• Explosions at Brussels airport and nearby metro station
• At least 31 dead and hundreds more wounded
• Isis claims responsibility
• Q&A: What we know about the attacks
• NZ advice for Kiwis travelling to Belgium
Airports that don't require people to undergo security checks until they move beyond the departure hall may be forced to consider a shift to screening at every entrance following the terror attacks in Brussels.
With two devices apparently detonated in an open-access or "landside" area of the Belgian capital's Zaventem airport, the safety of remaining public spaces at hubs with otherwise stringent security arrangements faces renewed scrutiny.
Since the September 11, 2001, US attacks security provisions within the aviation industry have focused on stopping terrorists boarding planes, with measures such as full-body scans, shoe checks and bans on everything from liquids to nail clippers applied only once the passenger is past the check-in desk.
At most terminals it's therefore a comparatively easy task for a would-be attacker to walk in off the street unchallenged and mix with legitimate travelers in some of the most densely packed parts of the airport.
"This will be a wake-up call for airlines and airports to tighten their security and introduce new procedures," said Mark Martin, a Dubai-based consultant to the airline industry. "They will have airport lock-downs and increase security in luggage, cargo, crew - every element will be under the scanner now."
Much may depend on where governments choose to draw the line. Attacks on airports are already rare, with only one comparable incident having occurred in the past decade, when 37 people were killed by a suicide bomber in the arrivals hall of Moscow's Domodedovo hub in 2011.
The aviation industry has shown that it can adapt quickly to new threats in the past, with an immediate ban on carrying liquids through the security barrier - later set at 100 milliliters - imposed within months of a foiled attack on trans-Atlantic flights in 2006, changing travel habits at a stroke.
Still, while it may be possible to make airports virtually impregnable, doing so could shift the threat to other public areas, such as railway stations. There's evidence that's already happening, with November's attacks in Paris targeting restaurants, bars, a music venue and the Stade de France sports arena, yet proving just as shocking as previous outrages involving downed airliners.
Airports Council International said in a statement that checks on people entering landside zones "could be disruptive and actually create new security vulnerabilities." Such spaces are currently no more regulated than theatres, department stores and museums, the industry body said.
Tighter restrictions could also undermine the ambitions of major airports to establish themselves as retail destinations for non-flyers. Singapore Changi, Asia's second-busiest international hub, has become a hangout for students lured by its McDonald's and Burger King outlets and local families attracted by its air-conditioned open spaces, all accessed without security checks.
US airports already monitor areas before security checkpoints with artificial intelligence programs that scan for people lingering unexpectedly.
Another red flag is the wearing of excessive clothing that could disguise weaponry or explosives, said Richard Bloom, chief academic officer at the Prescott, Arizona, campus of Embry-Riddle Aeronautical University. Profiling techniques based on ethnicity or prior travel patterns have a mixed track record, and random screening may be just as effective, he said.
Stricter airport security regimes could see the wider introduction of measures that greet travelers almost at the curbside in a minority of countries.
In India, passengers must present their ticket and proof of identity on arrival and have their bags screened and sealed. The documents are still examined at the check-in counter, followed by security scans and the removal of jackets, shoes, phones, watches, belts and wallets familiar to travelers worldwide.
In disputed Kashmir and in the northeast further security checks apply before passengers board, requiring them to arrive hours before flying. People seeing off friends and family must also buy passes that aren't available at times of heightened security, while travelers can't leave the terminal once they enter.
Embry-Riddle's Bloom said that deploying metal detectors across the airport may be warranted, though many potential weapons no longer feature metal, and would-be terrorists are adept at keeping ahead of security measures.
At Paris's Charles de Gaulle and Orly airports, where controls were already at the highest level after the November attacks, extra border police began patrolling public areas, joined by riot police, according to an airport employee.
The Department of Homeland Security said it's monitoring the situation in Brussels after the attacks, which killed at least 31 at the airport and a subway station, and "will not hesitate" to introduce additional measures if needed.
Significantly upgraded security is likely to come at some expense and hurt travel times. "The long-run issue is about security costs and how that slows down flying," said Cantor Fitzgerald analyst Rob Byde. "More security checks could mean a slower turnaround of aircraft. That could be damaging.