The technology exists to safeguard the world's air transport system against threats like the Yemen-based mail bombs, but the cost may be too high to be practical.
Analysts warn that the cost of screening every piece of air cargo in a bid to prevent terrorists from downing airliners might bankrupt international shipping companies, hobble already weakened airlines and still not provide full protection.
"In a worst case, it would stop world trade," said James Halstead, a longtime consultant with the Aviation Economics firm. "UPS and FedEx would probably go bust. We'd have a full-disaster scenario."
The cost of the extra effort, he said, "would be almost too much to consider."
Many countries already conduct extensive checks of cargo, Halstead said. But the increasingly sophisticated technology used by terrorist groups makes further refinements extremely difficult.
Authorities "do as much checking as they can in many places, but it's the danger of these small items that is the problem," he said.
The problem is compounded by the frequent use of passenger flights to carry cargo, some of which has not been properly screened.
In last week's narrowly averted plot, one device almost slipped through Britain and the other seized in Dubai was unwittingly flown on two passenger jets. Investigators were still piecing together the potency and construction of the two bombs, which they believed were designed by the top explosives expert working for al-Qaida in the Arabian Peninsula, the Yemen-based faction thought to be behind the plot.
More than half of the cargo flown into the United States comes via passenger planes, making cargo bombs a tempting way for terror groups to attack civilian passengers.
As of Aug. 1, all cargo on passengers planes in the U.S. is required to be screened. It's estimated that the new rule will cost $700 million and require 9,000 employees in just the first year, according to the Airforwarders Association.
Cargo flights departing for the U.S. from another country are handled differently. Currently, American authorities do not get details about what's on a cargo plane until four hours before it's supposed to land in the U.S. Once it lands, officials physically screen packages that warrant a closer look based on intelligence.
Jayson Ahern, former acting commissioner of Customs and Border Protection, said new rules should be adopted to ensure that the U.S. gets information on what cargo planes are carrying before they leave for the U.S.
Having the information before the plane leaves would give officials a chance to flag packages that might require extra attention.
The Yemen-based branch of al-Qaida has expanded its use of PETN plastic explosives in the last year, posing severe detection problems, particularly in locations where shipping companies and airlines use older equipment to scan cargo.
Some next-generation machines can pick up traces of chemical explosives, but the costs are extremely high. Swabbing packages individually for explosives is considered the most effective way to scan, but that's not a practical option for the millions of packages that crisscross the globe every day.
A comprehensive switch to these swabbing devices would create massive delays of everything from clothing to iPods. Relatively inexpensive cargo transport would become a thing of the past, with draconian implications for world trade.
The cost of these machines would likely be in the billions of dollars, and would be economically impossible for some countries.
"The technology exists," said aviation systems expert Philip Butterworth-Hayes. "It's horrendously expensive and will take many years to install at all the various cargo depots and freight-forwarding places. If you add up all the places cargo can access the airside at airports, there are many thousands of places, and to put screening units in all those places is very complicated."
He said that to put up-to-date screening equipment in every location would not be possible in the next two years even if money was available.
Butterworth-Hayes said governments — not airlines or shippers — should pay for the new equipment since national security is at stake.
European airport operators have cautiously endorsed more screening measures but are urging that national governments shoulder at least a portion of the rapidly expanding costs of aviation security, which in Europe are borne by the airport companies.
"The economics of security is definitely an angle we've been concerned with for some time," said Robert O'Meara, spokesman for the European branch of Airport Council International.
Statistics show that security currently accounts for 35 percent of airport operating costs in Europe, in contrast to just 5 to 8 percent before the 9/11 attacks. Forty percent of all airport employees at the continent's 313 airports are now security-related staff.
Any additional requirements for the closer screening of air cargo will likely drive up those costs still further.
Magnus Ranstorp, a specialist with the Swedish National Defense College, said it would be too expensive to try to establish a foolproof system because of the constantly changing tactics used by terrorist groups.
Instead, he said, efforts should be made to keep cargo off passenger planes — a change that in itself would prove extremely costly for airlines, which derive substantial revenue from cargo — and eliminating cargo service from high-risk countries, like Yemen.
"You just have to be more rigorous, because these groups are constantly coming up with innovations," he said. "They will move onto something else."
Jill Lawless in London, Slobodan Lekic in Brussels, Samantha L. Bomkamp in New York and Eileen Sullivan in Washington contributed to this report.