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75
Sewer air may possibly serve to carry organisms derived from
sewage, though probably to a very limited extent. Since some
infective diseases are sewage borne, there must exist a still more
remote chance that they may be thus conveyed. I would add my
opinion that where sewer air escapes freely into the open air, it is
at once so freely diluted that the chance of disease being thus
conveyed becomes almost negligible. It is further more than
probable that the effect of sewer air escaping in concentrated form
into houses is due as much, or more, to some traces of a chemical
poison too minute for chemical recognition, than to any pathogenic
micro-organisms which it may convey.
The subsequent experiments carried out by Dr. Andrewes on
behalf of the Local Government Board afford much stronger evidence
than the Hampstead experiments did in disproof of the
opinion that sewage organisms are not found in sewer air. This
later evidence emphasizes the bacteriological importance of sewer
air, and does not justify the optimistic view held by Dr. Andrewes
in 1905. The conclusions he has now reached are expressed in
the following extract from his report to the Local Government
Board.
Sufficient evidence has been brought forward in Sections III.,
IV., and V. of this report to justify the conclusion that under many
ordinary circumstances characteristic sewage bacteria are to be
found in the air of drains and sewers. The view, till recently
current, that sewage does not readily give up its bacteria to sewer
air is shown to be incorrect. The evidence is of more than one
kind, and it all points in the same direction.
(1) In Section III. I have shown that the streptococci of drain
air when tested as to their biological characters, are found
to correspond, almost in their entirety, with those of
sewage and only to a slight extent with those which chiefly
abound in fresh air. The types of streptococci which are
most abundant in fresh air were not found in the air of
sewers and drains. The evidence obtained from the
streptococci of drain air in the City of London is in
harmony with that previously obtained from the streptococci
of the air of main sewers in Hampstead.
(2) In Section IV. I have shown that bacilli of the colon group
obtained from drain air, correspond essentially in their
biological characters with those of sewage, and that such
bacilli can only be obtained from fresh air with great