View report page

203
These results suggest something in the nature of a statistical artefact—sulphur dioxide
which apparently of itself had no bearing on mortality suddenly becoming highly
significant in association with length of residence. The part played by each component
of the combined index of residence and smoke was tested by the calculation of first
order correlation coefficients which are as follows:—
r3b.2 = Males + 0.232.
Females + 0.435.
r2b.3 = Males + 0.667.
Females + 0.726.
[The subscript 3b.2 means the correlation between 3 and b (smoke and bronchitis)
when 2 (per cent, born in London) has been held constant at a mean value
similarly, 2b.3 means the correlation between 2 and b (per cent, born in London and
bronchitis) when 3 (smoke) is held constant.]
Thus in the first instance, when percentage born in London is held constant, i.e.,
the same proportions applied to all eight areas, there is no significant correlation
between the index of smoke and bronchitis, but in the second case, when the amount
of smoke is held constant, there is a much higher value of r for the correlation of
bronchitis with percentage born in London, approaching significance in both
instances. This is held to be a sufficient indication that duration of exposure to
the London atmosphere is more important in relation to bronchitis than variations in
the degree of pollution within London—London's air is bad for bronchitis no matter
in what part of London a person lives. This is not to say there are not variations in total
mortality within London, Martin (1955) has shown how mortality varies in London
boroughs—a variation that has, however, declined over the past forty years. It could
be argued from these figures that while the case against London is proven, that against
its atmosphere is not, but it is difficult to see what other facet of London life could be
pertinent in the context of bronchitis. Social class has already been discounted because
of the near parity of London vis-á-vis the country as a whole, and as regards smoking
habits although these doubtless play a part, it has been shown in the respiratory field,
lung cancer and bronchitis, that the degree of urbanisation is a relevant factor. The
coefficients for cancer of the lung are interesting (the figures for females can be ignored
because the relatively small number of deaths on which the mortality ratios are based
makes then unreliable); for males there is significant correlation with percentage born
in London and near significance with social class. Stocks and Campbell (1955) found
an association between cancer of the lung in males and urbanisation ; they estimate
that three-eighths of male lung cancer deaths in Liverpool were due to this factor
(one half of the deaths were attributable to smoking habits). Eastcott (1956) has drawn
attention to the differential death rate from cancer of the lung between immigrants
from the United Kingdom in New Zealand and native New Zealanders—a difference
that increased with age at immigration, the inference being that there is something in
the environment of the U.K. that planted the cause of the cancer early in life; the
difference is unlikely to be due to smoking habits because there is a higher consumption
of tobacco per head of population in New Zealand than in the United Kingdom though
the author does not state the type of tobacco nor its method of consumption. The near
significance with social class is in accordance with expectation; the occupational
mortality tables of the Registrar General show a rising gradient in male mortality
ratios for cancer of the lung with increase in the numerical social class (i.e., increase
with lower social class) for men aged 20-64 years, and over half the deaths from this
cause occur between ages 45-64 years. All the calculations have been based on mortality
of London residents. It could be argued that total mortality is not the right index to
use because any long-term effect of atmospheric pollution will be reflected in deaths
at later ages. This aspect has been considered and the standardised mortality ratios
calculated for ages 45-64 and 65+ years separately. With two exceptions (lung cancer
in females aged 65+ and cardio-vascular diseases in males aged 45-64 years) there
was no difference in the ranking order between the age group standardised mortality