminished inor ganic dusts, but it appeared to increase the total number of fungus spores.
The most intensive sustained analysis of bacteria and moulds in the atmosphere was made in Paris during the last quarter of the nineteenth century. Largely through the influence of the chemist, J. B. A. Dumas, the Observatoire Montsouris was launched as a State institution in 1871 to make records needed for meteorology and agriculture. The Observatoirc was housed in a palace in the Parc Montsouris, about 5 km. south of the centre of Paris. One of its tasks was to be the microscopic and cul tural study of the organic and inorganic dust in the air, including both lNIucedincac (moulds) and bacteria.
Observations were started in 18i5 by M. Schoenauer. He was re placed after a year or two by Pierre Miguel (b. 185o, d. 1922), the dis tinguished bacteriologist, who continued in charge of the work for over a quarter of a century. During the course of the survey, various methods were tested and discarded or improved; but all aimed at estimating the number of particles of various types contained in a measured volume of air. Moulds were at first estimated microscopically in a 2.4-28 hour deposit, obtained by impinging the air to be sampled on a glycerined glass slide which was placed horizontally 2 to 3 mm. above a downward-facing orifice. The diameter of the orifice was from o•5 to o•75 mm. Suction of 20 litres per hour was maintained by a water-operated pump (Miguel, 1879). Miguel found that this apparatus yielded about too times as many particles as the aeroconiscopes designed by Maddox and Cunning ham, though for qualitative work away from the laboratory he still used a wind-operated trap of the Maddox type.
Bacteria, especially bacterial spores, could not be satisfactorily counted microscopically and Miguel was forced to estimate them by cultural methods. At first he drew known volumes of air through liquid media (sterile beef extract, etc.), partitioning the liquid either before or after exposure into 5o or too vessels, and adjusting the volume of air sampled so as to leave from a quarter to a half of the vessels sterile—in order to get a reliable estimate of the number of bacterial particles in the volume of air sampled. The numbers of microbes in the air varied greatly in the same place at different times, and this variation was studied in relation to season, weather, district, and altitude. Miguel was the first to make a long-term survey of the microbial content of the atmosphere by volu metric methods.
In the Parc Montsouris, out-of-doors, Miguel estimated that the mould spores averaged about 30,00o per cubic metre in summer, some times rising to 200,000 in rainy weather. In prolonged dry weather they decreased in number, and were only about i,000 per cubic metre in winter, with very few indeed when snow was on the ground. While rain was falling the numbers of mould spores usually decreased considerably, but afterwards their numbers recovered quickly—in fact, much more quickly than did those of particles of inorganic dust. Resting stages (eggs) of infusoria were estimated at about t or 2 in io cubic metres of air. Pollen grains in June may make up 5 per cent of the airborne organic particles, while starch grains near habitations may account for per cent. Bacterial numbers out-of-doors in the Parc Montsouris were at first estimated at about ioo per cubic metre; but improved culture media increased this figure by a factor of 7 to lo times. The numbers of bacteria in the centre of Paris were, perhaps, to times as high again as in the Parc Montsouris, with larger numbers inside dwellings, and still more in crowded hospitals. The work showed signs of settling into a steady routine with the publication of Miquel's Les organisnres vivants de l'atmosphere, Paris, 1883.
However, in 1883 and 1884 Miguel was stung into a burst of renewed activity by the intrusion of a rival centre for the study of hygiene which had been established in Berlin under W. Hesse, who used the new solid media which Miguel abhorred. With the collaboration of de Freudenrich in field work, Miguel studied the microbial population of the air at high altitudes in the Alps by volumetric methods (1884, p. 524); with the help of a sea captain, M. Moreau, the air over the sea was studied on voyages to Rio do Janeiro, Odessa, Alexandria, and La Plata; the micro-organisms brought down in rain-water were caught, precipitated, and counted; hourly variations of fungus spores and bacteria in the air were studied on improved volumetric traps with sticky slides, or on paper impregnated with nutrient media and moved by clockwork. At Montsouris, fungus spores showed a diurnal periodicity with two maxima at about 8 and 20 hours, regardless of wind velocity. When he pressed the study of changes in spore content of the air with passage of time still further, Miguel found that the hourly reading was merely a smoothing of still shorter-term variations.
Trapping airborne bacteria at Montsouris on a moving paper disc imbibed with nutrient agar, Miguel (1885) observed a regular diurnal periodicity—with two maxima at approximately 7 and 19 hours averaging about 75o per cubic metre, and with two minima at approximately 2 and 14 hours averaging about 15o per cubic metre. This periodicity was not related to wind direction, and was not altered by moderate falls of rain. In the centre of Paris the bacterial content also showed two maxima and two minima, but there the minima were about equal to the maxima at Montsouris, and the times of the maxima were closely related to activities in the city such as sweeping the street, and to the passage of horse-drawn traffic.
Miguel appears to have been overwhelmed by the richness of the information on the mould spore flora provided by his apparatus, for he promptly abandoned it, merely remarking `the micrographer who has the leisure could make some nice [curieuse] studies of this subject'. It was, however, not abandoned before the main elements in the mould-spora had been discovered by this excellent method.
Interest in the mould-spora waned when it became clear that the devastating epidemic diseases prevalent from time to time in cities were not fungal in origin but were due to bacteria, and attention became ur gently focused on drinking water as the source of many of the current epidemic fevers abounding in Paris. The laboratory at Montsouris then became the centre for the bacterial analysis of samples of drinking water sent from wells in Paris and other parts of France.
Meanwhile, in Germany, the work of W. Hesse (b. 1846, cf. had proceeded along similar lines. Hesse's apparatus for air sampling con sisted of a narrow horizontal tube, 7o cm. long and 3.5 cm. wide, con taining a layer of Koch's nutrient gelatine. A known volume of air was aspirated slowly through the tube, and micro-organisms settled and grew on the medium. Most colonies developed near the entrance to the tube, and Hesse assumed that by the time the slow stream of air had reached the end of its 7o cm. course all micro-organisms had been precipitated by gravity. Hesse found that moulds penetrated much farther into his tubes than did the bacteria, and made the important deduction that mould-germs as found in the atmosphere are on the average lighter than the bacterial germs. This led him to conclude that, whereas fungus spores were usually present in the air as single particles, the aerial bacteria mostly occur in the atmosphere either as large aggregates, or attached to relatively large carrier particles of dust, soil, or debris (Hesse 1884, 188S). He also observed that most colonies consisted of a single species—bacteria usually in small colonies of pure culture, and fungi as isolated spores— and deduced that the airborne germs are not in the form of aggregates of different types.
Hesse's method was also used in London by Frankland (i886, 1887) and Frankland & Hart (1887) on the roof of what is now known as the Old Huxley Building of the Imperial College of Science and Technology, and elsewhere. Simultaneous comparisons were made between the number of micro-organisms per io litres (as indicated by colonies growing on Hesse's tubes of peptone gelatine) and the number deposited on horizon tal dishes of the same medium, expressed as the number deposited per unit area per minute. Tests were made both outdoors and inside crowded or empty buildings. Frankland noted that the number of colonies was greater when the mouth of the tube faced the wind rather than in other directions, so he standardized his method by always turning it at an angle of 135° to the wind. A control tube facing the wind but not aspirated was always used, and sometimes it had a substantial number of colonies. Frankland seems to have been the first to realize that aerodynamic effects are of major importance in techniques for trapping the air-spora.
These methods for studying the air-spora were continued into the present century, notably by Saito (1904, 1908, 1922) in Japan, and by Buller & Lowe (191i) in the Canadian Prairies.