Cold weather reduces emergency department visits in Mexico by more than heat increases them.

Cold kills, but it does not fill emergency rooms.

Mortality has a well-documented U-shaped relationship with temperature: both cold and hot days raise deaths, in Mexico as elsewhere (Cohen and Dechezleprêtre, 2022). Emergency visits do not. An additional cold day reduces them by 9.1 percent on the day and by 16 percent accounting for the following 30 days, while an additional hot day raises them by up to 3.7 percent.

The cold effect is not over on the day.

An additional day below 10°C reduces visits by 9.1 percent on the day itself and by 16 percent once the following 30 days are accounted for; most of the difference builds up over the first two weeks.

Cold empties the emergency room, except for respiratory cases.

Accounting for 30 days, an additional cold day reduces visits in every diagnostic chapter but one, from 11 percent for endocrine and metabolic conditions to 83 percent for infectious and parasitic diseases. Respiratory visits are the exception: they fall 7.3 percent on the day and rise 34 percent cumulatively, consistent with infections spreading after cold exposure.

The young stay away longest; the old barely change.

Accounting for 30 days after a cold day, visits fall 23 percent for ages 12 to 20 and 19 percent for ages 20 to 40, against a statistically insignificant 1.4 percent for the over-80s. Same-day reductions run from 7 to 12 percent across ages; the gap opens in the weeks that follow.