Indonesia, Cambodia, Singapore and Australia are among the countries that have introduced or stepped up the use of the scanners in the past few days after warnings that the flu could cause a global pandemic.

United States officials have faced criticism that they are not using more thermal imaging devices, which have become a familiar sight for travellers through Asia since the outbreak of Severe Acute Respiratory Syndrome (SARS) in 2003.

Hong Kong, which was one of the hardest hit by SARS, has thermal cameras pointed at visitors entering the southern Chinese city by land, sea or air.

Swine flu is believed to have killed as many as 84 people in Mexico -- eight of them confirmed -- and already spread to the United States, Europe, Israel and New Zealand, and governments are desperate to stop the virus spreading further.

But as the multi-coloured screens become more common, their effectiveness is being questioned.

"If a person has been exposed or infected... the person might not be symptomatic at the airport," World Health Organisation spokesman Gregory Hartl told journalists in Geneva earlier this week.

"Border controls don't work. Screening doesn't work."

Matthew Leung, one of Hong Kong's most senior government health officials, agreed that scanners had limitations.

"I don't think any single measure can be 100 percent effective and certainly I would agree with the WHO's assessment that infra-red screening at the border cannot be completely foolproof," he said.

Leung said the scanners should only be seen as one of an array of measures that governments should take.

The cameras use thermal imaging to assess the skin temperatures of people as they pass through a checkpoint, and transform that data into a coloured image on a screen.

The machines will be set up so that when someone with a high temperature, normally above 38 degrees Celsius (100 degrees Fahrenheit), passes through the testing area, his features will be highlighted in a particular colour, often red.

Those monitoring the screen would then pull over the person for further investigation and see whether the higher temperature is because of a fever, or whether the person is just out of breath or agitated.

Despite the doubts, manufacturers are benefiting from the heightened concern.

Singapore Technologies Electronics, which sold around 200 of its Infra-red Fever Screening Systems during SARS, said it had received a "small number" of orders and some enquiries in recent days.

It said the great advantage of the 50,000-US-dollar machines was that they would measure temperatures accurately without having to delay passengers' journeys.

"It is a quick and non-intrusive system for mass screening of people from a distance of more than one metre," said Lee Fook Sun, the company's deputy president of operations.

Colum Murphy, author of "Flu Action Plan: A Business Survival Guide", said the lesson from the SARS epidemic was that governments should do everything they can to avoid panic, and scanners could help calm nerves.

"Anything that can minimise fear and help people feel more secure is advantageous," said Murphy, deputy editor of the Far Eastern Economic Review.

"And even if they only manage to uncover one case and delay a possible pandemic by a few days, then the scanners have bought governments more time."

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