Flashover likely caused Switzerland bar fire that killed 40

The cause of the fire that left dozens dead at a bar in Switzerland in the early hours of Thursday is still under investigation. But Swiss authorities have said that it's likely that the explosion reported from the site was caused by a flashover.

A flashover is a common and deadly phenomenon in which a fire in an enclosed space rapidly spreads, causing nearly everything in the room to ignite almost simultaneously, according to fire experts.

Authorities said this likely happened at Le Constellation, the bar in the ski resort town of Crans-Montana, where flames ripped through the building, caused at least one explosion and killed about 40 people. Beatrice Pilloud, the local attorney general, said at a news conference Friday that initial evidence suggested that the fire started from sparklers attached to bottles of Champagne that were held too close to the ceiling.

When a flame in a room is not quickly put out, hot gases rise to the ceiling and spread heat throughout the space, fire experts say. Temperatures can rise quickly to as high as 1,000 degrees Fahrenheit, a point at which everything in the room can start burning at once, according to the National Fire Protection Association.

"All the wood, all the seats, all of the decorations and everything else in the room would be heated to the ignition temperature," said Steve Kerber, executive director of the Fire Safety Research Institute in Maryland. "If you have very combustible materials, like plastics, it happens very fast."

In a flashover, a flame can spread from a candle to a sofa, then to the carpet and the rest of the room in three to five minutes, depending on the type of materials involved, Kerber said.

It would burn the people inside, too.

Even a firefighter in full protective gear would be unlikely to survive a flashover, according to the National Fire Protection Association. "Those are temperatures well beyond what you could get away with breathing," Kerber said.

Flashovers occur frequently in fires. "There are flashovers that happen every single day all around the world," Kerber said. "Every major fire has a room or multiple rooms that transition through flashover."

A flashover was involved in the 2003 nightclub fire in Rhode Island that killed 100 people, a disaster that Kerber said the blaze in Switzerland appeared to resemble. In that case, pyrotechnics ignited by a band playing at the club, the Station, set soundproofing foam lining the walls and ceiling ablaze.

But flashovers are preventable. Sprinklers can stop fires from reaching that point, Kerber said, as can adherence to fire codes, including not having flammable material on ceilings in enclosed spaces.