AEROBIOLOGY 
collected over water or for aquatic forms collected 
inland. The latter technique has been successfully em- 
ployed by ZoBell and Mathews [40], Rittenberg [26], 
and Jacobs [14] in their analyses of data on the micro- 
bial populations of marine air. 
In the investigation of the local dissemination of 
organisms the problem of identification of origin is 
usually less difficult. In such cases the collections can 
frequently be made near or above a source which is 
readily identified because of a local anomaly in surface 
cover or because the spore formers or pollinators can be 
isolated with a reasonable degree of fineness. Proctor 
and Parker [24] have suggested the artificial intro- 
duction into the atmosphere of an easily identified 
organism for studying dispersion but, because of the 
rapidity with which small organisms are dispersed 
throughout large volumes of air and the concurrent 
impracticability of introducing sufficiently large num- 
bers of organisms to allow reasonable chance for a 
sample recovery, it appears that such a method would 
be limited to studies of purely local transport. The use 
of stained insects has been employed for the latter 
purpose with some success [30]. 
From the standpoint of the meteorologist, who is 
primarily concerned with the atmospheric processes 
involved in the dissemination of organisms and less 
interested in the results of such dissemination, it ap- 
pears that the most pressing need in aerobiologic re- 
search is the establishment of a standard list of “‘bio- 
logical indicators” or ‘‘markers” (as they are called 
by Stakman [31]). This- list should contain properly 
described and representative spores, pollens, molds, 
bacteria, and, perhaps, insects that can be easily identi- 
fied and whose local sources have been determined. 
The sources represented by the type specimens should 
be mutually exclusive as to character of surface and/or 
geographic location. In the case of fungi, considerable 
care must be exercised in selecting species that produce 
a vast number of spores in a relatively short time; they 
should not multiply on dead vegetation or survive in a 
given region from one year to another [8]. 
From the standpoint of agriculturists, medical re- 
searchers, and workers in related fields, who are more 
interested in the results of dissemination than in dis- 
semination processes, the primary need in aerobiology 
is the perfection of techniques for collecting and identi- 
fying organisms of pathogenic importance and in the 
determination of circumstances governing their sur- 
vival and multiplication after they cease to be air- 
borne. This segregation of interest is meant as no 
reflection upon the attitude of the meteorologist, for 
it is taken without question that the physical processes 
governing the distribution of pathogenic and allergenic 
forms by the atmosphere are identical to those govern- 
ing the distribution of the nonpathogenic and non- 
allergenic species. 
The Atmosphere as an Environment for Organisms. 
From the pathological standpoint the viability of or- 
ganisms transported by air currents is as important a 
consideration as is the distance to which they are 
carried. As stated by Stakman [31], “To show that 
1105 
propagative bodies of pathogens are disseminated by 
the wind is only the first step in finding out what needs 
to be known. Are they viable at the end of their trip; 
how long will they remain viable, even if on the host 
plant,.if conditions are not favorable for germination 
and entrance into the host soon after their trip; do they 
belong to a virulent race; is the host plant in susceptible 
condition; are [meteorological] conditions favorable for 
rapid multiplication and spread if initial infection oc- 
curs? These are only a few of the most important 
questions that must be answered if the problem is to 
be studied in its broadest aspects.” 
It is, first of all, obvious to the meteorologist that the 
organisms must be able to withstand great extremes of 
temperature, pressure, humidity, and solar radiation if 
they are to survive transport over great distances. 
From the standpoint of temperature it may be argued 
that because solid particles (thus all organisms) in the 
air radiate and absorb very nearly as a black body, they 
must undergo extreme temperature changes which are 
not represented by temperature changes within the air 
mass. However, the heat capacity of the organism, or a 
particle with which it is associated, must be very small, 
thus it can be assumed that the temperature of the 
organism at any instant must be very nearly the 
temperature of the ambient atmosphere. It is therefore 
doubtful if such organisms are ever subjected to ex- 
cessively high temperatures. Certain organisms prob- 
ably succumb to the very low temperatures that exist 
at high altitudes [84] although there are some forms 
which survive temperatures approaching absolute zero 
[39]. 
Bacteriologists have shown that exposure to ultra- 
violet radiations for short periods is lethal to most 
bacteria. In this connection, however, it is of particular 
interest to note that bacteria which are killed when 
suspensions (or agar plates streaked with suspensions) 
are exposed directly to sunlight are the same organisms 
which have been recovered from air by exposing agar 
plates for different periods of time. It appears that at 
least the air-borne parent cells are relatively resistant 
to sunlight. Whether this resistivity is due to extrinsic 
or intrinsic conditions is not clear. Some results [29] 
indicate that certain individual bacteria within a species 
survive dosages of ultraviolet radiation which are or- 
dinarily lethal to the species, but that the resistivity 
of the individual bacterium does not persist in the 
subculture progeny. On the other hand, the writer will 
show later that many bacteria suspended in the at- 
mosphere must be associated with a dust particle of 
some sort (such as surface debris or sea salt) or a 
water droplet, and this particle may afford sufficient 
protection against the lethal type of radiation. Never- 
theless, the bacterial counts should show a diurnal 
variation due to this cause. 
There is the possibility that desiccation of the or- 
ganism in dry air may render it nonsusceptible to 
ultraviolet radiations because of its reduced water con- 
tent. There appears to exist, however, a marked 
difference of opinion among investigators concerning 
the effect of humidity. Rentschler [25] concludes from 
