Most black henna contains more than five times the permitted levels of chemicals, experts warn.
A maximum 6 per cent concentration of p-Phenylenediamine (PPD) is permitted in products in the UAE.
But Khalifa Al Romaithi, the director of public health at Abu Dhabi Municipality, said the concentration of PPD in most black henna was "more than 30 per cent".
PPD has not been approved for use on the skin by the EU or the US Food and Drug Administration. A study by the US Department of Health and Human Services in April last year found that PPD has "potent skin-sensitising properties".
Reactions can be severe, with blistering and seeping wounds. If not treated immediately, people develop a permanent scar in the shape of the tattoo, known as a "ghost scar", said Dr Safwan Khraisheh, a dermatologist at the capital's Gulf Diagnostic Centre Hospital.
PPD has also been linked to other illnesses, including asthma, lupus and non-Hodgkins lymphoma.
Products that usually contain PPD include sunscreen, printer ink, photography developing agents, tyres and leather and textile dyes.
Dr Minal P Patwardhan, a specialist in skin diseases and cosmetology in Dubai and Sharjah, once saw "an entire class of college students" with allergic reactions to black henna.
"About 10 to 15 of them had black henna done and their skin was swollen in the design they had done," she said. "It's very common."
Another client, Vardhini Kothari, suffered a reaction to black henna at a party in Dubai nine years ago.
And Nada Fedaa, 21, an Egyptian living in Abu Dhabi, had black henna painted on her skin two months after her wedding last July. She said it burnt a little during application but she thought that was normal.
After the henna had dried and been washed off, the tattoo was swollen. "It was so painful ... it was unbearable," she said.
