New California tool ranks extreme heat danger by neighborhood. Here's how it works

It’s going to be hot this summer. And a new tool can help you figure out the risk that the temperature can pose to your health.

The California Communities Extreme Heat Scoring System — which the state is calling CalHeatScore — can tell you the level of heat-related health risk you face and point you to cooling resources in your area.

This pilot program ranks risk from extreme heat into four categories, from mild to severe. A ranking of “severe,” for example, could correspond with a higher temperature inland, as opposed to at the beach.

Why it was developed

In 2021, the California Department of Insurance recommended that the state create a heat warning system similar to those for hurricanes or tornadoes. The group later published a report that found that extreme heat events from 2013 to 2022 resulted in 5,000 hospitalizations and nearly 460 deaths. A 2022 state law required the development of the system.

Walker Wieland, who manages the project in the state’s Office of Environmental Health Hazard Assessment, calls extreme heat a “silent killer.”

Heat “doesn't have these scenes of catastrophe like you see with wildfires or hurricanes,” he said.

But scientists say it is no less dangerous.

“ People still don't often see extreme heat as really risky, so we need to really reach out, especially to people who are going to be more vulnerable,” said David Eisenman, professor of medicine at UCLA and co-director of the Center for Healthy Climate Solutions.

He led the team that developed the statistical methods behind the heat score system.

CalHeatScore incorporates the number of emergency room visits for four heat-related illnesses: heat stroke, heat exhaustion, kidney failure and dehydration. Crunching that data, plus temperatures, is how the tool can issue public health warnings — going beyond anything a phone weather app can do, for example.

Heat scores are also broken out by ZIP code — an especially important distinction in Southern California, Eisenman explained.

“You can have, as in Los Angeles, one set of weather on one side of the county and all kinds of different weather 15 miles away,” Eisenman said.

The graph shows how health risk increases with temperature. The blue dots represent the number of heat-related hospital visits at a given temperature, which is charted along a baseline value and incremental increase on the Y-axis.

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California Office of Environmental Health Hazard Assessment

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Levels of risk

CalHeatScore established a baseline of risk for heat-related illness, which is level zero. As risk rises, so does the heat score, from 1 to 4.

A four represents the highest level of risk on CalHeatScore's scale.

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California Office of Environmental Health Hazard Assessment

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Each day, you can check the ranking for your ZIP code, as well as the forecast for the following week. CalHeatScore also lists nearby cooling centers, where people can get a reprieve from heat waves.

The people who developed the tool also hope it can aid public health response.

“ We need to take those warnings and turn them into actions that people can take for themselves — as well as actions that responsible public officials can take for their communities,” Eisenman said.

For those who run hospitals, schools, or outdoor workplaces and warehouses, the tool provides a demographic breakdown of each area, including factors such as the number of outdoor workers and those who have insurance.

“ There's so many mediating factors between a person, a hot day, and whether or not that person becomes ill,” said V. Kelly Turner, associate director at UCLA’s Luskin Center who was involved in the creation of the tool.

California envisions the heat-scoring system being used by a wide range of people and institutions.

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California Office of Environmental Health Hazard Assessment

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Application and future uses

CalHeatScore is still a pilot program.

California’s Division of Occupational Safety and Health, for example, relies on data from the National Weather Service and uses temperature cutoffs to guide public and employer advisories.

Wieland, Turner and Eisenman believe CalHeatScore can be incorporated into responses by public officials.

“CalHeatScore can be a  great planning tool to allocate resources or maybe could be a tool to trigger certain thresholds that are involved in the local emergency response and local planning,” Wieland said.

His team has begun outreach with local governments, including Los Angeles and San Mateo counties.

In the meantime, Wieland is building a mobile app that can send notifications. His team will host a webinar series and regional workshops to explain the tool.

CalHeatScore, though available for use this summer, is still in its early stages of product development.

Turner highlighted the long-term nature of the project.

“ It's a stepping stone to the future in which heat warnings aren't about the actual temperature threshold,” she said. “They're highly personalized — because exposure to heat and whether or not you get sick depends on factors that are more personal than universal.”