This story originally appeared in the California Health Report.
Another summer of extreme heat is sending many Californians to hospitals and emergency rooms in growing numbers. It is one of the deadliest climate hazards, yet its true human toll remains obscured.
As a public health practitioner who studies health impacts from heat, I find the trend unmistakable and concerning. Behind the numbers are people without access to cool places, workers laboring in unsafe conditions, children attending schools without enough shade and residents managing chronic health conditions in homes that trap heat. In too many cases, we can only infer their stories.
Even worse, many never show up in the surveillance data at all. They struggle to survive these conditions year after year, yet remain hidden.
Tracking California, an independent research program at the Public Health Institute, has conducted statewide surveillance of heat-related illness for over two decades. Our state-level data shows that the health burden is getting worse. Hospitalizations and emergency room visits for heat-related illness have risen roughly 30 percent from the first decade of our data (2005-14) to the most recent one (2015-24). Rates were highest in 2024, California’s hottest summer on record.
But heat doesn’t affect all Californians the same way. Our analysis found that between 2019 and 2020, unhoused residents accounted for 1 in 10 heat-related ER visits statewide, despite being a small fraction of the population. On hot days, unhoused individuals were 38 percent more likely than housed individuals to end up in the ER for heat-related illness.
While this analysis reveals a disparity that earlier surveillance missed, the picture is still incomplete. Data from health systems that support California’s most vulnerable residents, including community clinics and safety-net providers serving low-income, uninsured and undocumented populations, are largely absent from heat surveillance. Without this data, prevention efforts risk missing the very people they are meant to protect.
Granular data matters because the health risks of heat are local. It is shaped by where people live, work and learn. Low-income households are more likely to live in poorly insulated homes that trap heat, and as California’s nights grow warmer, residents in these homes will have less time to recover before the next day’s heat arrives. Agricultural workers labor in extreme temperatures with minimal protection. Some schools lack sufficient shade or cooling infrastructure, putting children at higher risk as daytime temperatures soar.
These risks vary across communities, which is why one-size-fits-all solutions do not work for everyone. For heat resilience programs to protect public health, they must be informed by local data that capture the specific vulnerabilities and lived experiences of the communities they serve.
At Tracking California, we partner with under-resourced, highly impacted rural and Tribal communities across the state to integrate data into heat prevention initiatives. Together, we are supporting the development of local cooling centers and resilience hubs, piloting heat resilience training programs and building locally relevant heat action plans shaped by the people who use them. Pairing data and science with local expertise and priorities, these programs reflect a novel approach in California that put the community at the center of heat resilience.
These collaborative models, linking local health surveillance with grassroots partnerships, can be scaled for better heat safety statewide. California is investing in heat-resilient infrastructure to make homes, schools, neighborhoods and workplaces safer. Data must be treated as essential to that infrastructure. Consolidation of heat-related illness data from community clinics and urgent care centers into state heat surveillance systems is an essential step. The data will tell us where heat impacts are rising, who is hit hardest and whether resilience investments are reaching those with the fewest resources to cope.
Every heat-related illness and death is preventable, but prevention requires visibility and planning. As the heat season grows longer and more extreme, better health data is one of our most powerful tools for saving lives.
Shubhayu Saha is the executive director of Tracking California at the Public Health Institute.
The California Health Report reports on communities across the state about health equity issues.
