🟠 Moderate Evidence
A nationwide ecological study published in PLOS Global Public Health has quantified a robust inverse relationship between monthly ambient temperature and ischemic heart disease (IHD) mortality across the United States from 1999 to 2020. The analysis of state-level mortality data reveals that colder months are consistently associated with elevated risk of death from IHD, while warmer months show protective effects—a pattern that persists across demographic and geographic subgroups.
Key takeaways
- Ischemic heart disease mortality increases significantly during colder months, with a 20% difference in relative risk between the coldest (−12.8°F) and warmest (89.2°F) temperatures observed
- The association was statistically robust (Wald p < 0.001) and consistent across age, sex, race, and climate subgroups in a 22-year nationwide analysis
- Lag effects were strongest in the month of temperature exposure, then declined—suggesting acute rather than long-term cold-related mechanisms
Study at a Glance
| Source | PLOS Global Public Health |
| Study type | Nationwide ecological analysis with distributed lag non-linear modelling |
| Sample period | 1999–2020 (22 years) |
| Geographic scope | All United States (state-level analysis) |
| Data source | CDC WONDER (mortality); National Centers for Environmental Information (temperature) |
IHD Mortality Risk Across Temperature Range
Relative risk of death by ambient temperature, United States 1999–2020
Source: Ahmed et al., PLOS Global Public Health, 2024 | Georgian Medical Journal News
Temperature and cardiac physiology: mechanisms under investigation
The inverse relationship between cold exposure and IHD mortality is biologically plausible, though the precise mechanisms remain incompletely understood. Cold ambient temperatures trigger vasoconstriction, increase blood viscosity, elevate blood pressure, and promote a prothrombotic state—all recognized risk factors for acute myocardial infarction. According to the American Heart Association, winter mortality from cardiac causes typically peaks in colder climates and seasons globally, supporting this mechanistic framework. The study’s finding that lag effects were strongest in the month of exposure and attenuated thereafter suggests acute physiological responses to cold rather than cumulative chronic effects.
The research team, led by investigators including Hafiz M. Ahmed and colleagues, employed a negative binomial regression with distributed lag non-linear modelling to adjust for state, year, and month fixed effects—a methodological approach that controls for seasonal and long-term trends in IHD mortality unrelated to temperature. The fully adjusted model explained 93.6% of the deviance compared to a null model, indicating strong model fit. This suggests that ambient temperature captures a substantial proportion of monthly variation in IHD mortality once structural confounders are accounted for.
Consistency across populations strengthens causal inference
A key strength of the analysis was its examination of temperature–mortality associations stratified by age, sex, race, and climate zone. The authors found that the inverse association remained generally consistent across all demographic and geographic subgroups, though some estimates were imprecise in smaller population subsets. This consistency suggests that the cold–mortality relationship is not an artefact of a single demographic group or regional climate pattern, but rather a robust phenomenon affecting diverse populations. Residents of colder climates showed similar associations to those in warmer climates, implying that the relationship is not simply driven by accidental misclassification of exposure or unmeasured confounding by climate adaptation.
The ecological study design—using state-level aggregated data rather than individual-level records—is both a strength and a limitation. State-level analysis enabled examination of a full 22-year national timeseries with complete mortality ascertainment via the CDC’s WONDER database, providing unprecedented temporal and geographic coverage. However, ecological designs cannot establish causality at the individual level and may be subject to ecological fallacy, in which associations observed at the population level do not necessarily reflect individual-level relationships.
Clinical and public health implications emerge as winters intensify
The magnitude of the temperature effect—a 20% difference in relative risk—is clinically and epidemiologically significant. In absolute terms, colder months are associated with thousands of excess cardiac deaths annually in the United States. These findings align with existing guidance from the American Heart Association recommending cardiovascular risk stratification and intensive monitoring during winter months, particularly for older adults and those with established IHD. The lag structure—peak effects in the month of exposure—suggests that acute cold-related interventions (such as indoor temperature maintenance, cardiovascular medication optimization, and cold-weather physical activity counseling) may be time-sensitive and impactful.
As climate patterns shift globally and cold-weather events intensify in some regions despite overall warming trends, understanding the temperature–mortality relationship becomes increasingly relevant for health policy and adaptation planning. The study provides quantitative evidence supporting investment in winterization programs, improved heating access for low-income populations, and enhanced cardiac care capacity during colder months. Such findings are particularly salient for healthcare systems in regions with severe winters and vulnerable populations with limited heating resources.
The study identified a strong inverse association between ambient temperature and IHD mortality, with colder temperatures associated with a relative risk of 1.10 (95% CI: 1.05–1.16) at −12.8°F and warmer temperatures with a relative risk of 0.90 (95% CI: 0.86–0.94) at 89.2°F, reflecting a statistically significant 20% difference in risk across the observed temperature range (Wald p < 0.001).
— Hafiz M. Ahmed and colleagues, authors of the study published in PLOS Global Public Health
What this means
Frequently asked questions
Why does cold increase heart disease deaths more than heat?
Cold exposure triggers immediate physiological responses including vasoconstriction (narrowing of blood vessels), increased blood viscosity, elevated blood pressure, and increased clotting tendency—all of which strain the heart and increase myocardial infarction risk. Heat, by contrast, causes vasodilation and reduced cardiac workload. The study found that warm temperatures were associated with lower IHD mortality risk, while cold was associated with higher risk.
Does this mean I should avoid going outside in winter?
No. The study measures population-level associations between ambient temperature and mortality; it does not imply that all outdoor winter exposure is harmful. Instead, individuals with known heart disease or risk factors should take sensible precautions: dress warmly, avoid strenuous exertion in extreme cold, maintain indoor heating, take prescribed cardiac medications reliably, and consult their doctor about winter-specific risk management.
How reliable is this finding given that it is an ecological study?
The study’s consistency across demographic and geographic subgroups, strong statistical fit (93.6% deviance reduction), and mechanistic plausibility strengthen confidence in the finding. However, ecological designs cannot prove causality at the individual level. Prospective individual-level studies would provide additional confirmation, but the current evidence is sufficiently robust to inform public health planning and clinical practice.
Future research should examine whether individual-level data—linking personal exposure histories and health outcomes—can confirm the causal mechanisms identified in this ecological analysis. Such studies might also investigate whether temperature-related IHD mortality has shifted over the 22-year study period as healthcare and heating infrastructure improved, and whether the relationship varies by specific cardiac pathologies (acute myocardial infarction, arrhythmia, heart failure) or medication use. These lines of enquiry will refine understanding of cold’s role in cardiac mortality and inform targeted prevention strategies as climate and population demographics continue to evolve.
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