AEROBIOLOGY 
By WOODROW C. JACOBS 
Headquarters, Air Weather Service, Washington, D. C. 
INTRODUCTION 
The field of extramural aerobiology is concerned 
with the distribution of living organisms by the exterior 
atmosphere and with some of the consequences of this 
distribution. It includes within its sphere of interest 
a study of the dispersion of insect populations, fungus 
spores, bacteria, viruses, molds, and pollens—in fact, 
all forms of life, both plant and animal, that are borne 
aloft and transported wholly or im large part by the 
atmosphere. 
The present-day field of aerobiology had its origin in 
the pioneer experiments of Spallanzani in 1776, and in 
the work of Pasteur, Tyndall, and others who used the 
method of the aerobiologist in combating the theory of 
the spontaneous generation of life and in developing 
the germ theory of disease. It has been only within the 
last fifteen or twenty years, however, that aerobiology 
has emerged as a specialized field of investigation. An 
examination of the very extensive literature on the 
subject accumulated during these later years reveals 
that activity in the field has been largely confined to 
the biologist and has seldom included the meteorologist. 
Few of the biological data appear to have been ana- 
lyzed from the standpoint of fluid mechanics which 
leaves the quantitative meteorological approach, based 
on theory and supported by meteorological-biological 
observations, as something that remains largely for the 
future. 
The biologist has directed his attention primarily to 
the problems of entrapment, identification, and enu- 
meration of organisms carried by air, but has more re- 
cently become interested in the atmosphere as the 
medium for the dispersal of the organisms. The mete- 
orologist, on the other hand, has as yet shown com- 
paratively little interest in the results of aerobiological 
investigation as a possible means for acquiring new 
knowledge on the nature of mixing processes in the 
atmosphere and on the movements of air masses. Never- 
theless, the ever-increasing concern of the botanist, 
zoologist, geneticist, agriculturist, allergist, the general 
public, and, more recently, the militarist, in the results 
of the atmospheric dissemination of organisms, whether 
the latter be injurious insects, allergens, or pathogens, 
shows clearly that the subject is one of great impor- 
tance and of wide general interest. 
BIOLOGICAL FACTORS 
Methods and Problems of Sampling and Counting. 
From the standpoint of the biologist the ultimate 
quantitative goal of atmospheric biological research is 
the determination and/or prediction of the type and 
number of viable organisms suspended in a given 
volume of air. For those microorganisms which may be 
readily identified by their shape or superficial surface 
characteristics and which cannot be grown on culture 
media, the adhesive-coated slide is widely used as a 
sample trap. A glass collecting slide is normally ex- 
posed horizontally to the air and, although particles 
are deposited by the combined action of turbulent air 
motion and by gravity, the technique is commonly re- 
ferred to simply as the “gravity slide” method of 
collection. The method has several important disadvan- 
tages from the meteorological point of view. First, com- 
paratively long exposures are required—24 hours being 
the time unit commonly used. The method, therefore, 
does not afford a means for determining the degree of 
atmospheric contamination at a given moment or 
through a given short time interval which, of course, 
limits the use of the data in the ‘“‘synoptic”’ sense. The 
second, and principal, criticism of the gravity method, 
however, lies in the fact that the catch does not repre- 
sent recovery from a definite volume of air. Attempts 
to convert gravity slide figures into volumetric terms 
have not been too successful. In spite of the obvious 
disadvantages of the method, a large proportion of our 
present statistics on pollen and fungus-spore distribu- 
tion has been obtained from these gravity slide samples. 
For the study of mold spores and bacteria which are 
too small for convenient direct microscopic examina- 
tion, it is necessary to examine cultured specimens and 
count the colonies of each type of specimen. This type 
of sample is obtained by exposing standard Petri dishes 
containing suitable culture media to the air for periods 
ranging from two minutes to one-half hour. This is the 
so-called “plate method” of collection and suffers from 
the same disadvantage as the gravity slide technique 
in that the results cannot be interpreted in volumetric 
terms. The samples obtained by this method represent 
organisms released from the air during the brief period 
of sampling and no record whatsoever is obtained during 
the long gaps between exposures. The method is also 
time-consuming and expensive, and to secure reason- 
ably complete data from only a few collecting locations 
requires the support of a large staff of laboratory 
technicians. 
Because it is necessary to identify and count the 
microorganisms after they have been captured, it ap- 
pears that some sort of impingement-slide method of 
collection offers the greatest promise of adaptability 
to aerobiological work. For surface collecting the equip- 
ment should be economical, simple to operate, and 
capable of sampling known volumes of air. Above all, 
the equipment, the techniques of its use, the method of 
counting, and the units of measure for reporting results 
1103 
