- He selected Denver to represent the atmospheric humidity in an elevated, semiarid
region to contrast with northeastern United States locations with a normal rainfall pattern.
In all these areas studied, where poliomyelitis epidemics occurred, Armstrong noted that
when the atmospheric relative humidity rose above a certain level during the polio season
months, the adjusted relative humidity of warmed inhaled air was 90 per cent.
In the various conclusions in his observations, Armstrong stated that in an attempt
to explain the seasonal incidence of epidemic poliomyelitis, a hypothesis was suggested
that required no assumption of an extra-human source or change of infectivity for the
virus nor any assumed alteration in the susceptibility of the population to infection. On
the other hand, the hypothesis attempted to relate the seasonal behavior of poliomyelitis
to generally observed alterations in the upper respiratory tract due to atmospheric
changes, notably, in temperature and relative humidity of inspired air. The upper
respiratory passages were viewed not only as a portal of entry for the virus but also as a
portal of exit for the virus most effective in transmitting the disease from person to
person.
He elaborated further that when air of usual temperature and humidity was breathed, it
was warmed to a rather constant temperature of 90F and through absorption of moisture
from the upper respiratory tract, its relative humidity was raised to approximately 90 per
cent. A definite correlation between the curve of relative humidity of atmospheric air
warmed to 90F and the curve of incidence for recognized poliomyelitis for the same area,
based upon either monthly or weekly intervals of time supported the view that a dry air at
90F tended to prevent infection with poliomyelitis in a population, while a moist
atmosphere at the same temperature tended to favor its spread. The incidence of cases
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