The most important step in conclusively determining whether chemical weapons have been used is for independent experts to get prompt, unfettered access to the site of the suspected attack, according to Physicians for Human Rights, the American organization that was one of the first to document Iraq's use of poison gas against its Kurdish population in 1988.
In the Syrian conflict, that step may also prove to be the most difficult, said Susannah Sirkin, a senior adviser at the organization in Cambridge, Mass. The areas where the weapons may have been used are hard to reach because of fighting and government restrictions on independent organizations that seek to enter the country.
"That poses a challenge," she said, "to anyone trying to assess what was delivered, whether or not it was chemical weapons, what it was, what injuries it has caused."
On March 19, the Syrian government and Syrian rebels accused each other of having used chemical weapons in the northern province of Aleppo. Physicians for Human Rights called for immediate investigations by independent experts of all sites where the use of chemical weapons was alleged, and both the rebel groups and the Syrian government requested an independent United Nations investigation.
Ms. Sirkin noted that the clock was ticking and that evidence could be deteriorating.
Under ideal circumstances, she said, investigators would conduct full clinical exams of anyone injured or killed, and analyze blood, urine, soil, residue, clothing and any remains of weapons delivery systems, like munitions or cannisters.
"That usually is not going to happen because you're in a war," she said.
If there is no direct access to the site, investigators try to gather as many interviews as possible from patients and other eyewitnesses.
Intelligence agencies would look at what chemical weapons the players were known to possess. If the use of the weapons was confirmed, they would look, in trying to determine who used them, at factors like how they were delivered, which combatants had that capacity, and intelligence on what orders might have been issued.
In a war environment, Ms. Sirkin said, there are many cases "that may look like chemical weapons, and legitimate observers, including medical observers, may legitimately believe that chemical weapons were used, and they may or may not have been."
"It's not easy to know unless you're an expert," she said.
For example, after reports of a poison gas attack in Tbilisi, Georgia, in 1989, people suffered convincing symptoms. But the reports were false, and the afflicted were later determined to be suffering from mass hysteria spurred by fear of the reported weapons.
Until a clear determination is made on whether chemical weapons were used, Ms. Sirkin said, "it's important for everyone commenting on a situation to be as precise as possible."
"One doesn't want to cause panic," she added.
On the other hand, she said, a small death toll or mild symptoms do not necessarily mean that banned weapons were not used, because there are many factors that affect their potency, including weather and volume.
Determining what was used is important not only from a political and human rights point of view, but also from a public health standpoint, she said, so that appropriate care can be delivered.
Everyone with access to a site should try to preserve anything that might be evidence, she said, because with chemical weapons used so infrequently, experts are still learning about new ways to detect them.
Even without immediate direct access to a site, investigators can make headway, though they may not be able to make a final determination. Ms. Sirkin said her organization was surprised in the Iraq case by how much it could find out by interviewing people who had fled the scene and then returning years later to gather samples.
She said she remembered being unable to enter Iraq in 1988 and wondering if the question of whether chemicals were used in the Kurdish areas would ever be answered.
Physicians for Human Rights ended up surveying refugees who had fled to Turkey. Based on reported symptoms and people's descriptions of "what they had seen, heard, smelled and felt," the group declared that it believed that mustard gas and a nerve agent had been used.
Four years later, when the group reached the site, it found trace breakdown elements of the nerve agent sarin. "We had no idea that traces could remain four years later," she said.
She described a fearsome list of symptoms that nerve agents can cause as they attack the nervous system, interfering with muscle control and organ functions.
"Within one minute," she said, "one could expect blurred vision, eye pain, watery eyes, drooling, nausea and vomiting, loss of bladder and bowel function, chest tightness and difficulty breathing, change in mental status, generalized muscle weakness and twitching."
"It would be best if these were not in anybody's arsensal, because they are so dangerous and frightiening," she said. "It's not for nothing that government leaders, the U.N. and the population of Syria believe that these weapons should never be used."
This article originally appeared in The New York Times.