We thank Peter Morfeld and Thomas Erren for their interest in the 2020 report of the Lancet Countdown.1Watts N Amann M Arnell N et al.The 2020 report of the Lancet Countdown on health and climate change: responding to converging crises. They argue that the use of attributable fractions is inadequate to estimate premature mortality from ambient air pollution or exposure to heat, and base their argument on the work by Hammitt and colleagues,2Hammitt JK Morfeld P Tuomisto JT Erren TC Premature deaths, statistical lives, and years of life lost: identification, quantification, and valuation of mortality risks. which states that mortality does not allow one to “distinguish between situations in which a few people die much earlier than they would have if unexposed and situations in which many people die a little earlier than they would have if unexposed”.2Hammitt JK Morfeld P Tuomisto JT Erren TC Premature deaths, statistical lives, and years of life lost: identification, quantification, and valuation of mortality risks. This takes us to the fundamental problem whereby one cannot know the age at which a person would have died if they had not been exposed (ie, the counterfactual age of death). Given this unavoidable limitation, premature deaths are defined at the population level, as a hypothetical quantity based on the population age structure as observed (under the influence of previous continuous exposure in the case of air pollution), and which contrasts the actual mortality with the expected mortality in the same period of time (a year, in our case) in a hypothetical exposure-free world. Any death occurring in this calendar year that is attributable to exposure is thus regarded as a premature death, acknowledging that death is unavoidable and can only be postponed. In this approach, the number of premature deaths estimated by the attributable fraction is by construction valid only for a snapshot in time, and should not be interpreted as an attempt to make a statement about changes in life expectancy due to an exposure.This discussion is not new, and enough literature has been published to resolve possible ambiguities.3Brunekreef B Miller BG Hurley JF The brave new world of lives sacrificed and saved, deaths attributed and avoided., 4Héroux M-E Brunekreef B Anderson HR et al.Response to “Quantifying the health impacts of ambient air pollutants: methodological errors must be avoided”. Although we acknowledge the limitations that we described earlier, the use of attributable deaths in a particular time period for quantifying the burden from given risk factors is well established, and the use of the term premature deaths for attributable deaths is standard in the literature. Indeed, this terminology has previously been used by WHO,5WHO
Burning opportunity: clean household energy for health, sustainable development, and wellbeing of women and children. Anenberg and colleagues,6Anenberg SC Miller J Minjares R et al.Impacts and mitigation of excess diesel-related NOx emissions in 11 major vehicle markets. and many others, and is now commonly understood by policy makers and other stakeholders. However, to avoid any possible misinterpretations, we will strive to make its definition clearer in our future endeavours.

We declare no competing interests.

References1.Watts N Amann M Arnell N et al.

The 2020 report of the Lancet Countdown on health and climate change: responding to converging crises.

Lancet. 397: 129-1702.Hammitt JK Morfeld P Tuomisto JT Erren TC

Premature deaths, statistical lives, and years of life lost: identification, quantification, and valuation of mortality risks.

Risk Anal. 40: 674-6953.Brunekreef B Miller BG Hurley JF

The brave new world of lives sacrificed and saved, deaths attributed and avoided.

Epidemiology. 18: 785-7884.Héroux M-E Brunekreef B Anderson HR et al.

Response to “Quantifying the health impacts of ambient air pollutants: methodological errors must be avoided”.

Int J Public Health. 61: 387-3885.

Burning opportunity: clean household energy for health, sustainable development, and wellbeing of women and children.

6.Anenberg SC Miller J Minjares R et al.

Impacts and mitigation of excess diesel-related NOx emissions in 11 major vehicle markets.

Nature. 545: 467-471Article InfoPublication HistoryIdentification

DOI: https://doi.org/10.1016/S0140-6736(21)00878-3

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