THE MICROBIOLOGY OF THE ATMOSPHERE 



of an individual patient who has become sensitive and reacts adversely 

 to substances, often in minute amounts, which normal individuals can 

 tolerate. The substance or allergen can be taken into the body, for 

 example in food, or by contact through the skin, or by inhalation from 

 the air. 



Hay fever was one of these puzzles. Long before Pasteur's epoch, 

 hay fever had been attributed to inhalation of pollen ; but it remained for 

 Charles H. Blackley (1873), a Manchester physician, to prove by in- 

 halation experiments on himself and others that this guess was correct, 

 and to demonstrate by trapping methods that pollen was at times present 

 in the air in large quantities. Blackley first tried Pasteur's gun-cotton 

 filters and obtained some pollens, but too few to satisfy him. Finally he 

 used four sticky horizontal microscope slides exposed under a roof 

 supported by a square central post. The slides were placed at 'breathing 

 level' (about 135 cm.), and he caught a maximum of 880 grains per sq. 

 cm. per 24 hours on 28 June 1866. In 1867 his maximum was only 106, 

 and in 1869 he placed his slides vertically in a vane shelter and gave no 

 numerical data. He found that rain reduced the number of pollen grains 

 caught to about 5 per cent of the number caught in dry weather. He ex- 

 plored the air above the ground up to 1,500 ft. by means of kites, and 

 found that vertical slides facing the wind caught nearly 20 times as much 

 pollen at the higher altitude as at breathing level. 



Blackley showed by means of his sticky slides that the air contains 

 enough pollen during the grass-flowering season for large quantities to be 

 deposited on exposed surfaces. He also gave himself an attack of bronchial 

 catarrh by inhaling Penicillmm and Chaetomiwn spores — an experiment 

 which he said was too unpleasant to repeat. 



According to Durham (1942), after Blackley's pioneer work no progress 

 was made with these studies until the period 1 910-16, when fresh in- 

 terest was aroused by the discovery that injections of pollen extracts can 

 be used to de-sensitize patients who are allergic to pollen. 



When the study of airborne allergens was again taken up in the present 

 century, it was unfortunate that the technique chosen should have been 

 the so-called 'gravity-slide' adopted by Blackley — a method which 

 Pasteur had abandoned in 1861 and which Miquel had roundly con- 

 demned as 'the simplest and most defective method' of collecting air- 

 borne particles. 



By the early years of this century it became possible to assess the 

 value of the ancient belief that the wind brings disease. Many diseases of 

 crop, but very few diseases of man, have proved to be caused by minute 

 particles carried on the wind. The particles are not some sort of invisible 

 atoms as Lucretius thought; indeed, among the motes in the sunbeam, 

 he may himself have been watching some of the baleful fungus spores and 

 pollens which cause crop disease and respiratory allergy. 



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