In the light of this weekend's bomb scares, all talk now is of how the security of air cargo can be improved.
In the UK, 60% of air freight is carried in passenger planes, the rest goes on specialist cargo planes.
But whilst all passenger baggage is subject to inspection by x-ray, only a small amount of air cargo is checked in the same way.
There is no standard procedure for checking air freight and regulations vary from country to country.
The United States had a target for screening 100% of air freight on passenger flights by August of this year, it is unclear yet whether this goal has been met.
Individual airlines have to meet their government's regulations, but some airlines go beyond these. British Airways for example says that all air cargo travelling on its passenger planes is screened.
Global distribution company TNT also says it x-rays all packages that pass through its UK depots.
In the UK, goods must either be screened or originate from a so-called known consigner, which is a form of vetting administered by the Department for Transport.
To become a known consigner, businesses have to meet a number of criteria which cover subjects such as security measures and staff recruitment checks. They also have to undergo site inspections.
They are typically shipping companies which have their own controls which may include scanning.
But goods from these companies are not ordinarily subject to further security vetting before they are loaded on to a plane.
Goods from unknown shippers are automatically screened either by a regulated agent or the airline before being loaded.
But even when cargo is scanned industry analysts say that the technology used is less sophisticated than that used for passenger luggage.
The Department for Transport would not give any details on the amount of air cargo that is scanned electronically but it is understood that all goods leaving the UK on passenger planes are scanned.
The British International Freight Association insists that the UK's regulations are stringent and that all flights emanating from the UK have to meet high safety standards.
Others though are more critical of the system and maintain that the shipping of air freight remains the Achilles' heel of airline safety.
Philip Baum of Green Light Aviation Security says that the current 'known consigner' system is highly bureaucratic and relies heavily on trust; i.e. the shipper trusting the information given to him by the company handing over the parcel.
And the systems in place in the UK and US are limited to flights taking off from those territories; goods on incoming flights, like the parcel from Yemen headed to the US would not have gone through these controls.
All aspects of air freight security are now under review by the British government and several steps have already been taken to tighten controls.
- guidance given to airport security staff on how to identify suspicious packages in future
- the suspension of unaccompanied air freight to the UK from Yemen and Somalia
- the suspension of the carriage of toner cartridges larger than 500g in passengers' hand baggage on flights departing from UK airports.
And in the US, the destination for the hidden bombs, steps have been taken to enhance security including heightened cargo screening and additional security at airports.
But in an environment of fear, many are calling for 100% x-ray screening of all air freight.
The US authorities are already lobbying for this, but there will be time and cost implications if this becomes standard around the world.
No silver bullet
According to the international air transport body, Iata, 26 million tonnes of goods travel by air each year.
The bulk of air cargo is made up of higher-value electronics, engineering and machine parts, pharmaceuticals and fruit and vegetables, but can also include scrap metal and cars.
Electronic screening of all air freight would involve taking it out of containers, breaking up pallets and unpacking of goods.
This would take a huge amount of time and manpower and would cause delays and extra costs.
Shippers use air freight because of its speed so some fear that any delays would be detrimental to the industry.
"It is unrealistic to ground all cargo flights or to scan everything," David Learmount of Flight Global says. "There is no silver bullet to this problem."
And the problem with x-rays is that explosives can be missed.
The printer cartridge bomb found at East Midlands Airport went through an initial scan without being picked up.
The current regulations governing air freight were introduced in the aftermath of the Lockerbie bombing in 1988.
At the time experts said that the screening of passenger baggage would pose huge problems for the industry, but that is now routine.
One of the key elements to making the system safer is increased vigilance.
"Goods and packages can be profiled in the same way as passengers," says Philip Baum. "You have to look at the sender and the recipient and ask questions."
Mr Baum believes that a fundamental overhaul of airline security is needed, not just a "knee-jerk reaction to the latest scare".
And any new systems that are introduced are only as good as the people that operate them.
As David Learmount of Flight Global says, "Lessons need to be learnt from this, but we cannot change things overnight."