It’s Not the heat, It’s the Vulnerability: Attribution of the 2016 spike in Heat-Associated Deaths in Maricopa County, Arizona

The Maricopa County (AZ) Department of Public Health has operated an advanced heat mortality surveillance program since 2006. In 2016, and then again in 2017, this program reported a record number of cases of heat-associated deaths. The spike in 2016 was particularly notable, representing more than a 75% increase in cases from the prior ten-year average. The summer of 2016 also brought record-setting high temperatures to the region. The month of June was the warmest observed in the county and the six-month warm season spanning May through October was the fourth warmest. We used time series models to estimate the extent to which the anomalously warm weather in 2016 contributed to the large increase in heat-associated deaths relative to previous years.

None of the statistical models we used suggested that 2016 should have been a year with an elevated count of heat-associated deaths in Maricopa County, despite the anomalous warmth. All combinations of exposure variables and model parameterizations we chose suggested that the number of heat-associated deaths in 2016 should have fallen near or below the historical average. This finding suggests that non-meteorological factors were responsible for the spike in heat-associated deaths in 2016. Among the plausible explanations is a change in the social service programs that provide shelter for homeless individuals. The number of heat-associated deaths among homeless individuals and the share of heat-associated deaths that those cases represented were both anomalously high in 2016. This pattern may be related to the observed 25% increase in the number of unsheltered homeless individuals in the community between 2015 and 2016. Non-meteorological factors may play a more important role in driving year-to-year and long-term trends in heat-associated mortality than commonly modeled or understood. Our findings suggest that heat-health risk assessments should strive to include temporal analysis of social vulnerability factors to be more informative.