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causes out the mortality ratio tor all causes, all ages, is 3 per cent. higher than in the whole
country and at ages 45-64 years it is 8 per cent, higher. For females, a similar though not
so pronounced pattern applies and the fewer cardio-vascular deaths more than offset the
higher ratios from diseases of the respiratory system, resulting in a female mortality
for all causes, all ages, lower than the national experience.
Two possible explanations of the differences in the mortality pattern of the county
and the country are variations in the choice of the underlying cause of death by
the certifying medical practitioner, and a differing social class composition (bearing in
mind that there is a class gradient in mortality from respiratory causes, particularly for
bronchitis). The first has been dealt with in more detail by Hewitt (1956) but, even
allowing for a difference in diagnostic fashion between town and country, differences
of this magnitude cannot be explained away. As regards the second, as the following
table shows, the relative proportions in the social classes for males (females are not
tabulated by social class tor London) are not dissimilar as between London and England
and Wales.

It social classes I and II, and IV and V are combined to form a three-fold classification
there is almost perfect correspondence between London and the country as a whole.
Such differences in social class as exist, in particular the difference between social classes
IV and V, are again insufficient to account for the differing mortality ratios in table I
above. The lower London ratios for mortality from cardio-vascular diseases can only be
explained by excess deaths from respiratory causes, leaving fewer available to die from
heart disease.
The excess mortality in London from respiratory diseases, the 'Smog' of 1952,
and the general emphasis given to atmospheric pollution led to a decision to investigate
matters more fully in London to see if any association could be found between pollution
and mortality—whether smoke as well as smoking was to be condemned.
Methods
Two aspects of atmospheric pollution have to be considered—the long-term and
short-term. As Wilkins wrote (1954) 'the problem of smog is, in some respects, a
short-term magnification of the general problem of atmospheric pollution, and becausc
the effects of normal pollution are ever present they undoubtedly represent, in the
long run, a greater damage and loss to individuals and to the nation than does an
occasional smog incident
Areas
In order satisfactorily to calculate any correlation between the effects of atmospheric
pollution and local mortality the County had to be subdivided. In the event the basis
chosen was a parliamentary constituency or a metropolitan borough, whichever was
the smaller, which resulted in 46 areas of roughly equivalent population.
There are in London numerous atmospheric pollution recording stations under the
aegis of the Department of Scientific and Industrial Research but the methods of measurement
differ not only as regards the instruments, which are a mixture of deposit gauges
for the measurement of solid deposit, lead peroxide cylinders for the recording of sulphur
dioxide and volumetric apparatus for the dual measurement of both smoke and sulphur
dioxide, but also as regards the interval at which they are read. There is no way in
which the readings from lead peroxide cylinders and deposit gauges can be reliably
compared with the readings from volumetric apparatus, which is to be preferred
for the quantitative assessment of the amount of smoke and sulphur dioxide in the
atmosphere, and the two first tend to be influenced by weather conditions. The difference
in the intervals of reading the instruments, while not affecting unduly any conclusions
Atmospheric
pollution
records