The severity of a burn is measured by the depth and size or the destruction caused to the skin.

A number of the victims, who were wearing summer clothing when they toured the active volcano on Monday, have suffered burns on up to 80 per cent of their bodies and are being treated in specialised units across NZ.

Many also ingested ash and volcanic gases, resulting in horrific injuries to their lungs and airways, leaving them unable to talk.

They likely suffered third-degree or "full thickness" burns, which damages tissue deep under the skin causing extreme pain although there might be numbness due to the destruction of pain centres in the body.

Treatment involves IV intake of fluids and antibiotics to prevent infection and later on, natural or synthetic skin grafts.

But there's also the possibility of fourth-degree burns, which affects the body right down to the bone.

The pain of burning is one of the most intense a human body can experience and also requires a sophisticated treatment regime.

The extent of adult burns is measured by rules assigning a percentage to nine sections of the body and adding up how many are affected.

Each body area is given an estimated percentage surface area:

* Anterior trunk, 18pct

* Posterior trunk, 18pct

* Legs and feet, 18pct

* Head and neck, 9pct

* Arms and hands, 9pct

* Genitalia, 1pct

So a person suffering burns to 20 per cent would commonly have burns to their legs and feet or trunk.

The larger the skin surface area burned, the more unpredictable the outcome.

Severe burns to more than 40 per cent of the human body can be fatal.

If they were lucky, some of the survivors might have escaped with second-degree burns, which while still extremely painful and disfiguring, can be treated by ointments and may not require a long hospitalisation.

Source: Various Australian hospital and US Health Department websites.

Australian Associated Press