Specific Objectives of the Project:
Using panel data at the individual and county levels, we propose to investigate the causal impact of extreme temperature events and drought on mental health in California with particular attention to agricultural communities. More specifically, we will:
● use spatially-explicit health data (~245,000 respondents over 18 years of age) from multiple waves of the California Health Interview Survey (CHIS) between 2005 and 2015 to assess the impact of extreme temperatures and drought on psychological distress in agricultural communities
● combine county-level time series on temperature, drought, and suicide rates over the period, 1999–2013 to determine if suicide rates respond to extreme temperatures and drought, and if this response varies between metropolitan and rural counties.

Summary of Results:
The main contribution of the project is in highlighting the relationship between heat waves and adult psychological distress. Previous research in this area, i.e., investigating the role ambient temperature plays in influencing mental health, has considered the impact of rising temperatures on hospitalizations for mental illnesses or suicides, yet there is a lack of attention to how warming affects mental health on a more basic level. In psychiatric research and practice, psychological distress is recognized as a precursor to exhaustion (Arvidsdotter et al. 2016). Furthermore, the majority of analyses in this area focus on non-consecutive instances of extreme heat and extreme cold, rather than heat waves, or cold waves. As sustained extreme weather events, heat waves draw down physiological and economic resources at a greater rate, leaving greater psychological distress in their wake. Consequently, the primary contribution of our study is in showing the type and aspect of heat waves that exerts the most adverse impact on psychological distress. To wit, we find that nighttime heat waves compound psychological distress while daytime heat waves do not. Furthermore, as the number of days in a heat wave increases, the greater the impacts on psychological distress.

A second contribution of the study is to show that non-wave extreme heat events do not compound psychological distress. This result is counter to the clear negative impact that extreme heat appears to have on other mental health outcomes such as emergency department visits or suicides (see for example, Mullins and White 2019 and Burke et al. 2018).

A third and methodological contribution of the study is its assessment of individual heat exposure at a finer geographical scales than seen in previous studies in the heat-health literature. By measuring temperature exposure at the respondent’s residential ZIP code area rather than their county, we ensure that intra-county differences in extreme temperature events are recognized.

From a policy perspective, one takeaway from our research is the possible welfare enhancing impact of a policy that reinforces messaging related to heat warnings and advisories with particular attention to local percentile thresholds. Such a policy is consistent with previous research, such as Guirguis et al.’s (2014) who found that in California, between 1999 and 2009, “there were 11 000 excess hospitalizations that were due to extreme heat over the period, yet the majority of impactful events were not accompanied by a heat advisory or warning from the National Weather Service. These results suggest that heat-warning criteria should consider local percentile thresholds to account for acclimation to local climatological conditions as well as the seasonal timing of a forecast heat wave.”

In terms of the mechanism at work here, we hypothesize the following. Heat exerts an adverse impact on mental health. Adult psychological distress is sensitive to sustained rather than scattered heat exposure, and to nighttime rather than daytime heat. A nighttime heat wave is particularly damaging to mental health. We believe that this may stem from the disruptive impact of nighttime heat waves on human sleep. Our hypothesis is consistent with Lohmus (2018) who, after reviewing experimental data on sleep and temperature, concludes that, “… a heatwave with greatly elevated night temperatures is probably more detrimental for sleep than high temperatures during daytime only.” Lohmus also observes that in people who are not acclimatized to warmer weather, “high nighttime temperatures are almost always deleterious to sleep quality.” Though more interdisciplinary research is often recommended, these preliminary pieces of evidence suggest that the mechanism by which (nighttime) heat affects adult mental health may travel through sleep patterns.

Negative / Incomplete Results. Unfortunately, due to the period over which the project was funded (and extended), challenges arose surrounding Covid-19 that limited the extent to which we could investigate all the objectives listed above. As illustrated in the results section above, while we were able to investigate the role of extreme temperature events on mental health (as measured by psychological distress), we were not able to (i) further disaggregate the analysis to fully investigate the relationship within agricultural communities nor (ii) perform a less granular and complete analysis of suicides and their relationship to extreme temperature events at the county level. Preliminary assessments were performed with lackluster results. That is, preliminary investigations of the relationship between psychological distress and extreme temperature events within agricultural communities did not show any strong relationship nor did preliminary analysis of suicide rates and drought when measured at the county level. The lack of statistically significant results does not suggest that such relationships do not exist, but rather indicate that with the data we had available and the specifications we were limited to, such relationships were not identified. We feel that both of these areas warrant further research.