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However, with these three exceptions, all the recording stations listed are located
within the defined associated areas. Having regard to the prevailing wind from the
south-west they may not be recording the pollution generated within the area but
the pollution blown over them; nevertheless this is the air that the residents have to
breathe. On these grounds the association of the areas defined on the map with their
respective recording stations is considered to be justified.
Population
The populations used as a basis for the investigation were, for complete boroughs,
the Registrar General's mid-1955 estimates of the home population, i.e., populations
current some 15 months after the investigation began and 9 months before it ended;
for parts of boroughs the populations were estimated by a conversion of the numbers
of electors as at November, 1955, into total population, on the basis of the ratio of
electors to total population ascertained at the time of the census, and adjusting the
numbers thus obtained so that the total for the complete boroughs accorded with the
Registrar General's mid-1955 estimate.
The age and sex composition of the area populations so assessed was calculated by
modifying that ascertained at the census* in the light of overall changes (notably fewer
children under five years and more in the age range 5-14 years) which, according to
the Registrar General's mid-year estimate of population, had occurred in the County
by mid-1955.
The social class composition and the proportions born in London have been taken
as those prevailing at the census date, again specially obtained for areas approximating
to the parts of boroughs.
Morbidity
No study of the effects of atmospheric pollution would be complete without a
consideration of morbidity. There are practically no pertinent data available collected
on a consistent basis; of the respiratory diseases pneumonia is notifiable but there is
reason to believe the notifications are incomplete. There are available the weekly
number of first applications for sickness benefit from the local offices of the Ministry
of Pensions and National Insurance, but they are not analysed at that stage by cause,
age or sex and, relating as they do, to the insured population they are not representative
of the total population; in fact the section of the population most vulnerable to the
effects of fog, namely the aged, would be completely missed in statistics from this
source. Moreover, the local offices of the Ministry are not coterminous with any
local government boundary and, therefore, figures from this source are not particularly
helpful from the epidemiological aspect. The most that can be obtained in the shape
of morbidity data is a summation of impressions from various sources at the time of a
'smog' incident—Ministry of National Insurance figures already referred to, demands
upon the District Nursing Service, demands for hospital beds, etc.; for a long-term
assessment of the prevalence of respiratory disease there is no information available.
From this aspect, therefore, the survey was confined to an examination of mortality.
Mortality
For the mortality data the published statistics of the Registrar General were
inadequate because they are based on metropolitan boroughs. Accordingly in 1953, I
approached the medical officers of health of the boroughs who kindly agreed to make
available to the department copies of the death returns received by them from the
local registrars; the collection, classification and tabulation of the data began in July,
1954. Deaths of London residents outside the county were obtained from the transferable
death returns of the Registrar General.
There are two aspects of mortality to be considered according to whether the longterm
or short-term effects of atmospheric pollution are under examination. For the
former we are concerned with the deaths of the residents of London, wherever they
may have died, so long as London was their normal place of residence and for the latter
all deaths occurring in London irrespective of place of residence.
* For parts of boroughs, for which the sex and age composition was not tabulated in the census volumes, information
for areas approximating to those under examination was obtained specially from the Registrar General.