160 



PEPI'Kll THEE AS A CAUSE OF HAY FEVER. 



It will bo seen that dust was almost invariably present on 

 the plates and usually in large amounts. Its presence in the 

 air no doubt aggravates the effect of pollen in causing hay-fever 

 by imtating and making more susceptible the mucous membranes 

 of the nose, throat and eyes : moreover its influence is probably 

 cumulative and increases as the dusty season is prolonged. 



Pepper Tree PoUcn: 



Of 27 plates on which pollen was found, no less than 23 had 

 pepper tree pollen on them ; and with one exception, that of a 

 grass whose pollen occurred on three occasions, no pollen other 

 than that of the pepper tree was found more than twice, single 

 grains omitted. 



Pepper tree pollen was, therefore, to an overwhelming de- 

 gree the chief pollen in the air of the town during the period and 

 none of the other pollens were found sufficiently frequently or in 

 sutiicient numbers to suggest that they play an important part 

 in causing the epidemics. 



In Dr. Scheppegrell's investigations (1917) he adopted the 

 number of pollen grains falling on a square centimetre in twenty- 

 four hours as the unit for expressing the density of a particular 

 pollen in the air. He found that with most pollens 25 grains per 

 square centimetre is sufficient to cause hay-fever in most subjects, 

 whilst 100 per square centimetre causes atttacks of considerable 

 severity. 



Table VII gives particulars of the density of pepper tree 

 pollen found at Bloemfontein on eight occasions in 1919. The 

 figure given in the last column was arrived at by working across 

 the microscopic field a varying number of times, and reducing 

 to the area and time adopted in the unit. 



Table VII 



Density of Peppeh Tree Pollen : Bloemfontein, 1919. 



