Mortality Related to Cold and Air Pollution in London After Allowance for Effects of Associated Weather Patterns

We looked for atypical weather patterns that could confound, and explain large inconsistencies in, conventional estimates of mortality due to SO2, CO, and smoke. Using Greater London data for 1976–1995 in the linear temperature/mortality range 0–15°C we determined weather patterns associated with pollutants (all deseasonalized) by single regressions of daily temperature, wind, rain, humidity, and sunshine at successive days advance and delay. Polluted days were colder (P<0.01 for SO2, CO, and smoke) and less windy and rainy than usual, and this cold weather was more prolonged than usual with 50% maximum temperature depression 5.9 days (95% interval 4.0–7.7) before high SO2, compared to 2.0 (1.6–2.3) days before average cold days. We also used multiple regression of mortality at 50+ years of age on all these weather factors and pollutants at 0-, 1-, 2- to 4-, 5- to 13-, and 14- to 24-day delays to allow for the atypical weather patterns. This showed cold weather associated with 2.77 excess deaths per million during 24 days following a 1°C fall for 1 day, but no net excess deaths with SO2 (mean 28.0 ppb) or CO (1.26 ppm). It suggested (P>0.05) some increase with smoke, perhaps acting as surrogate for PM10, for which data were too scanty to analyze.