By John Eldridge, Editor of Jane's Nuclear, Biological and Chemical Defence

Considerable media coverage recently has focused on 'chem-bio detectors', with calls for them to be deployed in places seen as vulnerable to attack by weapons of mass destruction (WMD) such as underground stations or airport lounges. In fact, there is a considerable difference between the principles of chemical and biological detection; the agents themselves are, after all, very different in character.

The aims of detection are to identify the agent and provide warning - to those who are unaffected - of the advent and the extent of a chemical or biological aerosol. A common feature of both chemical and biological detectors is the fact that they spot the harmful agent by sampling their environment. Some detectors continuously sample the surrounding air, using a variety of technologies to identify and quantify the proportion of agent present in the airstream. Other systems are designed to detect harmful agents in the soil or in water.

In biodetection, the silver bullet sought by everybody is to achieve agent identification that is instantaneous and specific. The precise identity needs to be known down to as far as the individual strain of disease bacterium or virus. There is enormous pressure to reduce system reaction time, thereby allowing swift action to minimise people's exposure to the agent. There are now handheld biodetection systems increasingly available to emergency services personnel, especially in the USA, which can give results within around 15 minutes of the agent being picked up in the air sampling unit.

Some modern systems use immunoassay techniques, which mimic the human body's creation of antibodies to ward off disease attack. Others use genetic analysis, picking out the precise base sequences for a particular disease using DNA 'probes'. A process called Polymerase Chain Reaction (PCR) allows much higher detector sensitivity by amplifying the sequences by as much as 108 times.

New techniques, involving mass spectrometry, will allow much greater ranges of agents to be quickly analysed and identified. The assay and DNA probe systems operate by using a test strip, which is placed in a reader and compared against library data held by a database that includes the commonly identified threat agents. Generally, database libraries in these systems can quickly be updated to include emerging threat agents or other agents that are not harmful to the human system but which could damage food chain crops or animals. All these systems have to process samples of the agent material and deliver a visual result, either in the form of a colour change to a paper sample, by detecting fluorescence or by presenting the data on screen.

There is, meanwhile another technology for detection. In the USA, where the most significant developments in bio-detection are occurring, cloud recognition systems are under trial with the aim of identifying clouds of biological or chemical agent that have been released against defending forces in the battle area or maliciously in a civilian environment. These systems have the benefit of allowing the deployment of sampling detectors towards the edge of the cloud to give precise data on density, extent and agent identity.

The two technologies used in tandem begin to deliver true stand-off warning of chemical or biological attack. Clearly a biological attack presents the greatest challenge, as the triggers that would prompt the defender to deploy detectors occur much later in the event cycle. In other words, people will become ill before it is realised that an attack has taken place, since the incubation period for disease is usually in the order of days.

In the USA, it is over the next few days that the number of anthrax cases will indicate whether the releases of aerosolised anthrax spores occurred simultaneously or whether it is part of a sustained campaign.