Although current technology can identify radiation and many biological and chemical warfare agents, detection equipment tends to be reactive -- engaged after an attack. Its most practical value is to provide critical information to first responders.

The exceptions are conventional-explosive and radiation detectors in use at airports, seaports and border checkpoints. Extremely sensitive truck-size "portal monitors" can catch even traces of radioactive materials passing through.

And 7,000 of the 9,000 border, port and airport inspectors wear pager-size detectors on their belts, inconspicuously scanning travelers arriving in the United States for dirty-bomb-grade radiation, according to Dean Boyd, spokesman for the new Bureau of Customs and Border Protection.

Government sources often won't comment on detection deployments. But here's a rundown on some of the detectors in place in the Washington area, as well as personal detectors you can buy:

* The Washington Metropolitan Area Transit Authority began testing a chemical detection system in Metro stations in 1999. By year's end, 12 of 47 stations will have detectors able to detect blister agents and nerve agents, which include tabun and sarin. The system alerts Metro personnel to the station involved, the location in the station and what kind of chemical is present. Lt. Leslie Campbell, counterterrorism coordinator for Metro police, says the remote analysis cuts 40 minutes off the arrival time of first responders.

Also, Metro police carry pager-size detectors that vibrate or sound an alarm when sensing radiation, and two dozen officer-canine teams roam stations to detect explosives.

* The Department of Homeland Security is equipping the Environmental Protection Agency with air filters that within 24 hours of a biological attack can identify the pathogen, how much was dispersed and how many people may be affected, says Homeland spokesman Brian Roehrkasse. After the EPA -- which was already monitoring its air -- the filters will be installed in other, unspecified "urban centers."

* D.C. police won't comment on "anything related to security," but area fire departments are equipped with hazardous-materials detectors. District firehouses have radiation detectors, including recalibrated Cold War-era Geiger counters, and District hazmat units have more sophisticated radiation and chemical detectors.

Fairfax County's hazmat teams have hand-held radiation detectors and chemical detectors that can sense "all military and toxic industrial chemicals," says Daryl Louder, hazmat program manager. Radiation detectors are deployed at the county's four hazmat stations, and the county is testing a home pregnancy test-style detector that changes color when exposed to a biological agent.

Montgomery County, too, is using an array of portable detection equipment for sensing biochemical agents and radiation.

Jim Schwartz, assistant chief of operations for the Arlington County Fire Department, says most of the metropolitan area counties have about "the same capabilities and are focused on the same risks and threat." In the past year or so, the counties have upgraded detection equipment and now monitor for radiation 24 hours a day.

* U.S. Park Police have put "air samplers" at undisclosed locations and have used radiological testers during some public events.

* The U.S. Postal Service is testing a system in Baltimore that senses anthrax and other biological agents. If effective, it eventually will be installed in all 282 mail-processing plants nationwide to protect postal workers as well as mail recipients.

As public concern has increased, so has the sale of retail detection devices for personal use. Many of them have been sold by survivalist companies and medical supply dealers for years. Some experts doubt that civilians need radiation or chemical detectors.

Further, they warn that civilians not experienced at reading radiation levels may panic if a highly sensitive and expensive meter signals a "false positive" alert when it is, in fact, registering only safe, low-level radiation which occurs naturally.

Here's a sampling of what is being marketed for personal security:

* VigiWATCH, a Swiss company that has made watches for more than a century and is known for its high-tech products, is selling a standard-size wristwatch for $1,100 that not only keeps time but also measures radioactivity reaching the wearer and cumulative doses.

* NukAlert, a $160 key-chain radiation detector, was put on the market last month by KI4U Inc., a licensed radiological laboratory in Gonzales, Tex.

The detector, which has a 10-year battery, "chirps" when it senses dangerous radiation; the more frequently it chirps, the higher the radiation levels. It was tested independently by an unaffiliated radiation lab to certify that it works. So far, 5,000 have been sold online, says KI4U President Shane Connor.

* USA GUARD, a Vermont company, sells an $80 pen-size radiation detector called "Pen Dosimeter" and a $350 MGP DMC 2000X pager-style radiation detector that are commonly used in hospitals and by the military. The company also makes an "M8" civilian version of military chemical-detector paper that changes color when it comes into contact with dangerous chemical agents. A book of 25 sheets sells for $10.

Connor warns that some personal detectors are too sensitive and can be set off by normal radiation levels. Better detectors are being developed.

Dwayne Lindner, who heads the biochem defense division at Sandia National Laboratories, which developed the system being used by Metro, says several national-security labs are developing a variety of "advanced detection systems."

One of Sandia's most promising systems, now in field trials, is a suitcase-size device called Microchemlab that is designed to detect a wide variety of threats, including chemical, biological and industrial (such as dioxins and sulfuric acid). Lindner expects it to be available in about 18 months.

Clockwise from top: MicroChemLab's chemical and biotoxin detector, the NukAlert radiation detector keyfob and USA Guard's chemical agent detection paper.