Is cold weather bad for your heart? The evidence points at exertion, not just the chill

Cold weather does track with more heart attacks — but the most useful studies suggest the danger is less the ambient chill than what people do in it. Two large population datasets, one Canadian and one Swedish, both find that colder conditions coincide with a higher incidence of myocardial infarction (MI) [s1] [s2]. The Canadian data go further and locate much of the risk in a specific, physical act: clearing snow.

The nationwide temperature signal

The Swedish study is among the largest of its kind. Researchers linked daily weather from the national meteorological service to every MI reported to the SWEDEHEART coronary-care registry from 1998 to 2013 — 280,873 patients in all, of whom 92,044 had an ST-elevation MI, with weather data available for 274,029 (97.6%) [s2]. The mean age was 71.7 years [s2]. Incidence of MI rose with lower air temperature, lower atmospheric pressure, higher wind speed and shorter sunshine duration, and temperature carried the most pronounced association: a one-standard-deviation increase in air temperature (7.4 °C) was associated with a 2.8% reduction in MI risk (unadjusted incidence ratio 0.972; 95% confidence interval 0.967–0.977; P<0.001) [s2].

That is a real but modest day-to-day signal, consistent across ST-elevation and non-ST-elevation infarctions and across regions [s2]. An observational correlation of this kind cannot prove that cold air itself triggers infarctions; colder days differ from warm ones in many ways, from behaviour to respiratory infections. What it establishes is that the incidence of heart attacks is not evenly distributed across the calendar, and leans toward the cold end.

Why snowfall sharpens the picture

The Canadian study isolates a mechanism the temperature data only hint at. Using a case-crossover design — each person serving as their own comparison — researchers examined MI hospital admissions and deaths in Quebec between November and April across 1981 to 2014, taking in 128,073 individual hospital admissions and 68,155 deaths [s1]. They then asked whether snowfall the previous day changed the odds, adjusting for minimum daily temperature [s1].

The result splits sharply by sex. Compared with no snow, 20 cm of snowfall was associated with an odds ratio of 1.16 for hospital admission (95% CI 1.11–1.21) and 1.34 for death (95% CI 1.26–1.42) due to MI the following day — among men [s1]. Among women, the same snowfall carried odds ratios of 1.01 (95% CI 0.95–1.07) and 1.04 (95% CI 0.96–1.13) — effectively no change [s1]. Snowfall duration showed a similar, smaller pattern [s1].

A risk that appears in men but not women, the day after snow falls, is hard to explain by cold air alone, which both sexes breathe equally. The far more plausible reading — and the one the authors raise — is snow clearing: heavy, sudden, isometric exertion, often by people who are otherwise sedentary, in the cold, first thing in the morning. Cold constricts blood vessels and raises blood pressure; strenuous shovelling adds a surge in cardiac demand on top. The snowfall study effectively photographs that combination.

What the two studies agree on

Read together, the picture is coherent. Cold days carry a modestly elevated baseline risk of heart attack across a whole population [s2], and layering intense physical exertion onto a cold morning — the shovelling scenario — is where a sharp, measurable spike appears [s1]. Neither finding says cold weather is dangerous for everyone, and neither is a randomised trial; both are observational, and the snowfall effect in particular is an association drawn from administrative records rather than a demonstrated cause [s1] [s2].

What it leaves a reader

The honest takeaway is about context, not temperature readings. For most people, a cold day is not a cardiac threat in itself; the evidence flags the intersection of cold and heavy, unaccustomed exertion, with snow clearing as the archetype [s1]. Who is most exposed — older adults, people with existing coronary artery disease, those unused to strenuous effort — is exactly the group in whom sudden exertion carries the most risk, and whether any individual should avoid such tasks is a clinical judgement rather than something these population studies can decide. Chest pain, breathlessness or collapse during exertion in the cold is a reason to stop and seek emergency care, not to push through.