With Syria continuing to reject responsibility for the attack, the OPCW is now the only organisation that can reach an impartial assessment.
How will the OPCW examine the site?
Although the Syrian regime has offered the OPCW access to what it said were 22 witnesses of the attack, an OPCW investigation necessarily relies on probes that can be conducted only on the site of a suspected attack. While in Douma, the investigators are expected to gather soil samples that could ultimately be used to determine which chemicals were used.
"What you're trying to do is a combination of things: You try to investigate the actual location that was impacted, which gives you a clue of what the site looks like and whether you can find remnants of the weapons themselves," said Ralf Trapp, an international disarmament expert who has worked with the OPCW.
"If no weapons are found, environmental samples of soil, rubble or vegetation can also be helpful to the investigation, even though chemicals eventually degrade due to environmental factors. Lastly, you can talk to eyewitnesses and take blood or urine samples. But all of this depends on how much time you have on the ground," said Trapp.
And how much time the OPCW has will largely depend on the regime accused of conducting the attack - which is now technically the investigators' "host country."
A number of chemicals have been used or stored in the region. The Syrian Government is believed to have used sarin and chlorine gas on a number of occasions. Separately, the OPCW has also accused Isis of using mustard gas.
There are growing suspicions that evidence of the incident may have been tampered with, however. "It is our understanding the Russians may have visited the attack site. We are concerned they may have tampered with it with the intent of thwarting the efforts of the OPCW fact-finding mission to conduct an effective investigation," Kenneth Ward, US ambassador to the OPCW, said in comments at a meeting of the OPCW in The Hague. Russia has rejected the accusation.
In a statement released today, the French Government echoed US warnings, saying that it was "very likely that proof and essential elements are disappearing."
How and why was the OPCW created?
The US-Russia confrontation over Syria's alleged use of chemical weapons shows how complicated OPCW's mission really is.
Throughout the 20th century, efforts to ban chemical weapons repeatedly failed, and even the 1925 Geneva Protocol banned their use but not the possession. To confront the issue, OPCW finally began its work in 1997 - based on a more expansive arms treaty known as the Chemical Weapons Convention - that had been signed by a number of countries four years earlier.
With about 500 employees headquartered in The Hague, the OPCW won the Nobel Peace Prize 15 years later, in 2013. OPCW Director-General Ahmet Üzümcü delivered a powerful explanation for why the vast majority of states had voluntarily offered to give up their chemical weapons stockpiles in his acceptance speech at the time. This kind of weapon, he argued, was one that went beyond even the worst horrors of war:
"Almost one hundred years since their first large-scale use on the battlefields of Flanders, it is worth reminding ourselves of the reasons why these weapons invoke such horror, right up to our own time.
"Chemical weapons stir the deep-rooted and pathological fear all humans share of being poisoned. They do not discriminate between combatant and civilian, nor between battlefield and village.
"You cannot see them.
"You cannot smell them.
"And they offer no warning for the unsuspecting.
"But their effects are devastating - burning, blinding or suffocating their victims. Death is rarely instant and never painless."
And yet, the OPCW mechanisms largely rely on the cooperation of their member states to allow inspectors to do their work. Syria only ratified the Chemical Weapons Convention in 2013, in the midst of its civil war and the same year the OPCW won its Nobel Peace Prize. But the chemical weapons attacks didn't stop.
Chemical weapons investigations polarise. Why is OPCW still considered impartial?
Could Syria have underestimated the international backlash this time? The answer may depend on OPCW's findings and how they will be interpreted by its member states.
Theoretically, there shouldn't be any reason to doubt the organisation's work. "As an international organisation, the OPCW is driven by its own rules. Its impartiality is fundamental," said Trapp. Russia is as much involved in the OPCW as the United States, and procedures are based on a convention all member states agreed on.
The organisation's rules now apply to 192 countries and cover 98 per cent of the world's population. Apart from conducting independent investigations, its mission consists of monitoring the destruction of chemical agents that has led to the removal of 96 per cent of the world's declared chemical weapons stockpile, according to OPCW's statistics.
But even though impartiality is the basis for OPCW's work, Russia has recently begun to cast doubt on some of the organisation's findings, which could ultimately pose a serious challenge to its broader mission.
Last week, Moscow rejected an OPCW release that appeared to confirm British authorities' findings on the attempted poisoning of a former Russian spy in Britain and his daughter. Russia said it would not accept the official OPCW report unless it was allowed to participate in the investigation.
In the case of the suspected Douma chemical attack, Russia will have to look for a different argument, however. Russia likely gained access to the attack site days ago, the United States said on Tuesday.
