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events in the individual areas around the recording stations (the areas used in the longterm
analysis) but the number of deaths is so small that, even on a weekly basis, there
is so much fluctuation that nothing useful could be extracted. Figures 3a-c give actual
figures for the County as a whole whereas figure 3d gives an estimate based on the
8 areas used for the long-term consideration of the effects of atmospheric pollution—
these areas, which by a happy coincidence are a representative cross section of the
London population as regards sex and age structure and social class, gave rise on the
basis of the first three periods, to about one-third of the deaths occurring in London.*
A seven day moving average was considered to be the best way of showing the shortterm
trend, avoiding any "favourite" day of the week there might be for dying and
producing a reasonably smooth curve. The use, in contradistinction to the long-term
analysis in which deaths of Londoners was considered, of mortality in London is
justified because in the short-term we are concerned with those at risk at the time of
the smog.
The choice of deaths in London means that their number cannot be related to the
population for the production of rates because, judging from the difference between
deaths registered in, and allocated to London, there is a yearly net surplus of some
3,700 deaths of non-Londoners in London hospitals. Hence for the purpose of estimating
the short-term effect of a smog one is forced to consider whether or not there was a
rise in the level of mortality.
Quite clearly there was a 'rise' in all four episodes but there is no apparent similarity
between them. In period 1 deaths continued at the higher level after the foggy days
were over and in period 4 the rise continued.
These differences are only to be expected because level of atmospheric pollution
is not the sole factor operating during the duration of the smog as defined, the temperature
and, most difficult of all, the course mortality would have followed in the absence
of a smog all render comparison difficult: more often than not cold and smog go handin-hand.
Russell (1924) found this last difficulty in his survey of fog in Islington and
St. Pancras over the years 1870-1900. However, the second and fourth smogs were
comparable in intensity of fog, duration and level of pollution but the former was
much colder than the latter.
In Figure 4 therefore the daily moving averages of deaths for each of these two
periods have been shown. The effects of the smog, judging from the slopes of the two
curves within the limits of the days concerned, are practically identical but the level of
mortality at the beginning of the smog is very much higher in period 2 than period 4
which prima facie is attributable to the lower temperature of the former and the mild
winter of which the latter formed part. The continued rise in mortality after the smog
in period 4 is probably nothing more than a rise to the normal winter level—for
the two weeks preceding the smog mortality was exceptionally low for the season
of the year; in fact throughout the mild winter of 1956-57 the number of deaths has
remained at an exceptionally low level. The fall in the curve at the end of the smog
in period 2 is probably due to the fact that concurrently there was a sharp rise in temperature
which masked the rise in mortalitv attributable to smoe alone.
Proportion
of old people
dying in
smog
The question also arises of whether a smog affects particularly any section of the
community. A partial answer to this question is revealed in Figure 5 in which is shown
the proportion of deaths in persons aged 70 years and over to total deaths in each of
the second, third and fourth periods together with the month of February, 1956, following
the third period when it was cold but not foggy. In the second period (January 1955)
when there was a long, moderate but very cold smog the proportion of deaths in the
over 70's was of the order of 55-59 per cent., in the third period (January 1956) when
there was a short, heavy and cold smog the proportion was 65-69 per cent.; in the
cold spell (February 1956) it was 55-59 per cent, and in the fourth period (December
*By the time of the last episode it had been decided to concentrate solely on the seven multiple circuit volumetric stations
and the surrounding areas.
205
Q*