The Swiss ski resort blaze quickly developed into a ‘flashover,’ authorities say. Here’s what that means

By Billy Stockwell, CNN

(CNN) — Swiss authorities have said that a deadly fire at a ski resort bar in Crans-Montana, Switzerland, rapidly developed into a “flashover” – a dangerous phenomenon in which everything in a room ignites almost simultaneously.

The blaze quickly grew into a “flashover fire” after likely being caused by sparklers in champagne bottles that came too close to the ceiling, Béatrice Pilloud, the attorney general for Valais canton, said Friday.

A flashover occurs when hot gases rise to the ceiling and spread across the walls, according to the US-based National Fire Protection Association (NFPA). The heat then intensifies until all combustible items in the room reach their ignition point and catch fire.

Authorities have also discussed “backdraft” as another potential contributing factor for why the fire spread so fast. A backdraft is an explosion that occurs when oxygen is introduced into a room full of hot gases, according to the NFPA.

As temperatures soar to as high as 1,000 degrees Fahrenheit, even firefighters in full protective gear are unlikely to survive, NFPA said.

Ripples of smoke

Independent fire consultant Stephen MacKenzie told CNN Friday a flashover creates ripples of smoke which spread laterally across the ceiling and start to “preheat” everything in front of it.

As the fire spreads, it starts to seek out more oxygen, MacKenzie said.

If an opening is made to admit oxygen – such as a door being opened as people try to escape – this could have created a “chimney effect,” he said, which accelerated the flow of smoke and combustible gases upward.

“You’ve got a hot gas layer developing, you’ve got the heat radiating down on you. People start to realize, I need to get out of here,” he said.

The fire will also have generated a mix of combustible gasses, MacKenzie said, which will start to ignite. “The smoke is actually on fire,” he said.

Asked how long it takes a flashover to develop, MacKenzie gave a grim forecast. “Seconds,” he said. “We’re looking at seconds to minutes.”

Medical staff said Friday that some of those in hospital appear to have injuries consistent with a flashover.

“Most of our patients appear to have been victims of the flashover, with injuries typical of this phenomenon,” a spokesperson at Geneva University Hospitals (HUG) told CNN, adding that a backdraft could also have been possible.

Dr. Robert Larribau, the head of the emergency services at the HUG, said those impacted by a flashover typically display severe burns “predominantly affecting exposed body areas such as the face, neck, and upper limbs.”

Meanwhile, backdraft injuries typically include “severe blast trauma, extensive thermal damage, and lethal inhalation of toxic gases,” he said in a statement.

CNN’s Martin Goillandeau, Caitlin Danaher, Mitchell McCluskey and Lauren Chadwick contributed reporting.

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