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HISTORICAL INTRODUCTION 



The air we breathe, like our food and drink, varies in quality from time to 

 time and from place to place. This fact was recognized many centuries 

 before industrialized man assumed the right to pollute the atmosphere 

 with poisonous chemicals and radioactive isotopes. 



In Britain we hold that, 'when the wind is in the East 'tis neither good 

 for man nor beast'. Some places are noted for invigorating air, and some 

 for relaxing air; but it is not yet clear whether these properties are 

 associated merely with differences in temperature, humidity, and move- 

 ment of a gaseous mixture consisting mainly of 78 per cent nitrogen, 

 21 per cent oxygen, and 0-03 per cent carbon dioxide with traces of the 

 inert gases, or whether some other factor or factors are involved. 



Speculations on the Origin of Disease 



Classical \NTiters believed that the wind sometimes brought sickness 

 to man, animals, and crops. Hippocrates, the father of medical science, 

 held that men were attacked by epidemic fevers when they inhaled air 

 infected 'with such pollutions as are hostile to the human race'. A rival, 

 though perhaps not entirely incompatible, view held that epidemics were 

 the result of supernatural agencies, and were to be m arded off or cured by 

 taking appropriate action. 



Lucretius in about 55 B.C. held quite modern views. He observed the 

 scintillation of motes on a sunbeam in a darkened room and concluded 

 that their movement must result from bombardment by innumerable, 

 invisible, moving atoms in the air. This brilliant intuition enabled him to 

 account for many interesting phenomena, including the origin of pesti- 

 lences. We now know that bodies which transmit human diseases through 

 the air are larger than those which Lucretius thought of as atoms — the 

 mosquitoes carrying malaria, for instance, or the droplets which spread 

 the common cold and influenza viruses indoors. But in his concept of 

 baleful particles carried in clouds by the wind, settling on the wheat or 

 inhaled from the polluted atmosphere, Lucretius touched on some of the 

 main problems existing in plant patholog}'' and allergy today. 



Early Microscopists and the Discovery of Spores 



After Lucretius, more than 1,500 years passed before men even 

 began to be aware that the air teems with microscopic living organisms. 

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