DONNELLY, Idaho (AP) — Authorities say 25 people were hospitalized for carbon monoxide poisoning while staying at a vacation rental home in an Idaho mountain town.

Donnelly Rural Fire Protection Chief Juan Bonilla says firefighters were called to the vacation home in Donnelly about 6:30 a.m. Monday, where they found the occupants complaining of headaches and nausea.

Two adults and two children were initially taken by ambulance to a hospital in McCall. Once healthcare workers realized they had symptoms consistent with carbon monoxide poisoning, fire crews tested the air in the home and found carbon monoxide levels that were above normal, Bonilla said.

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That’s when the other 21 residents were taken to the hospital as well. Officials say 21 of the people staying at the home have since been treated and released.

St. Luke’s Hospital spokeswoman Laura Crawford said the other four were transferred to Boise-area hospitals for specialized treatment in hyperbaric chambers. The chambers use high-flow oxygen to help clean the carbon monoxide from a patient’s system, Crawford said.

“We averted a horrific tragedy,” she said. “Once we learned we were dealing with possible multiple carbon monoxide poisonings, we immediately activated our mass-causality incident command.”

Carbon monoxide is an odorless and colorless gas that is produced when fossil fuels are burned. Authorities recommend that homes that use fossil-fuel heating or appliances — such as gas, propane or oil — have carbon monoxide alarms installed to warn occupants if a leak has occurred.

The Centers for Disease Control and Prevention says symptoms of carbon monoxide poisoning include headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion.

It wasn’t immediately clear what caused the carbon monoxide leak in the Donnelly home, though Bonilla said the home’s heating element used propane fuel. The home’s owner called a propane service technician to examine the home after the leak was discovered.

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