CalHeatScore Tool
The Reality of Heat in California
Heat is not just a weather inconvenience—it is a dangerous climate hazard affecting Californians today. As climate change continues to drive increasing temperatures across the state, communities across the state are feeling the impacts of extreme heat.
CalHeatScore: A Heat Warning Tool for Everyone
The California Communities Extreme Heat Scoring System – CalHeatScore – is a public health webtool built to help Californians stay informed, ready, and safe when temperatures rise. First released on December 31, 2024, CalHeatScore’s mission is to reduce heat-related illnesses and save lives by scoring upcoming heat and connecting communities to locally relevant resources.
CalHeatScore uses a heat-health ranking system to translate weather forecasts into heat scores, ranging from 0 (Low) to 4 (Severe) to indicate increasing levels of human health risk from extreme heat impacts. The heat scores are updated daily, and the ranking approach is based on historical health responses to local heat conditions. Because California has a diverse range of climates and communities across the state and even within counties, risk from extreme heat is calculated at each zip code. Read the methods document for more detail on how CalHeatScore was built.
CalHeatScore shows daily rankings of forecasted heat impacts across California ZIP codes.
Check out the CalHeatScore Tool!
CalHeatScore Tool (feature)CalHeatScore Interpretation Table
| CalHeatScore | Impact Level |
Meaning*
In your zip code…
|
|---|---|---|
| CHS 0 | Low | Little to no heat-related health impacts are expected. |
| CHS 1 | Mild | Warm day. A minor increase in heat-related health impacts is expected among those sensitive to heat. |
| CHS 2 | Moderate | Very warm day. A moderate increase in heat-related health impacts is expected among those sensitive to heat. |
| CHS 3 | High | Hot day. An increase in heat-related health impacts is expected for everyone, especially those without access to adequate cooling and hydration or those engaged in strenuous physical activity outdoors. This level of heat can pose a significant risk to health. |
| CHS 4 | Severe | Extreme heat day. A major increase in heat-related health impacts is expected for everyone, especially those without access to adequate cooling and hydration or those engaged in strenuous physical activity outdoors. This level of heat can pose a very serious risk to health. |
Notes
1. Heat-related health impacts are defined as including, but not limited to, dehydration, heat illness, heat stroke, and acute and chronic kidney failure.
2. Populations sensitive to heat are those with greater risk of heat-related health impacts. They include, but are not limited to, children, older adults, pregnant people, those with chronic health conditions and disabilities, workers and those engaged in strenuous physical activity, people who lack access to cooling and/or hydration, unhoused individuals, and those who are not used to the expected level of heat.
*Increased risk estimates for CalHeatScore are based on an analysis conducted using emergency department visit data from summer months (i.e., May to October) of 2008-2018. The general applicability of these results may be limited. See the CalHeatScore Methods Documentation for more details.
Data Access
Live Data Access via CalHeatScore API
Users can access live CalHeatScore data via the CalHeatScore REST API. Detailed documentation on how to access, configure, and use the API is located on our website’s API documentation page.
Historical Data Access
Users can access past CalHeatScore data via the CalHeatScore Historical Data Archive. The data shared in the archive provides historical scores on a monthly basis. For more information on the data, please refer to the Historical Data readme.
User Guide (coming soon!)
For general guidance on the CalHeatScore tool, please refer to the CalHeatScore User Guide (link coming soon). The guide provides a tool walkthrough, individual heat-health risks to consider, and institutional considerations to make when using the tool. CalHeatScore has also prepared a video tutorial for users (Youtube).
CalHeatScore: Frequently Asked Questions (FAQs)
What is CalHeatScore?
The California Communities Extreme Heat Scoring System (CalHeatScore) is a public health tool that was released on December 31, 2024 to help protect people from extreme heat. CalHeatScore provides the public with information to help plan and prepare for extreme heat events, including information about life-saving resources to help protect at-risk populations. CalHeatScore uses historical climate and health data, along with daily weather forecasts, to score upcoming heat based on intensity and the potential impact on health. CalHeatScore creates an easy-to-understand scale of 0 to 4 for all ZIP codes in California to communicate heat risks. CalHeatScore also connects people to locally relevant resources, such as cool centers, so that they can be prepared when dangerous heat occurs.
Why is CalHeatScore being developed?
CalHeatScore was developed to reduce heat-related illnesses and save lives by translating meteorological, weather and health data into easy-to-understand heat risk alerts. This tool raises awareness of extreme heat events and empowers parents, seniors, caregivers, and communities to plan for, mitigate, and survive extreme heat events. CalHeatScore is committed to prioritizing the most vulnerable populations and ensuring that everyone has the information they need to stay safe during extreme heat.
How does the CalHeatScore tool work?
CalHeatScore is a public health tool that translates local weather and health data into information that the public can use. Here’s how it works:
- Localized Heat-Health Thresholds: Provides temperature thresholds that indicate the health risks of heat that are specific to California communities at the ZIP code level.
