Human Rights Watch (HRW) says the Israeli military recently used white phosphorus munitions in Lebanon and Gaza.

Here’s what you need to know about the chemical substance:

What did HRW say?

The rights group said late on Thursday that it had verified Israel’s use of white phosphorus munitions through interviews and videos showing the chemical substance was fired on two locations along the Israel-Lebanon border and over the Gaza City port.

“White phosphorous is unlawfully indiscriminate when airburst in populated urban areas, where it can burn down houses and cause egregious harm to civilians,” Lama Fakih, Middle East and North Africa director at HRW, said in a statement.

In an apparent denial of HRW’s report, the Israeli military said on Friday that it has made no use of white phosphorus in its Gaza war this week.

“The current accusation made against the IDF [Israel Defence Forces] regarding the use of white phosphorus in Gaza is unequivocally false,” it said in a statement.

What is white phosphorus?

White phosphorus is a wax-like, toxic substance that burns at more than 800 degrees Celsius (nearly 1,500 degrees Fahrenheit) – high enough to melt metal.

Its ability to ignite fast-spreading fires and create thick smoke over wide areas has made white phosphorus a substance of choice for militaries to create smokescreens. The smoke tends to last for seven minutes.

It is often colourless, white or yellow, and has a garlic-like odour.

White phosphorus munitions are difficult to extinguish, continuing to flare until the phosphorous has been burned up or until it is no longer exposed to oxygen.

It can be deployed through artillery shells, bombs, rockets or grenades.

“Airbursting white phosphorus spreads the substance over a wide area, depending on the altitude of the burst, and it exposes more civilians and infrastructures than a localized ground burst,” Ahmed Benchemsi, communications director for HRW’s Middle East and North Africa Division, told Al Jazeera.

Is white phosphorus harmful to humans?

White phosphorus can burn the skin down to the bone, and the chemicals can be absorbed by the body, causing dysfunction in multiple organs, including the liver, kidneys and heart.

“The burns have a double effect. They have a local effect because of the burn itself, which is generally quite severe and very deep, and then the second effect is metabolic, which can kill the patients,” said Roman Hossein Khonsari, professor of maxillofacial surgery and plastic surgery at Necker-Enfants Malades Hospital in Paris.

He said metabolic disorders can include abnormal potassium levels that cause heart failure.

Khonsari, who worked in Yerevan during the war between Armenia and Azerbaijan, said that if burns are not identified by doctors as being caused by white phosphorus, the victim may not receive the necessary care for organ failure risks.

Khonsari also explained that phosphorus burns continue to pierce the skin and will reach the bone unless the substance is properly washed out.

White phosphorus, which can stick to many surfaces, such as clothes, can also reignite if it comes into contact with the skin again.

It can also be lethal if inhaled, and the fumes can severely irritate eyes and make them sensitive to light.

Is white phosphorus banned?

White phosphorus is not explicitly banned by international conventions — though its use in a dense, civilian population is considered illegal by many legal scholars.

Protocol III of the 1980 Convention on Certain Conventional Weapons prohibits incendiaries or the use of other substances to attack civilian populations. But to be considered an “incendiary weapon”, an item needs to be “primarily designed” to set fire or cause burns.

White phosphorus is also used as a smokescreen, to signal or to illuminate a target. This allows militaries to argue that its use is not covered by Protocol III , to which Israel is in any case not a signatory.

In its manual on rules of warfare, Israel has said: “Phosphorous is no different from petrol reacting to a lighted match, and what differentiates it from chemical weapons is that its reaction is not directed against the human physiology.”

Khonsari, however, explained that burns caused by substances like petrol do not tend to be lethal if they cover a small area of the body, unlike those caused by white phosphorus.

Has Israel used white phosphorus in Gaza?

In addition to HRW’s latest claim of its use, a 2009 HRW report found that Israel extensively used white phosphorus munitions during its Operation Cast Lead in Gaza, which lasted from December 27, 2008, until January 18, 2009.

At the time, Israel shifted between confirming and denying its use of white phosphorus munitions.

In 2009, military spokespeople first said that it was being used to mark targets but later denied that white phosphorus was being used at all.

In the weeks after, Israeli officials ordered an investigation into the use of white phosphorus but maintained that it was in line with international standards.

The Israeli military “repeatedly exploded white phosphorus munitions in the air over populated areas, killing and injuring civilians, and damaging civilian structures, including a school, a market, a humanitarian aid warehouse and a hospital”, according to the 2009 HRW report.

It added that the forces had a nonlethal alternative available and used the white phosphorus airbursts even when no Israeli forces were present on the ground, suggesting that it was not used as an obscurant but instead as an incendiary.

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