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These two latter stations are fitted with eight port valves
which automatically change every twenty-four hours, thus
ensuring complete records over weekends and holiday periods.
The two stations are also part of the National Survey being
conducted by the Department of Scientific and Industrial
Research.

In the Wan-en Spring Laboratory Report for 1966 one of the
most illuminating sections is on trends in pollution by smoke
and sulphur dioxide. The important reduction in smoke emission
that has taken place in recent years both in and outside London
are cited. In London a decrease in both emissions and ground
level concentration to about one third of that ten years ago has
taken place.
Emission of sulphur dioxide in London has increased by
30% since 1952, due mainly to conversion to oil firing, but
ground level concentrations have not increased, and in some
cases have reduced by 20% as a result of smoke control work.
There can be no doubt that this considerable improvement
in the London Area and confirmed by the absence of "Smog" in
recent years, is due to the introduction of Smoke Control Areas.
It is therefore most important that this work affecting the health
of the community at large should proceed apace and not be
delayed by financial considerations.