What is the super fog that caused fatal US pile-up?

At least seven killed in accident involving 158 vehicles in Louisiana

Wreckage of a massive pile-up in the US state of Louisiana that took place in poor visibility. AFP

At least seven people were killed and dozens more injured in a 158-vehicle pile-up in what Louisiana officials described as "super fog".

The fatal crash occurred in the New Orleans metropolitan area on Monday morning, with first responders sorting through the wreckage in search of more victims until the evening. State police said the death toll could increase.

At least 25 people were taken to hospital with injuries, from minor to critical. Many others involved in the crash sought their own medical assistance, state police said.

Louisiana State police said responders would be better able to assess the vehicles once a tanker carrying hazardous liquid was removed from the scene.

Footage provided by the police department shows the massive pile-up on the I-55 motorway, with some cars burnt-out. The road is expected to be closed for the foreseeable future.

What is super fog?

The pile-up was caused by a weather event known as a super fog, which can significantly limit a driver's ability to see the road.

Super fog forms when a mixture of smoke and moisture mix with "cooler, nearly saturated air", the National Weather Service said. In windy conditions, super fog can move through low-terrain areas such as drainage ditches.

Because it can reduce visibility to three metres or less, super fog is very dangerous on motorways.

Drivers who encounter such weather on the road are advised to turn on their headlights, reduce speed and increase the distance between themselves and other vehicles.

Motorists are also advised to use their headlights at low beam, as high beams will cause a glare and make it more difficult to see the road.