of black smoke over this period was 13 µg/m3. The median number of deaths in London was 172 per day. A similar association was found for ozone, but smaller and less consistent effects were observed for nitrogen dioxide and sulphur dioxide.
• Conclusion: Daily variations in air pollution within the range currently occurring in London may have an adverse effect on daily mortality.
To estimate the total number of deaths likely to be caused by particulate air pollu- tion in London (the total burden of deaths), you will have to make an important assumption: that there is a causal association between black smoke and mortality.
The steps to be taken to determine numbers of avoidable deaths are listed below:
1 Calculation of the percentage of deaths. Deaths associated with air pollution are expressed in terms of the percentage increase in mortality for each µg/m3 increase in the concentration of black smoke. Assuming that there is a linear relation between mortality and black smoke, you know that the percentage increase in deaths following an increase in black smoke concentration from 8 to 22 µg/m3 (= 14 µg/m3) was 1.7 per cent. This means that the increase in daily deaths per µg/m3 of black smoke is
1.7 / 14 = 0.121%
2 Calculation of the number of deaths. To understand the public health implications of black smoke, you would want to know the actual number of deaths involved.
Use the median number of deaths per day (172). The average increase in daily deaths per µg/m3 of black smoke is thus
172 × 0.121 / 100 = 0.208
3 Calculation of number of avoidable deaths per day. To understand the relationship between mortality and exposure better, you would want to calculate how many deaths you could avoid if there were no particulate air pollution.
You can simulate this by extending the linear relationship between mortality and black smoke down to a concentration of 0 µg/m3. You will now be able to estimate the average number of deaths per day that would have been avoided if there had been no particulate air pollution on a day:
Number of extra deaths per µg/m3 of black smoke = 0.208 × the actual concen- tration of black smoke = number of deaths avoided if the concentration had been 0.
Note that the average of deaths avoided at 12 and 14 µg/m3 is equal to that avoided at 13 µg/m3.
You can also calculate the average number of deaths avoided on days with concentrations of black smoke of:
(a) 13 µg/m3:: 0.208 × 13 = 2.70 deaths avoided.
(b) 12 µg/m3:: 0.208 × 12 = 2.50 deaths avoided.
(c) 14 µg/m3:: 0.208 × 14 = 2.91 deaths avoided.
Average: (2.50 + 2.91) / 2 = 2.70.
4 Calculation of number of avoidable deaths per year. If the daily concentrations of black smoke have a symmetrical (even) distribution, then over the course of a year, exposures greater than the median (more than 13 µg/m3) will be matched by exposures less than 13 µg/m3. We can use this information about the distribu- tion of exposure to calculate the average number of daily deaths which could be avoided over one year if there was no exposure to black smoke. If the exposure were reduced to 0 µg/m3 the number of deaths avoided would equal the number
Outdoor air pollution 119
avoided on a day of 13 µg/m3. Using this argument we can calculate the number of deaths that would be avoided over one year if there was no exposure to black smoke:
Days in a year (365) deaths avoidable at 13 µg/m3 (2.70) = deaths avoidable/
year. 365 × 2.70 = 986 deaths could be avoided in one year if there was no black smoke exposure.
5 Calculation of percentage of avoidable deaths. The number of deaths avoided per year should now be changed to a percentage of the total number of deaths:
Total deaths expected in one year 365 × 172 = 62 780.
Deaths due to black smoke = 986.
Deaths due to black smoke / total of all deaths (986 / 62 780) = 1.6%.
This means that in one year 1.6 per cent of the deaths are the result of exposure to black smoke.
6 The main assumptions about the data include the following.
(a) The relationship between black smoke and death is causal, i.e., black smoke causes extra deaths.
(b) The relationship between amount of exposure and number of deaths is linear – you assumed that the relationship was a straight line, but the dose–
response curve could be a number of shapes.
(c) Particulate air pollution is adequately indicated by measured black smoke – that black smoke measures the aetiologically important fraction of particu- late pollution.
(d) The slope of the linear relationship is 1.7 per cent.
(e) The distribution of daily black concentrations is symmetrical – there could be a large number of days when the exposure is quite low, and just a few when it is very high.
(f) When we state ‘deaths caused by air pollution’, we include deaths that may have been brought forward by only a short time, possibly only a few days.
Activity 8.5
If you were explaining the risks of air pollution from black smoke based on this example, what information would you wish to provide when communicating to:
1 an expert committee considering standards for particulate air pollution 2 the general public?
Feedback
1 (a) You would want to elaborate the measures of uncertainty in your calculations.
You could start with using the confidence limits of the slope, but ideally, uncertainty should be reflected in each of the assumptions above.
(b) Include estimates of the relationship between black smoke and daily death that have been calculated for other places, or by using other assumptions.
2 To present this information to the public, you would want to give them a simplified version of the information you provided to the experts. You could also include some estimates of risk from other exposures or behaviours (such as risk due to traffic accidents, occupational exposures, smoking, and radon exposure). By showing other
120 Environment, Health and Sustainable Development
risks (a comparative risk analysis), you would enable the reader to put the significance of black smoke exposure in context.
Summary
This chapter has covered the main sources and health effects of outdoor air pollu- tion. Different methods for estimating the potential burden of disease from air pollution were discussed. Time-series analysis and cohort studies were covered in more detail, and a step-by-step guide covered the analysis and uncertainties of a specific air pollution episode. Research evidence demonstrates that both acute and chronic effects of air pollution lead to increased rates of mortality and morbidity.
The effects of outdoor air pollution are likely to be different in low and middle income countries. Standard-setting and public health concerns were also discussed.
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Further reading
McGranahan G and Murray F (2003) Air Pollution and Health in Rapidly Developing Countries.
London: Earthscan.