Commentary on Dennett <i>et al</i> .: When heat and the opioid epidemic collide ‐ implications for climate‐resilient public health strategies

Extreme heat driven by climate change amplifies the risk of drug overdose mortality, particularly among those already facing multiple sources of vulnerability. Integrating substance use considerations into climate adaptation strategies is essential to reduce the growing health burden in affected communities. Climate change and the spread of unregulated synthetic opioids represent two of the most pressing public health challenges facing communities across the United States (US). Understanding how these crises intersect is essential, as many of the populations most affected by one are also disproportionately burdened by the other [1, 2]. In their article, Dennett et al. [3] quantified, for the first time across all counties in the continental United States, the effect of extreme heat exposure on drug overdose mortality at the monthly level. They found that an increase in heat (using a composite index to also consider humidity metrics) leads to an increase in all drug overdose mortality during summer months. They estimated that approximately 150 excess drug overdose deaths occurred per year are because of heat exposure. In other words, implementing targeted prevention programs for individuals at risk of drug overdose during periods of extreme heat could substantially reduce the overall burden of overdose deaths. Any heat-related death or hospitalization is evitable [4]. Different adaptation strategies exist to prevent such impacts, but the intersection with drug use and overdose survivability is needed. Overall, we can distinguish two main types of adaptation strategies toward extreme heat: (i) long-term strategies focusing on housing, infrastructure (including healthcare services), urban planning and educational efforts; and (ii) seasonal and emergency responses through heat action plans and early-warning systems. For long-term strategies, an essential component concerns urban design and micro-built environments, which strongly influence spatial variations in the temperatures that communities experience [micro-heat islands, (MHI)] [5]. Emerging evidence also indicates that these environmental features can affect overdose survivability, although findings remain mixed [6]. For instance, some studies have reported positive associations between the presence of tree canopy or parks and fatal overdoses [7]. These results highlight the need for urban planning approaches that not only mitigate MHI through increased green space, but also account for potential impacts on drug use behaviors and overdose risk. In parallel, many local authorities have implemented heat action plans coupled with heat early-warning systems. Heat action plans include various community-level measures that can be active during the whole summer or activated in conjunction with early warning systems (typically based on National Weather Service’s local thresholds) [8]. Examples of such measures include opening community-based cooling centers or organizing home visits by healthcare professionals for vulnerable individuals. However, current heat action plans hardly incorporate targeted interventions for people who use drugs, representing a critical gap in existing adaptation strategies. To better inform how to update existing heat adaptation policies or inform pioneering ones, it is, therefore, crucial to: (i) systematically characterize underlying mechanisms; and (ii) adopt an intersectoral perspective to understand which individual and contextual characteristics modulate the heat-overdose risk. In their article, Dennett et al. [3] describe some possible pathways through which heat can lead to increase in opioid-related overdose, including physiological and behavior mechanisms as well as polysubstance use (including co-use of opioids, stimulants or psychotropic medications). Yet, other mechanisms are possible and need to be documented. These mechanisms include social isolation and disruptions of the drug supply (especially for opioid use disorder medications), harm reduction services or other overdose prevention programs. Moreover, climate change increases the likelihood that extreme heat events will coincide with other climate-sensitive hazards, such as wildfires or hurricanes, which can further elevate the risk of opioid-related overdose [9, 10]. Adopting a research framework that addresses compounded climate hazards [11] is, therefore, essential for understanding and mitigating their cumulative impact on people who use drugs. Furthermore, understanding which individual or ecological characteristics modulate the heat-opioid overdose nexus is critical to inform such adaptation strategies. In their article, Dennett et al. [3] showed that the effect varied by drug type (opioids, cocaine or psychostimulants) and that the effect was higher in counties with greater levels of social vulnerability as well as in suburban and urban counties. Yet, in future research, it would be important to also document how the effect of extreme heat varies over smaller spatial units as done for other outcomes [12] and the role of other potential effect modifiers, such as housing insecurity, as well as other environmental justice metrics [13]. The convergence of climate change and the opioid epidemic emphasizes the urgency of addressing overlapping public health crises through integrated adaptation strategies. By revealing how extreme heat intensifies overdose mortality, Dennett et al. [3] highlight the need to consider substance-use vulnerability into climate resilience planning and policy design. Tarik Benmarhnia: Conceptualization (equal); writing—original draft (lead). None.