Image caption Katie Piper was attacked with acid in 2008 by a jealous boyfriend

The acid attack on two young British women in Zanzibar has cast a spotlight again on a sinister crime. How often do such assaults happen?

Anyone who throws acid in someone's face intends to scar them for life.

It is a crime with a marked gender skew. Experts say that women and girls are victims in 75-80% of cases. Of the female victims, about 30% are under 18.

The case of Kirstie Trup and Katie Gee, British tourists in Zanzibar, who had acid thrown on their faces, chests and hands, has caused revulsion.

The artistic director of Russia's Bolshoi Ballet was attacked in January. Now after 18 operations he is still almost completely blind, according to reports.

Another high profile case was that of Katie Piper, who in 2008 was the victim of an acid attack orchestrated by her jealous boyfriend. Since then she has had nearly 100 operations and become a campaigner.

Image caption Bolshoi Ballet artistic director Sergei Filin was attacked in January

NHS statistics for England do not separate out acid attacks. In 2011-12 there were 105 hospital admissions in England for "assault by corrosive substance", but the category covers more than just acid.

The first acid attacks Sulphuric acid, historically called "vitriol" by European alchemists, only became widely available in England during the 1740s, initially for use in manufacturing. "Vitriol" began to be used as a weapon in Western Europe and the US, to such an extent that by the 1830s a Glasgow periodical felt strongly enough to write: "The crime of throwing vitriol has, we grieve to say, become so common in this part of the country, as to become almost a stain on the national character."

But 1,500 cases are recorded around the world every year, according to the Acid Survivors Trust International. "That is likely to be massively underreported," says Jaf Shah, ASTI executive director. "Most victims are fearful to report it to the police for fear of reprisal."

The lack of solid reported figures makes it hard to say whether acid attacks are on the rise globally, Shah says.

India has an increasing problem with acid attacks. ASTI estimates that 1,000 take place there every year. Eight weeks ago the country's supreme court criticised the Indian government for failing to act on the problem.

Mohammad Jawad, a plastic surgeon who helped rebuild Katie Piper's face and works with victims in South Asia, says the crime is about trying to destroy someone's identity.

Image caption A man protests in Srinagar, in Indian-administered Kashmir, in January 2013

"The attacker is saying: 'I don't want to kill her, I am going to do something to distort her.' It's a walking dead situation for the victim and often a grey area in the eyes of the law."

It is not about religion or culture, he argues. "It takes place in parts of the world where women are not empowered. It's an extreme form of domestic violence."

Jawad says it is vital to treat acid burns fast.

Qisas: An eye for an eye? Ameneh Bahrami, 24, was attacked with acid in 2004 by a university classmate

She had repeatedly spurned his offers of marriage

Blinded in both eyes, and after treatment in Spain she returned to Iran to bring him to justice in 2007

Took up a case under the Islamic Sharia code of qisas (retribution)

Won case in 2008, with attacker sentenced to blinding by acid

After numerous delays and widespread international coverage, she pardoned him in 2011 Read more about Bahrami's quest for retribution before eventually pardoning him

"The first few hours are pretty important." In thermal burns the damage is done once the burn is inflicted. But with acid, the burning continues on the skin until the acid is neutralised.

Katie Gee and Kirstie Trup reportedly ran into the sea to wash the acid off their skin before it could do any permanent damage. Although there are bacteria in the sea, on balance this was a good decision, says Jawad.

Saltwater in the form of hypertonic saline (very salty saline) is used to neutralise the acid. Cleaning agents can suck the acid out of the tissue by a process of reverse osmosis, Jawad says.

Acid attacks appear to be disproportionately common in South Asia. As well as India, Shah suspects there are "very high" numbers in Afghanistan but there are no official figures. Bangladesh and Pakistan also have considerable numbers.

Shah says that unlike India, where there is no dedicated measuring of acid attacks, Pakistan and Bangladesh have made it a specific offence. The most common motives are rejected marriage proposals and sexual advances.

In Bangladesh, reported attacks have fallen since the government tightened up the rules on the sale of acid and introduced the death penalty for attackers. There were 492 attacks in 2002. Last year it was under 75, Shah says.

Pakistan has also tightened up its laws. There are 250-300 recorded attacks there a year. Legislation in December 2011 has increased reporting by 300%, Shah says.

The prevalence of attacks in South Asia can be explained by the easy availability of acid, suggests Shah. Acid is widely used in the cotton, rubber and jewellery industries. Hence attacks are also seen in rubber-producing areas like Cambodia.

Image caption Bangladeshi acid attack victims mark International Women's Day in 2005

Acid can be sold for as little as a dollar or 50 cents a litre, says Jawad.

In the developing world acid attacks cause greater damage. Water might be hard to come by for someone seeking to wash away the acid. Burns units are few and far between. Shah recalls how in Nepal a woman had to walk "in absolute agony" for 24 hours before she could receive treatment.

The problem is not just South Asia. There have been notable attacks in Iran. There were 150 recorded attacks in Colombia last year. And recent reports suggest that attacks are on the rise in Italy.

Even where laws are tightened, convictions can be hard in male-dominated societies. In Pakistan, a woman died after an acid attack and left a mobile phone video message denouncing her attackers. But the suspects - her husband, mother-in law and father-in law - were acquitted.

It can take years for victims to recover, says Shah. They may need dozens of surgical procedures. Then there is the psychological trauma. "If victims have been ostracised from the family structure their exclusion is very severe."

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