Heat timeline and local impact

Heat rarely appears alone on a death certificate. More often, it acts as the trigger, like worsening a heart condition, complicating drug intoxications or pushing a compromised respiratory system past its limit. Whether that death gets classified as heat-related depends on the medical examiner, the information available and how the death certificate is ultimately coded. Those decisions vary across counties, states and time.

This is why official heat-related death counts are widely understood to be undercounts. Excess mortality analyses, when you compare total deaths during a heat event to what would statistically be expected, more consistently captures a higher number of deaths than cause-of-death coding alone. This gap between counted and excess heat-related deaths represents lives lost to heat that formal surveillance simply can’t record. 

Undercounting is not uniform, however. It is more common in places where heat-related death investigations are not standardized across all medical examiners in the jurisdiction and over time. Standardizing these investigations is particularly challenging if extreme heat only occurs episodically, rather than consistently on an annual basis. Understanding these limitations is essential to understanding heat-related mortality counts.

Accurately counting heat-related deaths requires more than a death certificate. It requires active investigation, standardized definitions and a system designed to find deaths that would otherwise go uncoded. That capacity took decades to build, but Arizona became a leader in developing it.

The science of heat-related death surveillance has evolved considerably since heat first became a tracked cause of mortality. Early classification relied almost entirely on death certificate coding, which depended on physicians and medical examiners identifying heat as a cause or contributing factor. Over time, public health researchers demonstrated that this approach missed a substantial proportion of true heat deaths, particularly where heat’s role was indirect but decisive.

Active case investigation — the process of systematically reviewing deaths during heat events against weather data, location of exposure and medical history — emerged as the more rigorous standard. Maricopa County has been at the forefront of developing and applying these methods, and its surveillance system has become a national model for counting  heat related deaths as a result.

The field continues to evolve. Excess mortality modeling, syndromic surveillance using emergency department visit data and remote sensing of environmental heat exposure are all advancing our ability to capture what traditional death certificate coding cannot.

Mortality counts tell us how many people died. They don’t tell us which neighborhoods have higher risk because of their housing type, shade or vegetation levels, or which communities face the greatest social and economic barriers to surviving extreme heat.

The Community Resilience Estimator (CRE) Heat Index was developed to answer those questions. Mapped at the census tract level across Arizona, it integrates the social, economic and environmental conditions that compound heat risk: housing quality, access to cooling, social isolation, age, income, and heat-amplifying physical infrastructure like impervious surfaces and absence of tree canopy.