If Iraq attacks coalition troops with chemical weapons, the strike will most likely come as a sudden furious rain of hundreds of artillery shells that saturate a limited area in a thick cloud of liquid poison and toxic vapors.

But if the allies' detection equipment and protective clothing perform as expected, the number of casualties may be of less military significance than the panic, delay and confusion that such an attack is almost certain to cause.

For all the horrors chemical weapons traditionally conjure, they have substantial limitations. In general, they have to be delivered in large quantities to be effective. The nerve gases, Tabun and Sarin, and the blood agent, hydrogen cyanide, are highly volatile, "non-persistent" gases -- they evaporate quickly and dissipate to sub-lethal concentrations in minutes or, at most, a few hours.

The thicker, "persistent" agents -- including the nerve agent VX and blister agents such as mustard and Lewisite -- remain active for days. But because they do not spread widely, large amounts are necessary to contaminate a sizable area.

The preferred method of chemical weapon delivery is by bomb or aerial spraying. But if the Iraqi air force remains subdued, "the artillery shell is the principal short-range chemical threat," said Brad Roberts, a research fellow at the Center for Strategic and International Studies here.

A 155mm round -- the largest artillery shell Iraq would be likely to use -- "would contain around 10 pounds of chemical agent," Roberts said, "of which 25 percent is destroyed on impact and of which no more than 15 percent carries farther than the perimeter of shrapnel," an area about 60 feet in diameter.

Thus, for maximum effect, chemical weapons must be delivered in salvos that rain hundreds of small warheads on a relatively compact area. Besides artillery, the only other weapons likely to deliver that volume are multiple rocket launchers.

Iraq has domestically produced and Soviet-made launchers that can fire dozens of rockets at once, each one carrying between 4 and 10 pounds of chemical weapon agents. It is also possible that some of Iraq's truck-mounted Frog missiles have been fitted with chemical warheads. Identifying the Weapon

If attacked by chemical weapons, allied troops will rely on two forms of early-warning devices. The most sophisticated is the West German-made "Fox," a sort of laboratory on wheels. Mounted on an armored, amphibious vehicle with a crew of four, it is "a complete nuclear, biological and chemical reconnaissance system," according to an Army spokesman.

Samples from the field -- a bit of potentially contaminated soil or a whiff of air -- are placed in a mass spectrometer, a device that determines the precise atomic composition of tiny amounts of material. Individual atoms and molecules of the sample are ionized -- given an electrical charge -- and then shot through a magnetic field, which bends their path into a curve shaped according to the particle's mass.

By monitoring the deflection pattern, the device can determine which kinds of atoms are present and at what concentrations. These data are then fed into the Fox's on-board computers, where they are correlated with weather information such as wind direction and humidity. The results are radioed to nearby troops, warning them which chemical agents to expect and when.

A less complex device, the M8 Automatic Chemical Agent Alarm, can be positioned up to 600 yards from a troop emplacement. Each company has four units, which function much like a smoke detector and can register faint concentrations of nerve agents in seconds.

Air entering the detector is exposed to alpha rays from a small radioactive source, which causes some of the molecules to be ionized. The airstream is then run through a series of baffles that remove the air molecules, leaving only the heavier ions of the chemical agent. These travel to a detector at the rear of the unit, where they cause an increase in current flow across an electrical gap. When the flow hits a critical level, the alarm is triggered.

The battery-powered sensors are placed upwind of troops, with wires running back to an alarm box that produces an audible warning (or blinking lights, if concealment is needed). The device, the Army says, is sensitive to concentrations from 0.1 to 1 milligram (mg) per cubic meter of air. By comparison, the lethal dose threshold for Tabun is 400 mg per minute per cubic meter; for Sarin, 70 mg; and for VX, 100 mg.

Even if detected early enough to enable troops to don protective gear, however, chemical weapon agents can still have disruptive impact. "Not only do troops have to pause to suit up," said Michael Eisenstadt of the Washington Institute for Near East Policy, but the cumbersome protective garments restrict peripheral vision, and their baggy uniformity makes it "more difficult to see who the leader is, posing command problems." In addition, gas-mask filters have to be changed periodically in uncontaminated areas.

Consequently it is desirable to get out of the suits as soon as possible. Allied forces have three devices to tell them when it is safe to do so. The simplest is a two-inch-wide strip of dye-impregnated paper that comes in 30-foot rolls. Soldiers tear off the adhesive-backed paper and stick it on the arms and legs of their protective suits. When the strips come in contact with chemical droplets as small as 200 microns (millionths of a meter), they turn red. Checking Contamination

For more detailed sampling of ground or vehicle contamination, troops are issued a 25-sheet booklet of papers as well as the M256 kit containing 12 kinds of test materials that turn different colors depending on the chemical agent present. Though it takes about 10 minutes to produce results, it is sensitive to nerve gas concentrations of .03 to .05 mg per cubic meter and mustard gas at 1 to 2 mg per cubic meter.

Each battlefield company also carries two battery-powered M1 Chemical Agent Monitors. Like the M8, they radioactively ionize incoming gas molecules, which are then propelled by an electric field through a tube and toward an electrode. Chemical agent ions are heavier than normal components of air and take longer to travel through the tube. The late arrival of molecules indicates that they are chemical agents and the number reaching the electrode indicates the relative amount on a light-emitting diode display panel.

Since mid-1988, when its war with Iran ended, Iraq is thought to have produced 4 million or 5 million pounds of chemical agents. Even that seemingly formidable supply, however, could be depleted in a relatively short time. At most, Roberts estimates, that is just enough to supply all of Iraq's front-line artillery units for one full day of chemical warfare.