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ventilation of the sewers, the better. The systematic
blocking up and closing of road ventilators tends to prevent
this free ventilation, and ought, therefore, to be discouraged ;
nor is there any necessity for the closing of an open ventilator
until other means have first been tried to abate any
nuisance arising from offensive smells in connection therewith—e.g.,
careful flushing and cleansing of manholes;
clearing away of sewer deposits, when found to exist;
increasing the number of open ventilators; erection of
shafts, but only in addition to, and not in place of, open
manhole ventilators, &c.
The necessity for diluting and oxidising sewer air as
much as possible is admitted when it is remembered that
putrefactive changes, taking place in the contents of sewers,
give rise to the evolution of certain organic and other gases
which, if allowed to accumulate under pressure, will tend to
escape into (or near) houses from the points of least resistance,
viz., defective house drains and traps, &c. It is true
that the continued inhalation of these gases for any length
of time in confined spaces tends to produce fever, anæmia,
loss of appetite, depressed vitality, prostration, headache,
sore throat, and general loss of health—a condition of things
that may be produced by such gases escaping through
openings into the sleeping and living rooms of houses, into
narrow streets near to houses, courts or alleys; but not
necessarily when escaping into a fairly wide thoroughfare
(not less than 30 ft.)—the rapid dilution and oxidation
making all the difference in this latter case. In sewers
which are properly laid, carefully planned, well flushed and
ventilated, these putrefactive gases are at a minimum;
but at a maximum in sewers badly laid, defective, unflushed,
and ill-ventilated.