Quantifying the health impacts of ambient air pollutants: methodological errors must be avoided
Assessing how much ambient air pollution contributes to premature deaths in Europe (Heroux et al. 2015) is certainly important. However, while the public health and policy challenge of air pollution is beyond dispute, the quantitative results may be not. Key results by Heroux and colleagues relied on a transformation of RR estimates (given in their Table 1) into numbers of premature deaths (reported on in the section 'Application of HRAPIE recommendations and implications for European Union air policy' However, this approach relies on a methodological error (Greenland 1999) Greenland showed that a calculation based on RR estimates, as performed in the EU research project, does estimate excess cases numbers but it does not estimate the number of premature cases or etiological cases. He illustrated the underlying problem with an application of occupational epidemiology to compensation decisions. We note that his statistical arguments apply to environmental studies too. Importantly, Greenland showed that the difference between the number of excess cases and the number of premature cases can be substantial and he wrote that this 'methodological error has become a social problem' In this vein, estimated numbers of premature deaths to be related to long term PM2.5 and short term O3 in year 2010 cannot be justified by the analyses the authors relied upon. That this methodological error is becoming somewhat pervasive (Erren and Morfeld 2011) is also evinced by recent analyses (Lim et al. 2012) referenced by Heroux et al. (2015) The formulas on p. 2237 support that the arguments of Greenland apply: Lim and colleagues relied exclusively on excess case statistics which do not allow to 'calculate the proportion of deaths or disease burden caused by specific risk factors' Calculations of years of life lost due to exposure potentially suffer from similar problems (Morfeld 2004) Heroux and colleagues concluded: 'The project results provide the scientific basis for formulating policy actions to improve air quality and thereby reduce the burden of disease associated with air pollution in Europe' To factually arrive at such much needed empirical basis, methodological errors must be addressed and avoided. Disconcertingly, ignoring possible impacts of identified errors may jeopardize important burden of disease research.
International Journal of Public Health