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Hello, this is Morales on the BBC World Service and I'm Ruth Alexander.

As governments around the world consider ways to ensure that their countries don't succumb to an outbreak of the Ebola virus,

some have introduced mandatory airport screenings for passengers arriving on flights from those West African countries that have been affected.

Airports in the United States, Canada, United Kingdom, France and the Czech Republic are taking temperatures and giving health questionnaires to people arriving from Liberia, Sierra Leone and Guinea.

But how effective is this type of screening and will it make the public safer?

We've been speaking to Dr Cameron Khan, an infectious disease physician at St. Michael's Hospital in Toronto, Canada.

He's also one of the authors of a paper that examined the potential of the Ebola virus to spread across international borders via commercial air travel.

In the first part of his study, he wanted to know how efficient screening of travelers is.

When countries are looking at whether or not they should be performing screening of travelers are really three options.

The first is what we call exit screening. This is screening of travelers as they are leaving Guinea, Liberia and Sierra Leone.

It turns out that there are only three points at which you would need to screen travelers, those are the capital cities of each of the respective countries.

The other option is to screen travelers on direct flights that are leaving the affected countries.

In that instance, you'd be screening the same travelers. It's just now that you'd be screening them at the point of entry, which happens to be in 15 cities across 15 different countries.

Finally, the other option which is relevant for most of the world is that there are no direct flights coming from those three countries, in which case you'd have to screen in about 1200 different airports and screen roughly 362 million travelers.

And that's because since there are no direct flights, you'd have to be screening travelers coming through connecting flights. Essentially any city would have to screen all international travelers in order to be able to identify individuals who had originated in West Africa.

About 99.9% of the travelers that would be screened would not actually have even originated from West Africa.

And as Dr. Khan explained, people leaving those countries have been screened at point of departure, assuming that those passengers who do show symptoms

prior to boarding are refused permission to fly. The entry-level screening can only identify those people who develop symptoms while flying. What are the chances of that happening? Or Ben Carter's been looking at this?

Hi Ruth, first we need to point out that the incubation period for Ebola, that's the time from exposure to the virus to a person showing symptoms, is on average about 9 days.

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