Nuclear weapons have a singular status as a terror weapon that chemical and biological weapons will never have. However, although groups such as al-Qaeda have tried in the past to acquire nuclear weapons - Osama Bin Laden proclaimed it a "religious duty" - the chances that they could ever get their hands on a working atomic bomb are slim.

Probably the closest terrorist groups can get is a radiological dispersion bomb - a weapon consisting of waste products from nuclear reactors wrapped around a conventional explosive. Such a device, made up of dynamite and Caesium 137 - a by-product of nuclear fission - was planted in Moscow's Izmailovo park in 1996 by Chechen Rebels as a warning to the Russian government.

The Centre for Defence Information (CDI) in the United States estimates that in a worst-case scenario in downtown Manhattan 2,000 people would die in the initial explosion and many more would be covered in radioactive isotopes.

The CDI's estimates of casualties exceed those of most observers. A larger consensus among experts imagines an explosion around the size of the one triggered by Timothy McVeigh outside the federal government offices in Oklahoma City in 1995. That blast killed 168 and injured 500. In comparison with September 11, those numbers are small. As Time magazine's Pentagon correspondent Mark Thompson says: "[A dirty bomb is] unlikely to kill 10,000 people, but any bomb that killed people and set off Geiger counters would terrify a whole city. It's ultimately a pure terror weapon."

Dirty bombs are not weapons of mass destruction in themselves. Their impact is largely psychological.

However the figures that the CDI predicts could be accurate if a radiological event produces a sudden evacuation as people flee from the epicentre. Casualties would be generated by panic, and there is the possibility that authorities would be willing to take extreme measures to stop those who had been contaminated leaving the area. In May, at the Police Federation's annual conference in Blackpool Bob Elder, chairman of the constables' central committee, said authorities might have to resort to "very unsavoury but necessary" crowd control measures, which led to claims that the police could ultimately shoot those running from the scene - although the Home Office has since flatly rejected this scenario.

Casualties will also be exacerbated by shortcomings in the emergency services. In November last year the National Audit Office identified substantial shortcomings in NHS emergency planning. Despite government investment of £100 million since September 11, the influential Public Accounts Committee reported in April that the DoH did not have a "full picture" of the country's readiness, and argued that staff may lack the "time, skills and resources" to mount an organised response. In the United States, there is only a single hospital emergency room dedicated to treating patients exposed to radiation hazards, at Oak Ridge, Tennessee.

Scientists disagree on whether sites around a dirty bomb attack would be uninhabitable. The Federation of American Scientists are far more sceptical, believing that due to isotopic reactivity "if decontamination were not possible, these areas would have to be abandoned for decades". However Steve Koonin, physics professor at the California Institute of Technology, argues that "long exposure to low-level gamma radiation, if you do the numbers, produces a miniscule increase in cancer rates - one extra cancer per 100,000 people".

Throw nuclear material and terrorism together and you generate a lot of fear. But chemical and biological weapons are certainly more lethal. Dirty bombs are weapons of mass terror, not mass destruction. It is far from clear whether these bombs would be at all effective. On a visceral level, terrorists couldn't do much better. On a practical level, weighing up the risks taken to the limited physical damage these weapons do, they probably couldn't do much worse.