Developing Warning System for Biological Attack Proves Difficult
The military has spent hundreds of millions of dollars to develop detectors. The truck-based system stationed at the Pentagon can identify four biological agents in less than 45 minutes, according to the latest annual report from the Pentagon to Congress on the status of chemical and biological defenses. Some air bases use a network of these sensors and compare their readings, to cut down on false warnings.

Another system tries to use light to detect aerosol clouds from miles away, but it cannot tell whether the clouds contain pathogens.

The Pentagon's inspector general last year criticized development of a new, more advanced system known as the Joint Biological Point Detection System for achieving only one of 10 critical goals. It broke down often, failed to identify lethal pathogens and sometimes gave false warnings when no danger existed.
http://www.nytimes.com/2001/10/28/national/28DETE.html?todaysheadlines&pagewanted=all