Heat waves and annual mortality among older adults (aged ≥65 years) in the USA

Background

Heat waves are increasing in frequency and intensity due to climate change. While acute mortality effects are well documented, the impact on annual mortality and the modification of this association by race, poverty level, and amount of green space at a national level are less understood. We aimed to quantify the association of heat waves with annual all-cause and cause-specific mortality among older adults in the contiguous USA.

Methods

We conducted a cohort study using an open cohort of 73 769 163 Medicare beneficiaries (aged ≥65 years) residing in 27 926 ZIP codes across the contiguous USA from 2000–18, with 668 448 618 person-years of follow-up. Heat waves were defined at the ZIP-code level as 2 or more consecutive days with a minimum temperature exceeding 2·5 SDs above the local 30-year summer mean. We used quasi-Poisson regression models at the ZIP-code-year level to examine the association between the annual number of heat waves and mortality counts, adjusting for individual and neighbourhood-level characteristics and secular trends.

Findings

An additional heat wave was associated with an increase in the annual mortality rate of 8·83 deaths per 10 000 person-years (95% CI 5·99 to 11·68). Across the contiguous USA from 2000–18, the 8307 observed heat waves were associated with an estimated 17 603 (95% CI 11 942 to 23 287) excess deaths. Were an additional heat wave to occur in every ZIP code each decade, we estimated 56 815 (95% CI 38 651 to 74 979) premature deaths would result. Increased heatwaves were associated with cardiovascular, respiratory, and neurological deaths. In subgroup analyses, the mortality increase was significantly greater for Black individuals (16·50 per 10 000 person-years, 95% CI 8·11 to 25·04) than for White individuals (5·85 per 10 000 person-years, 2·79 to 8·93). The association was stronger in high-poverty neighbourhoods (11·09, 4·00 to 18·29) and was protective in neighbourhoods with abundant green space (–13·51, –24·92 to –1·79).

Interpretation

Heat waves are associated with a substantial increase in annual mortality among older US adults, with disproportionate impacts on Black communities and communities on low incomes. Our findings suggest that climate change poses a significant chronic risk to public health, highlighting the need for urgent mitigation and targeted adaptation measures to address these inequities.