(This op-ed originally was published by Reuters.)

The Syrian government’s reported use of sarin in its war against rebel forces is ominous.

It suggests dissemination of the nerve agent could become more frequent there – whether by the Syrian military or by opposition forces in possession of captured stockpiles. If this happens, many more people are likely to suffer the agonizing effects of the chemical.

This could weaken the international taboo against such weaponry. No wonder President Barack Obama has warned that Syrian President Bashar al-Assad’s use of sarin would be a “game changer.”

Sarin is considered a weapon of mass destruction. As with all chemical agents, its effectiveness depends on the purity, the means of dissemination, and the vulnerability of the exposed population. At worst, chemicals can be devastating agents of death, even if less expansive in their effect than a biological release or a nuclear detonation.

A drop of liquid sarin can be lethal after skin contact or inhalation of its vapor. The colorless, odorless material blocks the transmission of impulses between nerve cells, which effectively paralyzes the organs of the body, including the lungs. Asphyxiation and death can result within minutes.

Unlike a biological agent, such as the bacterium that causes plague and can lead to an epidemic, a chemical does not pose the danger of an infected person transmitting the disease to others. Nor does a chemical release threaten the massive destruction and widespread radiation that would result from a nuclear explosion. Still, the effects of a chemical attack can be horrendous.

The largest use of chemicals as weapons took place during World War One. In 1915, German troops first released chlorine gas in the direction of French forces. The wind was at the Germans’ backs, so the gas soon enveloped the stunned French troops. They gasped for breath and suffered “agony unspeakable,” according to one account.

Additional chemical agents, including most famously mustard gas, later were used by all the major powers in the conflict. By the war’s end in 1918, these agents had killed almost 100,000 people. An additional 1.3 million victims suffered blindness, burned lungs, and other serious injuries.

The gruesome effects of the gas attacks prompted a postwar international agreement, the Geneva Protocol, which banned the use of poison gas in war. These weapons remained largely unused after that, though some countries continued to stockpile them.

In 1938, German scientists, in search of new insecticides, developed a family of toxic materials, including sarin, that attacked the nervous system. Sarin was far more potent than any previous warfare agent – and unknown outside of Germany until after World War Two.

Sarin since has been stockpiled as a warfare agent by several powers, but Iraqi President Saddam Hussein was the first to use it in warfare. He attacked Iranian troops with the chemical during the Iran-Iraq war in the 1980s. In 1988, Hussein also ordered low-flying Iraqi airplanes to spray sarin over his own people in the Iraqi Kurdish town of Halabja. Between 3,500 and 5,000 Kurdish civilians suffered agonizing deaths in what has been labeled a genocidal massacre.

Seven years later, in another case of terrorism, the Japanese cult Aum Shinrikyo released sarin in the Tokyo subway, killing 12 and sickening perhaps a thousand people. Seconds after exposure, froth bubbled from victims’ mouths and their bodies began to convulse. Initial estimates of 5,500 sick victims were attributable to panic among the many who jammed hospitals thinking, erroneously, they had been affected.

The year before, in Matsumoto, seven people with similar symptoms died from what was thought to be an accidental release of insecticide. Only after the Tokyo attack did authorities learn that the cult also had disseminated sarin there.

By then, impetus for a strengthened ban on chemical weapons was growing. The Chemical Weapons Convention, which went into force in 1997, prohibits the development as well as the possession of these weapons, and it provides for onsite inspections to verify compliance. More than 185 countries have signed the agreement. Syria is among the handful of countries that have not signed, and it has refused to allow inspectors on its territory.

Gas masks and antidotes, including atropine, can offer some protection against sarin. But as the Iraqi and Japanese experiences showed, the unprotected are vulnerable to the worst consequences of exposure. The introduction of chemical weapons can only compound the misery of war already inflicted on the Syrian population.

Obama has implied that Assad’s use of chemical weapons would warrant a forceful intervention. Based on photographs, victims’ reported symptoms, and the manner of shelling, U.S. intelligence officials now believe with “varying degrees of confidence” that Syria has used these weapons.

Obama is demanding further proof. But additional evidence, which can be collected from human tissues or soil samples, likely will be hard to attain. In part, that is because sarin dissipates in a few weeks. The trip wire for forceful action should be Assad’s refusal of inspections by outside authorities.

If Assad is permitted to get away with chemical murder, the consequence will be tragic not only for Syrians. The norm against unconscionable weaponry is likely to erode further, and inhibitions about the use of biological and nuclear, as well as chemical, weapons also may diminish.

Dr. Leonard A. Cole is director of the Program on Terror Medicine and Security at the University of Medicine and Dentistry of New Jersey, an expert on terrorism, and a former president of the Jewish Federation of Northern New Jersey. He lives in Ridgewood.