In 2024, Phoenix Fire Department became the first major urban fire service in the United States to systematically deploy prehospital cold water immersion as standard treatment for heat stroke. Over 300 patients treated in a single summer. The hottest year on record in Phoenix. And heat-related deaths actually decreased. The peer-reviewed data is now published — and every EMS agency should study it.

The context: Phoenix as a heat stress laboratory

Phoenix, Arizona, is the largest US city exposed to extreme, sustained heat. July 2023 was the hottest month ever recorded in an American city: average temperature 102.7°F, a 31-day streak of highs at or above 110°F, peak at 119°F. Arizona recorded 645 heat-related deaths in 2023 — a 52% increase from 2022.

The emergency medicine teams at Valleywise Health Medical Center in Phoenix responded. Dr. Geoffrey Comp and Dr. Paul Pugsley — emergency physicians at Valleywise and faculty at the University of Arizona College of Medicine-Phoenix — developed an in-hospital cold water immersion protocol in 2023. In 2024, they partnered with the Phoenix Fire Department to extend it into the prehospital setting.

The protocol: simple enough for field deployment

Dr. Pugsley stated it directly to the Associated Press in June 2024: "This cold water immersion therapy is really the standard of care to treat heatstroke patients."

The protocol: as soon as a patient presents signs of heat stroke — elevated temperature and neurological changes — paramedics initiate cold water immersion on scene or in the ambulance, before transport. Equipment: a lightweight bag filled with ice slurry, in every unit, operable by trained paramedics without physician supervision.

According to CBS News (July 2025), patients with core temperatures of 108°F, 109°F, or 110°F were arriving at the hospital at 101-102°F — entirely because of cooling initiated before arrival.

The data: 300+ cases, published in peer-reviewed literature

The retrospective study published in February 2026 in Prehospital Emergency Care — "Fighting Fire with Ice: A Multisite Collaboration to Evaluate the Impact of Prehospital Cold Water Immersion on Heat Stroke Patients" — documents the outcomes:

A companion paper by Comp, Pugsley et al. was published in Annals of Emergency Medicine in January 2025, documenting the in-hospital protocol that preceded the field program.

Three operational conclusions for every EMS agency

Prehospital CWI is safe and effective in real-world EMS deployment This is no longer a theoretical recommendation. It is a documented outcome from 300+ cases across a large urban EMS system, validated across multiple hospital systems.

Time to cooling matters more than time to hospital Every minute of uncooled hyperthermia above 104°F increases the risk of organ damage and death. The patient who arrives cooled has a fundamentally different prognosis.

Equipment design determines protocol adoption The Phoenix program worked because the equipment was in every unit. The same logic applies to any agency — including fire services and SMUR teams in France, where the summer 2025 data documents 5,700 heat deaths and 24,000 emergency visits.

What fire services and EMS agencies should implement

  1. Pre-season training for all crew members on EHS recognition and CWI application
  2. Equipment in every unit — not stockpiled in a depot
  3. Clear protocol that does not require physician authorization to initiate cooling on a suspected EHS case
  4. Hospital coordination to ensure continuity of cooling from field to ED
  5. Data collection to document outcomes and support protocol validation

Sources

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