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191
The results that follow are based on the period 1st October, 1954, to 30th September,
1956, the choice of the period being designed to cover the experience of two complete
winters.
Results
(i) The long-term effect
The detailed figures of the atmospheric pollution records are given in Table III,
by winters and summers separately and for the whole period of two years. Looking
at the figures it will be seen that they are reasonably consistent, winter by winter and
summer by summer; there is a difference in the proportions of sulphur dioxide to
smoke in the figures from the Chelsea and County Hall stations as compared with the
other five. Chelsea has a high content of both smoke and sulphur dioxide in the atmosphere,
probably due to the proximity of three power stations, whereas the County
Hall station shows the lowest figure for smoke but the second highest for sulphur
dioxide; the South-western ambulance station has the highest average for smoke and
a high average for sulphur dioxide (the last two stations are only 2½ miles distant from
each other and the figures for each demonstrate the difference to be found in so short a
distance).
Indices other than the means of daily average figures have been suggested;
Pcmberton and Goldberg (1954) experimented with means of the maximum daily
readings. Consideration was given to using the proportion of days when pollution
was about twice the winter average (taken as days when smoke exceeded 100 milligrams
per 100 cubic metres of air and sulphur dioxide exceeded 20 parts per 100 million parts
of air). The result of this exercise was that there was no significant difference in ranking
order of the stations by the proportion of days when pollution was high and by the average
daily figures (for smoke R (Spearman) = + 0.96 and for sulphur dioxide R = +0.88).
In Table IV are set out, for the different areas, the populations by sex together with
the percentage in the Registrar General's social classes IV and V, the percentage born
in London, the average pollution during the whole period of 24 months and standardised
mortality ratios* for the causes of death already listed in Table I; the sexes have been
separated because of the different mortality experience already demonstrated in that
table. Social class has been included as an index of socio-economic conditions and the
percentage born in London as an index of duration to exposure—many people who die
from respiratory disease will have suffered from it for a long time and as the ideal of
having, in association with every death, the life history of the person concerned is
wholly impracticable, the percentage born in London has been used as a rough index
of the duration of exposure of the population of the areas to the London atmosphere.
This index takes no account of migration inside the County nor do current figures of
atmospheric pollution necessarily represent the past because of changes in the nature
of the activities, commercial, industrial or residential, of the area, but they seem the
best available measure. The atmospheric pollution figures for the 24 months have been
used because of the consistency in the coefficient of variation over the periods shown in
Table III and similarly (although the standardised mortality ratios were calculated for
each winter and each summer separately) because the pattern of mortality was similar
for both winters and both summers, it was decided to use the ratios calculated for the
whole two year period, thereby reducing the error inherent in any such calculation.
* These ratios are based on the mortality experience of London as a whole because the appropriate mortality data
for England and Wales are not available over this period of 24 months.