AEROBIOLOGY 
particles as an average for all levels gives a figure of 
only 4.7 collectable bacteria (or molds) per cubic meter 
of air. If one considers the great number of bacteria 
that must exist in the lowest layers of air, these results 
show that either a very small percentage of the or- 
ganisms survive or that the original number is dis- 
persed rapidly throughout exceedingly large volumes of 
air. It is, perhaps, also possible that the processes 
governing the removal of organisms from the atmos- 
phere are more effective than has previously been 
assumed. 
The processes governing the exchange of spores and 
pollens between the earth and atmosphere are not 
wholly analogous to those governing the exchange of 
bacteria and molds between soil and air. The pollens 
and spores of fungi present a wide range of types with 
respect to their morphology, physiology, and mode of 
production and liberation [15]. Consequently they vary 
greatly in adaptation to aerial dissemination. In the 
case of certain fungi, very elaborate mechanisms exist 
for the forcible ejection of the fungus spores from their 
sporophores or spore mother-cells into the air. The force 
with which the spores are ejected varies greatly with 
- different species, the most common ejection distances 
being of the order of one or two millimeters to several 
centimeters. Although many fungi produce an almost in- 
’ comprehensibly large number of spores,” none of the 
latter appear to be provided with special adaptations 
for flotation as is the case with some seeds and insects. 
Of the various classes of organisms so far discussed, 
only the msects are capable of aerial locomotion. Their 
initial presence in the lower layers of the atmosphere 
and, to some extent, their vertical distribution and 
local dissemination are due to the flight activity of 
the insects themselves. Even here, however, the activity 
of the insect is dependent in some degree upon such 
meteorological factors as temperature, wind, and hu- 
midity [10, 20, 33, 37]. Some nonflying insects are 
specially adapted to aerial dissemination, good ex- 
amples being several species of spiders whose young spin 
webs from some elevated object to be later carried with 
the wind. Glick reports that at times great masses of 
such webs are found floating in the air, even at alti- 
tudes of 7000 to 10,000 ft. 
A large number of seeds are particularly well adapted 
to aerial dissemination through very effective struc- 
tures that increase their frictional resistance to air. 
Because of the size of the seeds and their very common 
occurrence, knowledge regarding them is more general 
than is true of smaller organisms. 
Viruses which are produced within the tissues of 
plants or animals do not appear particularly well adap- 
ted to aerial dissemination except as they may be 
carried by insect vectors or birds. There is the pos- 
sibility, however, that such virus material may oc- 
2. Christensen [3] cites as an example of the remarkable 
power of reproduction of many plant pathogens, that Ustilago 
zeae (corn smut) in two weeks may produce a gall of 20-40 
cubic inches, each cubic inch of which contains about six 
billion spores. One acre of corn with 10 per cent infection 
would produce 5 X 10 spores during the same period. 
1107 
casionally be associated with bits of organic matter or 
microorganisms in the atmosphere. Additional research 
on the possible (free) aerial dissemination of virus cub- 
stances is needed. 
In the case of marine bacteria, it is only when the 
sea surface is stirred sufficiently to produce spray that a 
mechanism exists for the introduction of the sea-surface 
organism into the atmosphere. The spray droplets will, 
of course, include any bacteria present in the sea water 
and subsequent evaporation of the droplet will leave the 
organism associated with a salt particle or droplet of 
concentrated sea water. Since such particles are ex- 
tremely effective nuclei for the condensation of water 
vapor in the atmosphere, they are more readily re- 
moved than are the nonhygroscopic dust particles which 
may contain soil bacteria. Since the number of marine 
bacteria in a cubic centimeter of sea water seldom 
exceeds 500, it can be computed (on the basis of data 
concerning evaporation from spray)? that the popu- 
lations of marine bacteria in the air must be sparse 
everywhere except, perhaps, at times of rough sea 
or in the vicinity of a coastal “breaker zone.” 
An understanding of the physical processes which 
serve to remove organisms from the atmosphere once 
they have become air-borne is as important to the 
aerobiologist as is a knowledge of the mechanism 
governing the liftmg of the organisms into the at- 
mosphere in the first place. Most investigators in the 
field of aerobiology have taken great pains to point out 
the usual small size and mass of air-borne organisms, 
and data concerning the computed or observed free 
rates of fall under the influence of gravity (in still air) 
appear in almost every analytical paper.t The present 
author has no desire to minimize the importance of 
small size and mass in furthering the dissemination of 
microorganisms for those are their primary adaptations 
to aerial transport. It should be pointed out, however, 
that of all the forces tending to move the single micro- 
organism vertically, the gravitational factor is merely 
another component of force additive to others usually 
of greater magnitude. The possibility that a single 
pollen grain, spore, or bacterium carried through con- 
vection to an altitude of several kilometers will ever be 
allowed to settle back to earth through the effects of 
gravity alone is exceedingly remote. Some organisms 
of near colloidal dimensions could be considered to 
remain almost permanently in the atmosphere were 
they not brought down to the surface again through 
turbulence or through capture by condensation or pre- 
cipitation products. 
Many investigators have made note of the fact that 
rainfall is an extremely effective mechanism for clearing 
3. On the basis of determinations of the sea-salt content of 
marine air [13], the author has previously computed that the 
number of marine bacteria near the sea-surface source averages 
about five per cubic meter of air [14]. 
4. The free rates of fall of the largest fraction of bacteria, 
spores, and pollens (at sea-level air densities) are within the 
range of 0.1 to 20 mm sec. In some cases the velocities ap- 
proach those predicted by Stokes’ law and in other cases sub- 
stantially reduced velocities are indicated. 
