I've just returned from five weeks away and I was in transit in airports on 11 occasions during the trip, from tiny Trapani in Sicily to the mega-glitz of Hong Kong. These days one has to be at the airport hours before an international flight so, conservatively, I estimate I spent 25 hours staggering around airport boutiques and duty free shops like a zombie, zonked out by sustained periods breathing in recycled air and soaking up artificial daylight.

They count on that, of course. Airports these days are more like enormous shopping malls than a place you pass through to go somewhere else. They bank on what I call the three dangers of duty free: boredom, stupor and guilt.

Boredom. You have hours to kill and no lounge access. You do the round of shops once and resist. You do it again and resist. The third time, after the flight is delayed once more, you idly pick up bottles of perfume, boxes of jellybeans, carved totems, painted clogs. Before you know it, you're standing in line waiting to buy something, anything. Because waiting in line is far more exciting than sitting on a hard seat staring at the departures board.

Stupor. You're beyond bored. You don't even know where you are or, if you did, you don't care. You've watched five movies on the flight, one of which is Confessions of a Shopaholic. You still think you're in it.

Guilt. You were so busy having a good time while you were away, you didn't give a passing thought to those at home. Now that you're at the airport and only a few hours away from facing them, serious guilt kicks in. If only you'd bought that Turkish delight in Istanbul, where it cost about $3 a box, rather than waiting for the airport, where it costs $20. But this is your last stand, unless you quickly grab a bottle of champagne on arrival at Melbourne Airport. That's no good, though, because the gift has to look authentic, as if you combed the bazaars of Byzantium for it. So the massively overpriced Turkish delight it is.

As a general rule, only fools and the undead do major shopping at airports. Yes, alcohol and cigarettes are usually cheaper at duty free shops, all that gold on display at Dubai is supposed to be good value and Jo Malone fragrance is reportedly less expensive at Heathrow than in the London stores. Small discounts can be found on luxury goods, if you play the exchange rate game properly. Some airports, like Rome, feature many of the high-end stores you find in the city and they heavily discount at sale time.

But duty free only makes sense if there was duty on the goods in the first place. Equally, some goods can be more expensive.

Airport rents are high and often the stores charge hefty exchange rates to convert from local currency. There are also those annoying credit card fees.

Certain items are packaged just for duty free stores and there are "airport exclusive prices" on some popular products, such as make-up and perfume. The trouble is, most of these bright, shiny, discounted things wouldn't be tempting if you weren't trapped like a rat in a maze searching for something stimulating to do.

It's probably smart to think ahead and make a mental list of what you genuinely want to buy at the airport and don't divert from it. (It's a bit tougher on parents with small kids, who want every dazzling thing.)

That way, the lack of fresh oxygen to your brain, the soothing muzak, the frazzled nerves about the forthcoming flight, and the signals the word "free" and "discount" send to a mind already overwrought by the cost of the holiday, won't combine to seduce you into buying a pair of camel-hide slippers or a cute puppy-shaped make-up kit you didn't need and never knew existed.

Which you will accidentally leave behind in the plane's overhead locker anyway.