With summer months approaching, soon too will household battles over thermostat temperatures. A new research review, authored by Drexel University public health researchers, looking at 29 papers, spanning five continents, may inform these debates with insights on how indoor temperatures impact health.
The latest paper follows a research review — highlighted last year by the Drexel News Blog — on the health effects from overheating indoors and how building design standards must evolve to protect health and safety. That research came from Simi Hoque, PhD, a professor and head of the Architectural Engineering program in Drexel’s College of Engineering, Leah Schinasi, PhD, an assistant professor who studies environmental and occupational health in the Dornsife School of Public Health, and Chima Hampo, a doctoral student in the College of Engineering. Now, the three researchers partnered with lead author Janelle Edwards, a doctoral candidate during the study and now a post-doctoral researcher in The Ubuntu Center on Racism, Global Movements, and Population Health Equity at the Dornsife School of Public Health, for the latest data on indoor temperatures and health.
The News Blog checked in with Schinasi, the senior author of the latest review, and Edwards, to ask what the data suggests may be an optimal maximum indoor temperature, why this research is so challenging and where research and policy must go from here.
Based on this latest data on high indoor temperatures and their effect on physical and mental health, what are the major takeaways for the general public?
Schinasi: In our paper, we identified and summarized published, peer reviewed studies that investigated associations between warm season indoor temperatures and variety of health or well-being outcomes: acute (short-term) cardiovascular or respiratory health outcomes, impaired sleep, impaired cognitive function, heat stress, thermal discomfort, impaired emotional health and depression or anxiety. The studies were conducted in variety of geographic locations, including the United States, Canada, Australia, Taiwan, China, Germany, the United Kingdom and Ethiopia.
Overwhelmingly, results from our synthesis of this literature suggested links between hotter indoor temperatures and poorer health outcomes.
Is there an approximate maximum indoor temperature you generally see as acceptable to prevent negative health impact?
Beyond examining overall associations, a key objective of our review was to identify safe maximum indoor temperatures for residential settings. That is, we aimed to identify the hottest indoor temperature that was both safe (and comfortable) for residents, after which declines in health, well-being or comfort started to occur. This question has direct implications for health advisories, such as recommended thermostat settings, and for policies requiring landlords to maintain indoor temperatures below certain thresholds during hot periods.
Our review highlighted two main findings. First, there remains considerable uncertainty, and a lack of empirical evidence, regarding what constitutes a safe maximum indoor temperature. Second, safe temperature thresholds seem to vary by setting and health/well-being outcome.
For instance, a study in Boston found that participants experienced disrupted sleep and reduced brain function at indoor temperatures of 22°C/71.6°F. In contrast, a study in New York City identified 26°C/78.8°F as the threshold associated with increased cardiovascular events, while research in Australia reported declines in self-rated health when temperatures exceeded 24°C/75.2°F.
These differences underscore the complexity of defining a universal “safe” indoor temperature. In reality, a single threshold may not be appropriate for all populations, settings, or health outcomes. But again, research on this topic remains limited and future may work may offer better clarity on this topic.
Why is it increasingly important to address the public health effects of rising indoor temperatures?
Edwards: Addressing indoor temperatures is becoming increasingly urgent as people spend over 90% of their time indoors, making the quality of indoor environments a major determinant of health and well-being. Yet, few enforceable regulations exist to ensure safe indoor temperatures, particularly in residential settings, both in the U.S. and around the world. As climate change intensifies, extreme heat events are growing more frequent and severe, often turning indoor spaces into dangerous environments, especially in poorly insulated or aging housing. These risks are not felt equally: low-income and historically marginalized communities are more likely to live in substandard housing with inadequate access to cooling or heating. This unequal exposure compounds existing health disparities and introduces new layers of complexity to efforts aimed at climate adaptation and health equity.
Schinasi: It’s also important to consider safe residential indoor temperatures because most adults, and especially those who are biologically sensitive, such as older adults and very young children, spend a majority of their time indoors and at home. Given this, it is critical to understand how to maintain safe and healthy indoor home environments.
What else is needed before action is taken at much greater scale on this issue?
Schinasi: In general, there’s a dearth of research that captures associations with indoor, rather than outdoor temperatures — our review paper captured only 29 papers and we were fairly comprehensive in terms of the health outcomes included. This represents a huge gap in research and thus understanding, particularly when we consider that most adults in higher income countries spend a majority of their time indoors and at home, even more than ever since the COVID-19 pandemic.
The studies captured in our review revealed substantial variability in what might be considered a “safe” or “optimal” warm-season indoor temperature. People experience and respond to indoor temperatures differently based on several factors. These include having underlying health conditions that impair the body’s ability to respond to heat, wearing bulky clothing, which can retain heat; and engaging in high metabolic output physical activities that cause the body to generate heat. Cultural norms and expectations around indoor comfort further shape individual responses to temperature. Together, these factors complicate efforts to define a single, universal threshold for safe indoor temperatures.
There remains a striking lack of empirical research examining the relationship between indoor, as opposed to outdoor, temperatures and mortality or morbidity outcomes. A major reason for this gap is the limited availability of publicly accessible data on indoor temperatures.
In the studies included in our review, researchers employed various strategies to address this data gap. Some collected indoor temperature measurements directly using thermometers or sensors placed in participants’ homes; others used physics-based models, or other modeling approaches, to estimate indoor conditions. Each of these methods has limitations, and further work is needed to improve data collection and modeling approaches.
We hope our review draws attention to these critical data gaps and encourages further research on indoor temperature exposures and health.
How can public policy and businesses better adapt right now to reflect this data on extreme hot temperatures and health?
Edwards: Summer indoor temperature threshold policies currently being proposed or recently enacted by governments (in places like Toronto, Canada and Dallas County, Texas) may be set too high. Many of these policies suggest indoor thresholds around 80°F (26.7°C), which many might agree is still uncomfortably warm. Our study found evidence of adverse health outcomes at indoor temperatures as low as 64.4°F (18°C).
While it’s encouraging to see governments beginning to hold landlords accountable for maintaining comfortable indoor temperatures for tenants during warmer seasons, our scoping review suggests they may need to reconsider what constitutes an appropriate threshold.
