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This is not to say that they should be regarded as negative evidence of an increase in
the amount of sickness, it is rather, for the reason stated, they are not sufficiently
sensitive for this purpose and in any case they will be affected in part by the use made of
hospital beds.
The sum total of these three sources of information is that they corroborate the
view that the smog of January, 1956, was the one which had the greatest effect on the
health of London.
Summary and conclusions
There is little doubt that living in London for a long period increases the risk of
dying from respiratory disease, particularly bronchitis : furthermore since these diseases
are often suffered over a number of years morbidity too is associated with the length of
residence. It must, however, be remembered that the index used—percentage born in
London—is somewhat crude and gives no indication of internal migration.
A significant relationship between mortality and the two types of atmospheric
pollution chosen—sulphur dioxide and smoke—has not been demonstrated though
smoke looks more dangerous than sulphur dioxide. This does not preclude there being
a critical level of one or of both in combination that would be directly associated with
increased mortality or morbidity, but equally there may be other types of pollutants
more dangerous than either. In the short-term, whilst it is difficult to disentangle the
effects of duration and intensity of atmospheric pollution and temperature from any
seasonal trend, the effect of the smog of January, 1956, suggests that the threshold level
above which a smog incident is likely to occur in London is when the mean level for
London attains a daily value of 200 milligrams of black suspended matter per 100 cubic
metres of air and 40 parts of sulphur dioxide per 100 million parts of air, i.e., about
four times the customary winter level.
The differential effect on old people when pollution reaches this level has been
shown, though the overall effect was much less than the catastrophe of 1952.
For the future it is proposed with the continued co-operation of the Medical Officers
of Health of the Metropolitan Borough Councils, fully to document each smog episode
as it occurs through the medium of this report but using as our criteria the higher levels
of pollution indicated in the last paragraph but one.
If, as to be anticipated, measures to diminish the amount of atmospheric pollution
reduce the amount of smoke it may be possible to assess with greater clarity the respective
parts played by sulphur dioxide and smoke which have both so far been present
concurrently in the atmosphere in roughly the same proportions during different
episodes of fog.
References
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Eastcott, D. F. (1956). 'The Epidemiology of Lung Cancer in New Zealand'. Lancet, 1, 37.
Hewitt, D. (1956). 'Mortality in the London Boroughs 1950-52 with Special Reference to Respiratory Disease'.
Brit. J. prev. soc. Med., 10, 45.
Hewitt, D. (1957). 'Vagaries of Local Mortality Rates under the 1953-54 Rules for Transfer of Deaths'.
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Lawther, P. J. (1956). 'The Clinical Significance of Dirty Air '. Public Health, October, 1956.
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Health and Med. subjects, No. 95.
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