- Daily Updates: Delivers updated daily forecasts of heat risk related to localized increases in expected heat-related illnesses.
- Local Population Profiles: Provides local sociodemographic profiles on some of those most impacted by heat, including children, older adults, outdoor workers, linguistically isolated, and those without health insurance.
- Resource Connections: Points users to locally relevant resources, such as the locations of cool centers.
What makes CalHeatScore distinct and suitable for California?
CalHeatScore is designed specifically for California’s unique needs:
- Locally Relevant Data: Provides ZIP code-level rankings of the severity of heat events that are tailored to California’s diverse microclimates and populations. Also includes supplemental data which identifies proportions of different populations more sensitive to extreme heat.
- Resource Integration: Connects users to locally relevant resources, such as cool centers. Centralizes access to heat-related resources across multiple government departments.
Who are the partners involved in developing CalHeatScore?
The Office of Environmental Health Hazard Assessment leads the effort to develop CalHeatScore with support from:
- Office of Land Use and Climate Innovation: Consults on communication strategies and partnerships.
- Department of Insurance: Provides data and insights from a 2024 report on insurance gaps relating to heat.
- Department of Public Health (CDPH): Offers recommendations on local applications of the tool.
- Office of Emergency Services (CalOES): Assists in communications and dissemination plans.
- In addition to these specific roles, all state agencies listed provide input on improvements to CalHeatScore.
- Additional collaborators that have contributed to the development of the CalHeatScore system include the UCLA Center for Healthy Climate Solutions and researchers at Oregon State University and Applied Climatologists, Inc.
How should CalHeatScore be used now?
CalHeatScore can be used to stay safe during extreme heat. Users are encouraged to:
- Access daily updates and heat rankings.
- Use the tool to find local resources like cool centers.
- Provide feedback to help improve future versions.
How can I provide feedback or get involved?
Users can share their experiences and suggestions to help improve CalHeatScore by emailing our program email: CalHeatScore@oehha.ca.gov. Opportunities to provide feedback will also be available at community outreach events.
CalHeatScore Informational Webinar Series
CalHeatScore hosted an informational webinar series from June 2025 to March 2026. This series was a partnership effort between OEHHA, the Governor's Office of Land Use and Climate Innovation (LCI), the Los Angeles Regional Collaborative for Climate Action and Sustainability (LARC), and the UCLA Luskin Center for Innovation.
Recordings of past CalHeatScore Webinars now available on the OEHHA YouTube channel.
CalHeatScore Webinar #1: June 12, 2025
This webinar provided background on CalHeatScore, examples of how to use the tool, and shared an overview of the public engagement process.
CalHeatScore Webinar #2: July 24, 2025
This webinar included an overview of heat warning systems around the world, a presentation on the opportunities and challenges of CalHeatScore, and a discussion on CalHeatScore development.
CalHeatScore Webinar #3: October 15, 2025
This webinar provided an overview of AB2238 (CalHeatScore’s enabling legislation), highlight state agencies that are integrating CalHeatScore into their programs, and outlined upcoming deliverables from the CalHeatScore team.
CalHeatScore Webinar #4: November 12, 2025
This webinar provided an overview of extreme heat planning at the state level, highlight California communities who are developing local extreme heat plans, and reviewed the purpose and proposed contents of the CalHeatScore User Guide.
CalHeatScore Webinar #5: January 28, 2026
This webinar introduced new supporting materials and how these resources can be utilized to advance extreme heat planning across the state.
CalHeatScore Webinar #6: March 4, 2026
This webinar provided an overview of recent and upcoming updates to the CalHeatScore tool as well as resources designed to support its use in heat-vulnerable communities. Participants also learned about additional opportunities for feedback and engagement with CalHeatScore staff.
Background
AB 2238
In 2021, the California Department of Insurance’s (CDI) Climate Insurance Workgroup recommended California build a system to rank heat waves to better communicate the deadly risks to Californians and help communities prepare, similar to how tropical storms and hurricanes are described by "category" level.
Assembly Bill No. 2238 (L. Rivas, Chapter 264, Statutes of 2022) codified CDI’s recommendation by requiring CalEPA to develop a statewide extreme heat ranking system. AB 2238 requires the Integrated Climate Adaptation and Resiliency Program (ICARP) to disseminate CalHeatScore information to communities and local leaders, and the Governor's Office of Emergency Services (Cal OES) to support ICARP’s dissemination and communications campaign. The Department of Public Health (CDPH) provides consultation on CalHeatScore and will support OEHHA to adapt the system for use at locally relevant scales. AB 2238 also required CDI to release a report on the insurance-related costs of extreme heat, described below.
