Wittingly or unwittingly, shop assistants are confusing customers with reassurances that their wares are safe - as Ho Chung, 30, found out to his cost. After a candlelit dinner to celebrate their third anniversary, he surprised his girlfriend with a pair of matching charm necklaces.

Soon a painful rash appeared around his neck.

'I was confused, as the shopkeeper claimed that an anti-allergic metal - 925 sterling silver - had been used and I thought I had measles,' the IT technician said.

Ho is among the quarter of the city's population who suffer from nickel allergy, which can be triggered by contact with as little as 0.5 micrograms of nickel.

The South China Morning Post tested 10 sets of earrings, necklaces and rings from local retailers and found four positive for nickel release. But the test, a swab with a nickel-sensitive chemical, has been shown to detect only nickel amounts well above overseas safety standards, meaning pieces that tested negative could still be hazardous.

Research by Pennsylvania State University in the United States and others have shown that the test, proposed by the European Union, detects only amounts of 10 micrograms or more of released nickel.

The EU in 1994 issued a directive restricting the amount of the metal released from products with prolonged contact with the skin to less than 0.5 microgram's per square centimetre a week. This was revised in 2005 to limit the amount in jewellery for body piercing to less than 0.2 micrograms. China followed the EU standard, while Britain's is even more stringent - 0.1 micrograms.

Test results showed no obvious correlation between nickel release and location. Two of the contaminated samples were from shopping hot spots in Wan Chai and Island Beverly in Causeway Bay, while the cheapest one was nickel-free.

'Nickel is prized for its toughness, corrosion resistance and silvery colour,' dermatologist Dr Leung Sze-kee said. 'It's not surprising most alloys contain nickel, a cheaper metal which is plated with copper - even 925 sterling silver and gold.'

Nickel can be found in everyday objects from coins to watchbands, zippers and spectacle frames.

Its use in jewellery is not bound by any Hong Kong law but is loosely regulated by the Consumer Goods Safety Ordinance, which stipulates suppliers should ensure products are 'reasonably safe' and contain safety labels on consumption and disposal.

The Customs and Excise Department said the EU standard 'can be of reference value' but had yet to be adopted in Hong Kong.

Yeung, a clinical assistant professor in HKU's department of medicine, said the best way to prevent allergies was simply to avoid nickel. 'Wear stainless steel earrings or paint clear fingernail polish on these nickel-plated metals,' he advised.

A clear labelling system that specified nickel release would be better protection, he said - a suggestion that did not find favour with the trade.

'This wouldn't work,' said Ivan Keung, owner of Silversmith, a jewellery retailer on The Peak. 'A labelling system simply scares away customers and causes unnecessary panic.'

But Ho wasn't impressed. 'It's a basic consumer right to know what we wear,' he said. 'There's no use crying now - our anniversary gift has gone into the bin.'