MUSTARD GAS
First used during World War I, mustard gas is a colorless, odorless liquid at room temperature and causes extreme blistering. The name stems from its color and smell in its impure state. It's not related to the condiment mustard in any way. It's commonly referred to as a gas because the military designed it for use as an aerosol.

Even slight exposure leads to deep, agonizing blisters that appear within four to 24 hours of contact. If it gets into the eyes, they swell shut, and blindness can result. If inhaled at high doses, the respiratory system bleeds internally, and death is likely.

Exposure to more than 50 percent of the body's skin is usually fatal. It also causes cancer.

It was the most common type of chemical weapon dumped. It was dumped in 1-ton canisters and artillery shells for decades. Mustard agent is heavier than seawater, so it sinks and rolls around on the ocean floor with the prevailing current.

It lasts at least five years in seawater in a concentrated gel.

NERVE GAS
The most deadly of chemical warfare agents, one drop of nerve gas can kill a person within a minute. Death comes through seizure.

It's colorless and odorless, with the texture of high-grade motor oil. It's easily spread through the air. It attacks the human nervous system, causing almost-instant spasms before preventing involuntary muscle actions, such as the heart's pumping.

The Germans developed nerve gas during World War II, and only a few countries are known to possess any of it now.

LEWISITE
Developed too late for use in World War I, Lewisite is a blister agent akin to mustard gas. It's oily in its pure form and can appear amber or black in its impure state. It smells a bit like geraniums.

It can easily penetrate clothing and rubber masks. Exposure results in painful blisters and lesions that begin within seconds and last for two to three days. Lewisite was meant to incapacitate enemy forces -- not necessarily kill -- and thus clog hospitals and cause terror.

Intense nausea, diarrhea and vomiting are common, and shock from low blood pressure is likely. Eye exposure can cause blindness. Extensive exposure can cause systemic arseniclike poisoning, leading to liver damage or death.

After an antidote was found during World War II, the Army decided that it wasn't as useful as other chemical weapons.

PHOSGENE
Used during World War I, it's a highly toxic gas that has no color but smells vaguely like moldy hay. It's particularly insidious, in that exposure doesn't result in symptoms until 24 to 72 hours later.

The gas combines with water in the respiratory tract to form hydrochloric acid, which dissolves lung membranes. Fluid then fills the lungs, and death comes from a combination of shock, blood loss and respiratory failure. Unlike nerve agents, phosgene must be inhaled to cause harm.

It slowly dissolves in seawater, eventually converting to its chlorine base and dissipating.

CYANOGEN CHLORIDE
A cyanide-based chemical weapon also used during World War I, this is a rapid killer that's easily dispersed. It's known as a blood agent, circulating quickly through the bloodstream on exposure through either inhalation or skin contact.

It's colorless but has a biting, pungent odor similar to almonds. But the aroma probably won't be detected because exposure causes almost-instant agony: The skin quickly turns cherry red. Seizures follow within 15 to 30 seconds, accompanied by vertigo and vomiting. Death is likely in six to eight minutes.

HYDROGEN CYANIDE
The gas used by the Nazis in their concentration camps under the infamous brand name Zyklon B, it's either colorless or pale blue and has a faint almondlike odor.

It's easily dispersed in the air and readily absorbed through skin contact or inhalation. At the cellular level, it cuts the body's ability to take in oxygen. In high doses, the effects are quick and catastrophic, including gasping for breath, seizures, the collapse of the cardiovascular system and coma. Death comes within minutes.

The gas is flammable and potentially explosive. It's lighter than air, so if it's released in the sea, it would bubble to the surface.

WHITE PHOSPHORUS This is a colorless, waxy solid with a garliclike odor. It reacts rapidly on contact with air, bursting into a flame that's difficult to extinguish.

Breathing it in small doses can cause coughing or irritation of the throat. Eating or drinking it in even small amounts can cause stomach cramps, vomiting, drowsiness and death.

It's heavier than water, so it sinks to the bottom.