A severe reaction to the PPD chemical in black henna as suffered by a 16-year-old Bangladeshi girl. Banned black-henna tattoos are still on offer at desert safaris and weddings despite renewed warnings that using hair dye afterwards can cause a severe allergic reaction.
Black henna often contains the chemical  p-Phenylenediamine (PPD), which gives the natural reddish-brown dye a black tint. It is also the main component in most hair dyes.
Once the body's immune system has triggered an allergic reaction to PPD, such as after having a black henna tattoo , further exposure can be fatal.
"If you then dye your hair, it could cause an acute reaction, which can make you go into anaphylactic shock," said Dr Fatma Mostafa, a skin specialist at Al Rustom Medical Centre in Dubai .
This can cause abdominal pain, difficulty breathing, dizziness and, in extreme cases, even death.
Dr Mostafa sees at least one patient a week with severe reactions to PPD.
The risks of black henna are well known to regulators and Dubai  banned its use in 2009, but the tattoos are still often offered on desert safaris and at weddings.
Dr Hussein Abdeldaiam, a dermatologist at Al Mafraq Hospital in Abu Dhabi, treated a woman last week who had applied black henna at her wedding celebration .
"She was in such a miserable condition," he said. "Her skin was completely swollen and weeping fluids."
People who have such reactions must "never dye your hair ever again", warned Dr Abdeldaiam. "You can never get rid of PPD and it gets worse every time you come in contact with it."
Natural henna - created by grinding the henna plant to make a paste or powder - is traditionally used to paint temporary skin decorations on women's hands and feet.
Allergic reactions to natural henna are rare. But when it is mixed with toxic chemicals, such as PPD and benzene, it can be harmful.
The addition of such toxins is still widespread in Dubai. Black henna is often applied free on desert safaris, but the director of public health and safety at Dubai Municipality, Redha Hassan Salman, said they focus inspections on beauty establishments.
"Checking desert safaris is not the system we follow," he said.
The municipality investigates complaints only if an official police report is filed.
"Our inspectors  don't go and follow people around on desert safaris," said Mr Salman. "It's not the way we do our business."
Some tour companies, which each take an average of 30 tourists a day on desert safaris, admit to offering free black henna tattoos as part of their tour packages.
Dubai Desert Safari Tours and North Tours both said they used black henna.
Karen Salvador, a marketing assistant at Fun Tours, said she had heard about the dangers of black henna but did not know which chemicals the company used.
Under a directive issued by the European Commission, all hair -dye products must contain the following warning on their packaging: "Do not colour your hair if … you have experienced a reaction to a temporary 'black henna' tattoo in the past."
Julie McCabe, 38, a British tourist who visited Dubai, is taking legal action in the UK against the cosmetics company  L'Oréal. She suffered a severe allergic reaction after colouring her hair in October with the company's Preferences dye.
"Hair-dye manufacturers have a warning on their packaging about black henna, so it is a potential issue that people are not aware of the connection," said her lawyer, Greg Almond.
The lawyer said Mrs McCabe, a mother of two from West Yorkshire, had been dying her hair for years without any problems. After the trip to Dubai she suffered a severe allergic  reaction, but he would not confirm if she had a black henna tattoo.
"People don't fully understand that this is such a potentially dangerous  chemical. Once you have been sensitised, you have to check everything for the rest of your life," he said.
By: Marie-Louise Olson