* Security tightened at some airports, borders
* Australian PM criticises lax European security
* Passengers often not checked on entering terminal
* Adding pre-terminal screening globally would be costly
Several countries have tightened or reviewed airport security following twin explosions at Brussels Airport, as Australian Prime Minister Malcolm Turnbull on Wednesday blamed Europe's porous borders and lax security for the attack.
Islamic State claimed responsibility for the bomb attacks in the departure hall of Zaventem airport, and a rush-hour metro train which killed at least 30 people.
Prosecutors said the blasts at the airport, which serves more than 23mn passengers a year, were believed to be caused by suicide bombers.
Turnbull waded into the global debate about protecting borders, reassuring Australians that "our domestic security arrangements are much stronger than they are in Europe where regrettably they allowed things to slip".
"That weakness in European security is not unrelated to the problems they've been having in recent times," he said in Sydney.
Authorities in London, Paris and Frankfurt responded to the attacks by stepping up the number of police on patrol at their airports and other transport hubs. Airlines scrambled to divert flights as Brussels airport announced it would remain closed on Wednesday.
"Two terrorists who enter the terminal area with explosive devices, this is undoubtedly a colossal failure," Pini Schiff, the former security chief at Tel Aviv's Ben-Gurion Airport and currently the CEO of the Israel Security Association, said in an interview with Israel Radio.
In the US, the country's largest cities were placed on high alert and the National Guard was called in to increase security at New York City's two airports.
A UN agency was already reviewing airport security following the downing of a Russian airliner in Egypt by a makeshift soda-can bomb in October last year. Islamic State has claimed responsibility for smuggling the bomb on board.
But despite attacks like a suicide bomb at Moscow's Domodedovo airport's arrival hall in 2011 that killed 37 people, there has been less attention focused on how airports themselves are secured.
"It strikes me as strange that only half of the airport is secure. Surely the whole airport should be secure, from the minute you arrive in the car park," said Matthew Finn, managing director of independent aviation security consultants Augmentiq.
The relative openness of public airport areas in Western Europe contrasts with some in Africa, the Middle East and Southeast Asia, where travellers' documents and belongings are checked before they are allowed to enter the airport building.
In Turkey, passengers and bags are screened on entering the terminal and again after check-in. Moscow also checks people at terminal entrances.
Israel's Ben Gurion Airport is known for its tough security, including passenger profiling to identify those viewed as suspicious, bomb sniffing devices and questioning of each individual traveller.
In the Kenyan capital Nairobi, where authorities are on high alert for attacks by Somali-based al Shabaab militants, passengers have to get out of their cars, which are then searched, at a checkpoint a kilometre from the main terminal.
But adding checks such as bag X-rays at terminal entrances could themselves create a potential target, one analyst said.
"Any movement of the security 'comb' to the public entrance of a terminal building would cause congestion, inconvenience and flight delays, while the inevitable resulting queues would themselves present an attractive target," said Ben Vogel, Editor, IHS Jane's Airport Review.