It is possible to sterilise letters and packages sent in the mail, but it is a procedure normally reserved for medical products and some types of food.

The energy of the beams and rays is absorbed by whatever they pass through and this kills harmful bacteria in a process which can be likened to "cold cooking".

These technologies were developed to sterilise medical and laboratory equipment and foods like herbs.

Each would have drawbacks if used on mail, and bulk sterilisation of mail would be expensive:

• Electron beam equipment is large and expensive
• Gamma ray equipment uses sticks of radioactive cobalt and can take several hours
• Gas sterilisation involves humidification of the package to allow the gas to permeate effectively
• Steam sterilisation is suitable for textiles and some plastics

IBA says its equipment could destroy anthrax in mail and it is keen to co-operate with postal services.

"The anthrax spore is like any other bacteria we treat today," said IBA's chief research officer, Yves Jongen. "It's a question of determining the right dose to ensure elimination of the deadly spore."

"Each technology has its advantages and in the current context of treating mail, we need to look particularly at logistical considerations and take maximum precautions," he added.

Concrete vault

IBA has a container-sized electron accelerator which can sterilise 240 kilograms of mail per hour.

It says that its largest machine could sterilise more than 20 tonnes of mail per hour, but has to be installed in a specially-designed concrete vault with thick shielding walls.

It is clear, however, that widescale sterilisation of mail would not be practical in the short term.

"These technologies are proven and effective in destroying bacteria, and can be implemented on a limited basis to address some of the current anthrax concerns," said Mr Jongen.

Professor Geoffrey Campbell-Platt, professor emeritus of food technology at the University of Reading, UK, said costs of irradiating food could be brought down to a penny or two per item in sufficient volumes, but irradiating mail would mean delay and expense as letters and packets were taken away for treatment.

"In the case of most of this equipment, there are special plants that do it, because the equipment needs shielding.

Investment, capital and space

"Gamma-ray irradiation with cobalt 60 is done in a chamber for perhaps three quarters of an hour or an hour, all depending on dose.

"The cobalt 60 has to go into a cold water pool and all that needs some investment, capital and space," he told BBC News Online.

Irradiation is typically used to sterilise items such as surgeons' gloves and plastic syringes which cannot be autoclaved.

Spices are also frequently irradiated because they are relatively valuable, and, despite having been dried, can still contain spores of various kinds.