Environment: air, heat, cold

The associations are among the best established in epidemiology; the counting is where the argument is. Fine particulate pollution shortens lives at every level yet measured, with no threshold found, and the global estimates of deaths attributable to it run into the millions a year; in Europe and North America concentrations have fallen for decades and the attributed deaths with them, while in much of Asia they have not. The estimates are modelled from exposure and a concentration–response curve, so they move when either is revised, and revisions have been large.

Temperature is the newer question. Multi-country studies of daily deaths find that cold kills far more people than heat in temperate countries — of the order of ten times more — because moderate cold on many days outweighs extreme heat on a few. But the two are moving in opposite directions. Europe's summers of 2022 and 2023 produced tens of thousands of heat-attributable deaths by the standard method, the highest on record, and the series of heat deaths is rising; cold-attributable deaths are falling as winters warm and housing improves. Whether the net effect of a warming climate on mortality in a given country is positive or negative over the coming decades is contested, depends on adaptation, and is exactly the kind of question in which the method chooses the answer.

Three things make the evidence uneven. Attribution methods differ between agencies, so heat-death counts for the same summer vary severalfold. Exposure is measured well where the harm is smallest. And the people most affected — the old, the poor, those in bad housing — are the least represented in the cohorts that supply the response curves.

Status

established

Direction

strengthening

Would change our view

A national series showing cold-related deaths falling by more than heat-related deaths rise, or the reverse, over a decade; an attribution study on a different method producing materially different counts for the same events; or a revision to the particulate response curve of the kind that has moved global estimates before.

Trackers

Gaps

Comparable heat-death series across countries; exposure data outside Europe and North America; indoor air and heat; Japan and Korea, where heat mortality is measured but rarely compared.