As the mean age at screening among the 3613 women who had never smoked was higher in those from lower occupational classes, all other risk factors were adjusted for age (table 1). Women in lower occupational classes were on average shorter and had poorer lung function, higher systolic blood pressure, and a higher body mass index than women from the other occupational classes. Body mass index and systolic blood pressure were strongly correlated (correlation coefficient 0.32, P<0.001). According to body mass index groups the mean systolic blood pressure (mm Hg) was 147.8 (95% confidence interval 146.6 to 149.1) in normal weight women, 153.6 (152.4 to 154.8) in overweight women, 163.0 (160.8 to 165.2) in moderately obese women, and 174.0 (170.1 to 177.9) in severely obese women. Significantly more women in occupational class IV and V reported bronchitis than women in occupational class I and II. No clear relations were observed between occupational class and cholesterol level, angina, or ischaemia on electrocardiogram.
Table 1 Age adjusted risk factors by occupational class in 3613 women from Renfrew and Paisley Study who had never smoked. Values are means (95% confidence intervals) unless stated otherwise
Overall, 43% (n=1555) of the women who had never smoked were overweight, 14% (n=515) were moderately obese, and 5% (n=194) were severely obese. The body mass indices of the women who had never smoked were compared with those of the 3590 current smokers in the full cohort (table 2). Overweight and obesity were more prevalent among never smokers than among current smokers in all occupational classes and among both never smokers and smokers in the lower occupational classes (fig 1). Severe obesity was more than twice as prevalent among women who had never smoked in the lower occupational classes than in women who had never smoked in the higher occupational classes and about seven times more prevalent than among current smokers in the higher occupational classes.
Table 2 Age adjusted proportions of overweight, obese, moderately obese, and severely obese by occupational class in 3613 women who had never smoked and 3590 women who were current smokers from Renfrew and Paisley Study. Values are percentages (95% CIs) (more ...)
Fig 1Overweight and obesity among women who had never smoked and current smokers by occupational class
The remainder of the results focus on the 3613 women who had never smoked. In the 28 years of follow-up, 1796 (50%) women died, with 51% of the deaths (n=916) due to cardiovascular disease and 27% (n=487) to cancer (table 3). Overall, 270 (39%) women died in occupational class I and II, 434 (47%) in III non-manual, 407 (56%) in III manual, and 685 (54%) in IV and V. Upward trends were observed in age adjusted mortality rates with declining occupational class for all causes, cardiovascular disease, coronary heart disease, stroke, and respiratory disease. These were reduced but remained after additional adjustment for body mass index. They also remained when adjustments were made for all risk factors included. Compared with women in occupational class I and II, age adjusted rates were about one third higher in women in occupational classes III manual and IV and V, decreasing to about 20% higher when adjusted for other risk factors. Age adjusted mortality from coronary heart disease was nearly 60% higher in occupational class IV and V but this decreased to 32% after adjustment for other risk factors. Age adjusted mortality rates in occupational class IV and V were higher by 50% for stroke and by 100% for respiratory disease, decreasing to higher by 32% and 78% after adjustment for other risk factors. Relations between occupational class and mortality rates did not differ significantly for all cancers, smoking related cancers, non-smoking related cancers, or breast cancer.
Table 3 Age and all risk factor adjusted relative rates of mortality by occupational class in 3613 women from Renfrew and Paisley Study who had never smoked
Among the 1913 women who had never smoked and for whom information was available on smoking habit of co-residents, additional adjustment for co-residents’ cigarette pack years had a slight attenuating effect on the age adjusted and the all risk factor adjusted mortality analyses (see web extra table 1).
In an analysis of the three lowest occupational classes (manual) compared with the three highest occupational classes (non-manual), the age adjusted relative rate for all cause mortality was 1.20 (95% confidence interval 1.09 to 1.32; table 4). The effect of including body mass index in the model was to reduce the association between occupational class and all cause mortality by 23.6%. Including percentage predicted forced expiratory volume in one second reduced the association by 21.8%, and including systolic blood pressure reduced it by 18.9%. Height, bronchitis, ischaemia on electrocardiogram, angina, and cholesterol level led to smaller reductions. All the risk factors explained nearly 64% of the association.
Table 4 Role of risk factors in explaining association between occupational class (manual v non-manual) and all cause mortality in 3613 women from Renfrew and Paisley Study who had never smoked
Upward trends between mortality and increasing body mass index were observed for all cause, cardiovascular disease, coronary heart disease, stroke, and cancer mortality (table 5). Compared with normal weight women, severely obese women had particularly high risks for all cause mortality (relative rate 2.07, 95% confidence interval 1.72 to 2.49). Adjustment for occupational class attenuated the relations somewhat, and although adjustment for all the available risk factors caused further attenuation, significant trends remained for all these causes (except stroke). Downward trends were observed between body mass index and mortality due to respiratory disease which strengthened after adjustment. Body mass index showed no clear relation with smoking related cancers, non-smoking related cancers, and breast cancer mortality.
Table 5 Age and all risk factor adjusted relative rates of mortality by body mass index in 3613 women from Renfrew and Paisley Study who had never smoked
Compared with normal weight women in occupational class I and II, age adjusted all cause mortality rates were not significantly increased among normal weight women in the lower occupational classes (table 6 and fig 2). Mortality rates were increased in the moderately and severely obese women and particularly in women in lower occupational classes. With the exception of women in occupational class I and II, moderately obese women had at least a 60% increased mortality rate. Severely obese women had around double the mortality rate of normal weight women. Adjustment for other risk factors had a large attenuating effect, but the increased risk in the moderately obese women and particularly in the severely obese women remained.
Table 6 Age and all risk factor adjusted relative rates of all cause mortality in 3613 women from Renfrew and Paisley Study who had never smoked by body mass index and occupational class
Fig 2Age adjusted relative mortality rates by occupational class and body mass index groups in women from Renfrew and Paisley Study who had never smoked, with normal weight women in occupational class I and II as baseline group