As part of the requirements of AB 2238, the CA Department of Insurance released a first-of-its-kind report: “Impacts of Extreme Heat to California’s People, Infrastructure, and Economy", which showcased how a single heatwave could not only result in hundreds of heat-related deaths and emergency department visits, but billions of dollars in economic losses. The total economic impacts across the Health and Safety, Manufacturing and Agriculture, Infrastructure, Energy, and Governance sectors for the seven studied extreme heat events amounted to $7.7 billion, affecting nearly the entire population of California. The California Department of Insurance is collaborating with OEHHA to iterate on the CalHeatScore model and explore how the tool can be used to provide insurance-related economic protections from extreme heat.
CalHeatScore is also required to include recommendations on thresholds or triggers for the implementation of policies that reduce health risks associated with extreme heat and recommendations for metrics to measure the short- and long-term impacts of extreme heat on human health. These are key components of CalHeatScore and will be addressed in future iterations of the tool.
CalHeatScore has been developed in response to AB 2238, considers the findings of the CDI Climate Insurance Report, and works to continue researching the issues and recommendations highlighted by the report. Links to all these partners’ heat-related websites and selected content is available in Resources section below.
AB2238 requires CalHeatScore to be based on...
- Available meteorological data on heat events
- Data on the associated health impacts,
- Measures of extreme heat severity and its health impacts,
- Locally relevant information such as urban heat island effects; and
- Public input.
Development of CalHeatScore
OEHHA’s Extreme Heat Ranking System Development Section (EHRSDS) developed CalHeatScore on behalf of CalEPA through a contract with University of California, Los Angeles, Center for Healthy Climate Solutions (UCLA-CHCS). In consultation with EHRSDS, UCLA-CHCS developed the underlying statistical model used in CalHeatScore in collaboration with Dr. John Molitor of Oregon State University and Dr. Larry Kalkstein of Applied Climatologists.
Our Partners
About OEHHA
OEHHA's mission is to protect and enhance the health of Californians and our state’s environment through scientific evaluations that inform, support, and guide regulatory and other actions. OEHHA supports the development and operation of CalHeatScore as part of California’s implementation of AB 2238. OEHHA has also developed related tools that synthesize environmental and socioeconomic data into statewide maps, such as CalEnviroScreen.
OEHHA has conducted studies on the health impacts of increased heat and tracked heat-related indicators of climate change. OEHHA’s studies have shown associations between heat and a range of negative health affects including death, low birth weight, and increased emergency room visits and hospitalizations for conditions ranging from heart conditions to mental health. These studies also have helped identify populations that may be more vulnerable to heat-related deaths and illnesses, including older adults, pregnant people, infants and children, and people of color.
This research also has helped inform some of California’s other extreme heat efforts: Preparing California for Extreme Heat: Guidelines and Recommendations (2013), CalEPA Urban Heat Island Index Report (2015) , CA Climate Adaptation Strategy , and the Extreme Heat Action Plan (2022).
Resources
Sign up for Emergency Alerts
- CalAlerts: Sign up for national and California emergency alert notification
- Listos: Sign up for County and City specific emergency alert notification
Additional Safety Tips and Resources
Know the Signs of Heat Illness
Know How to Stay Cool
Find your nearest cooling center on the CalHeatScore map or at County Cooling Centers and Resources
If you have signs of heat-related illness, and the strategies listed above or at the resources below are not providing relief, please contact a medical provider or go to the nearest emergency department as soon as possible
Tips for staying informed and safer during extreme heat
- Heat Ready CA
- CDPH Heat and Summer Safety
- CDPH Tips for Preventing Heat-Related Illness
- CalOES Cooling Centers
- OEHHA Fact sheet “Pregnancy and Heat – Protecting yourself and your baby”
- OEHHA Fact sheet “Children and Heat – Protecting infants, kids, and teens”
- About Heat and Your Health | Heat Health | CDC
Additional extreme heat programs, tools, and plans
- California Department of Insurance (CDI)
- Governor's Office of Planning and Research (OPR)
- Extreme Heat Planning
- Heat Mapping Tools
- OEHHA Research
- Centers for Disease Control: Temperature Extremes | Climate and Health | CDC
Heat and Communities
The health impacts of extreme heat are not felt equally across communities. Social determinants of health—such as income, access to resources, and living conditions—play a critical role in how well individuals can cope with rising temperatures. The impact of extreme heat is further compounded by long-standing systemic inequities, including racial and environmental injustices, that leave certain communities more exposed and less resilient. For more information on the impact of heat on communities, please refer to this informational reference page.
Listserv
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Contact Us
OEHHA is seeking feedback to refine and improve CalHeatScore. We invite community members, local and Tribal governments, researchers, nongovernmental organizations, and others to share their ideas and input on CalHeatScore via our program email.
Walker Wieland
Walker Wieland is the manager of the Extreme Heat Ranking System Development Section. The CalHeatScore program email is: CalHeatScore@oehha.ca.gov.
For media inquiries, please contact Amy Gilson: Amy.Gilson@oehha.ca.gov.