Nick Watts and colleagues1Watts N Amann M Arnell N et al.The 2020 report of the Lancet Countdown on health and climate change: responding to converging crises. did wide-reaching analyses to track the emerging health effect of climate change. Among other indicators they estimated “Indicator 1.1.3: heat-related mortality” and “Indicator 3.3: premature mortality from ambient air pollution” and concluded: “average heat-related mortality per year…a total of 296 000 deaths in 2018” and “premature deaths…from ambient PM2·5 have increased…from 2·95 million deaths in 2015 to 3·01 million deaths in 2018”. The Article1Watts N Amann M Arnell N et al.The 2020 report of the Lancet Countdown on health and climate change: responding to converging crises. estimated numbers of deaths by attributable fractions, which disallows identification of the number of premature deaths due to exposure.2Hammitt JK Morfeld P Tuomisto JT Erren TC Premature deaths, statistical lives, and years of life lost: identification, quantification, and valuation of mortality risks. The resulting bias is not quantified by typical confidence or uncertainty intervals and it could be substantial (in fact, the numbers of premature deaths due to exposure could be much larger than calculated by the authors).2Hammitt JK Morfeld P Tuomisto JT Erren TC Premature deaths, statistical lives, and years of life lost: identification, quantification, and valuation of mortality risks., 3Uncertainties in the GBD 2017 estimates on diet and health. Greenland and Robins4Conceptual problems in the definition and interpretation of attributable fractions., 5Estimability and estimation of excess and etiologic fractions. identified this methodological error decades ago.

Research into how health and climate change are linked is too important to ignore, rather than acknowledge and factor in, well known limitations of epidemiological data.

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.

Uncertainties in the GBD 2017 estimates on diet and health.

Lancet. 39418024.

Conceptual problems in the definition and interpretation of attributable fractions.

Am J Epidemiol. 128: 1185-11975.

Estimability and estimation of excess and etiologic fractions.

Stat Med. 8: 845-859Article InfoPublication HistoryIdentification

DOI: https://doi.org/10.1016/S0140-6736(21)00884-9

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