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195
It is customary to take the degree of C02 present in air as
indication of its purity or otherwise. Carbonic acid gas (or carbon
di-oxide) is a normal constituent of the atmosphere, forming 3
volumes out of every 10,000 volumes of fresh country air. In
ordinary town air this amount is therefore always present, but to
it has been added products of respiration and combustion, and
accordingly there is always present in such air a "permissible"
impurity, that is to say a degree of impurity which is unavoidable.
Dr. Chaumont fixed this at .02 per cent., or 2 parts in every
10,000. Anything beyond this would be an impurity. Carnelly,
Haldane and Anderson fixed the standard of permissible impurity
at 6 parts per 10,000 in dwellings and 9 in schools. The general
rule now is that C02 is reckoned an impurity if amounting to
more than 5 parts in 10,000 of air (i.e., 3 parts in fresh air
plus 2 as permissible impurity). Angus Smith found it present
as follows:—
Volumes of CO2
in 10,000
volumes of air.
On mountains and moors in Scotland (mean of 75
analyses)
3.36
In the streets of Glasgow (mean of 42 analyses) 5.02
London. E.C. (mean of 12 analyses) 474
Manchester streets, ordinary weather 4.03
During fogs in Manchester 679
Dr. Russell found the average for London air at St. Bartholomew's
Hospital was 4 parts per 10,000, and during foggy
weather 7 per 10,000. In Finsbury street air, it will be seen
that Mr. Colwell found CO2 to be present to the extent of 4.3
per 10,000 (mean of six analyses), in a typical above-ground
Bakehouse it was 4.9, and in underground Bakehouses it varied
from 12.0 to 17.5 per 10,000. His results may be set out as
follows:-