Screening people leaving a country where there's a serious flu outbreak makes more sense than checking air travellers as they arrive at their destination, a Canadian study suggests.

Recognizing that people coming down with an illness can be infectious before they develop symptoms, researchers came up with a tool cities can use when deciding to screen travellers departing an affected area.

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A health official takes the body temperatures of arriving travellers in Kathmandu in 2009. It would have been more effective to screen passengers leaving Mexico where H1N1 originated, researchers say. (Gopal Chitrakar/Reuters)

Dr. Kamran Khan, an infectious disease physician at St. Michael's Hospital in Toronto, uses global air traffic patterns to predict the spread of H1N1 swine flu that originated in Mexico, looking for lessons for the new bird flu outbreak in China.

Since about 90 per cent of international trips by air last less than 12 hours, it's unlikely that travellers incubating an infection will board a plane with no symptoms and develop an illness during the trip.

Khan concluded it would be more effective to screen passengers at six airports in Mexico as they left the country instead of those arriving at potentially 4,000 airports elsewhere in the world. That would maximize the public health benefit while minimizing disruption to people.

"This may seem like an intuitive finding, however, in fact our practice tends to be largely in the opposite. We tend to be screening people as they arrive and not when they're departing," Khan said in an interview.

In the May issue of the World Health Organization's Bulletin, the researchers advised countries to take steps for preparedness and response as close as possible to the source of any future epidemic threats.

Quarantine officers at major Canadian airports are looking for passengers with symptoms such as coughing or fever, the Public Health Agency of Canada said.

International co-operation is essential when screening at the source, Khan said.

The study was funded by the Canadian Institutes of Health Research.

With files from CBC's Kim Brunhuber