A "vast body of evidence" supports claims that a chemical attack took place in Douma earlier this year, the UN Commission of Inquiry on Syria said yesterday. 

The reported attack, on April 7, led to airstrikes by American, British and French forces six days later. As on all previous occasions, the Assad regime denied using chemical weapons and was vociferously supported in that by its ally, Russia, which claimed a fake chemical attack had been staged "for the benefit of extremists and their foreign backers".

Yesterday's 24-page report from the UN Commission was a general update on the course of the war and its effects on civilians during the last six months but in a section headed "Ongoing Investigations" it referred specifically to Douma:

"Throughout 7 April, numerous aerial attacks were carried out in Douma, striking various residential areas. A vast body of evidence collected by the Commission suggests that, at approximately 7.30 pm, a gas cylinder containing a chlorine payload delivered by helicopter struck a multi-storey residential apartment building located approximately 100 metres south-west of Shohada Square."

The Commission also said it had "received information on the death of at least 49 individuals, and the wounding of up to 650 others" but added that it "cannot make yet any conclusions concerning the exact causes of death, in particular on whether another agent was used in addition to chlorine that may have caused or contributed to deaths and injuries".

There are several points worth noting in this:

1. Stating that a helicopter was used for the attack clearly indicates the Assad regime was responsible, since rebel fighters have no aircraft.

2. In stating unequivocally that chlorine was used, the Commission has gone further than the preliminary report issued by OPCW investigators in July which said only that samples collected from the scene had tested positive for "various chlorinated organic chemicals".

3. The Commission is unsure what caused the deaths and injuries. It implies that at least some were due to chlorine but is non-committal about whether some other substance was also involved. 

4. The Commission's report mentions a single gas cylinder, dropped around 7.30 pm, though the OPCW is currently investigating two cylinders found at separate locations in Douma.

The mention of only one cylinder in the report is especially interesting because it suggests the Commission may have doubts regarding the other one –  and there are certainly some puzzling questions about it which have not yet been answered.

Images circulated on social media show one cylinder which had apparently landed on a balcony, smashing a hole through the floor and thus presumably allowing gas to enter the building. This is the cylinder mentioned in the Commission's report.

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Images show the other cylinder lying on a bed in a top-floor apartment, with a large hole in the ceiling a few feet away. The problem with this is that if it entered the room via the hole it would have to have shunted sideways in order reach the bed. A possible explanation is that unknown persons had placed it manually on the bed.

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The OPCW is still investigating both cylinders and the damage surrounding them. Referring specifically to the "bedroom" cylinder, its July report said:

"It is planned that a comprehensive analysis will be conducted by suitable experts, possibly in metallurgy and structural or mechanical engineering, to provide an assessment of how the cylinders [sic] arrived at its location, in addition to the observed damage to the bed and other furniture of the room, the roof, and the cylinder itself."

There's little doubt, though, that the OPCW will eventually conclude that both cylinders were chemical weapons – because it has done so with similar objects in previous reports.

Although the cylinders themselves appear to be gas containers of a type used for industrial purposes, images show they had both been fitted with a metal harness to provide them with wheels, lugs for lifting, and tail fins.

The addition of tail fins is especially important because, according to a previous OPCW report, this shows the cylinders were intended to be dropped from a height, with the fins stabilising their descent. In other words, the fins confirmed they had been adapted for use as a weapon. 

The OPCW also said that if weapons dropped from the sky were "not designed to cause mechanical injury through explosive force" it was "reasonable" to conclude that they were chemical weapons – and this is clearly the case with the Douma cylinders. The key question, therefore, is not whether they were chemical weapons but how they came to be in the positions shown in the images.

Answering this is particularly important in the case of the "bedroom" cylinder because it has been the subject of much speculation by supporters of false flag theories. Hopefully, the OPCW's final report will provide some clarity.

If the cylinder did not land on the bed of its own accord there are two other possibilities: that it landed on the floor directly below the ceiling hole and was later moved on to the bed, or that it was brought into the bedroom from somewhere else. But much as these alternative possibilities excite false flag theorists, they don't actually disprove the cylinder's use by the regime as a chemical weapon.

If, for whatever reason, rebels had brought it into the building from outside and manhandled it up the stairs to the bedroom, there's still the question of where it originally came from – and the most likely explanation would be that they had retrieved it from the scene of some earlier chemical attack (of which there have been plenty). 

Testing for chlorine

Unlike nerve agents, use of chlorine in an attack is difficult to detect after the event. It's a very common element found naturally in many different compounds, so the question is how to distinguish chlorine released by a chemical attack from chlorine that already exists in the environment.

The OPCW considered what to do about this when embarking on its first chlorine-related investigation in 2014. Its initial plan was to collect multiple soil samples from an area of open land surrounding the alleged chlorine release point. "The expectation was that chlorine would deposit on soil and be transported into the soil column, where it is converted to a variety of products, including chloride ion," its report said.

Chloride occurs naturally in soil but the OPCW reasoned that if an attack had taken place levels of chloride in the upper layers of soil would be highest at the impact point and higher in places that were downwind of the impact point than those that were upwind.

At the time this could not be put to the test because investigators were unable to gain access to the sites. Some of the later investigations did, however, find "elevated" levels of chlorine-related substances which could not be explained as a natural occurrence.

While such findings can be strongly suggestive of a chlorine attack, the evidence they provide is circumstantial; on their own, they can never be conclusive in the way that tests for nerve agents can be.

There's a further problem when it comes to testing people exposed to chlorine. Their symptoms may give a strong indication but proving it through laboratory tests is a different matter. Exposure to a nerve agent can be detected in the human body for some time afterwards but when the OPCW began looking for ways to test people for chlorine it found none.

One option it considered was testing for raised levels of 3-chlorotyrosine and 3,5 dichlorotyrosine but that method had only been explored by research scientists using biopsies from the nasal tissue of rats. The OPCW decided that trying it on humans in field conditions would be "near impossible" and probably not worthwhile because there could be other reasons for raised levels of these chemicals besides chlorine exposure.

In the Douma investigation, the OPCW says "various chlorinated organic chemicals" were found in samples from the two cylinder locations. This could indicate use of chlorine as a weapon but it is by no means conclusive and the report adds: "Work by the team to establish the significance of these results is ongoing." The additional work will probably try to establish whether the levels of chlorinated chemicals were higher than would be normally expected. 

The limitations of laboratory testing are not as big a problem as they might seem, however. Although it's obviously desirable to find out what chemical was used, the main task of investigators is to ascertain whether a chemical attack took place – and for that it is not necessary to identify any particular substance. Legally-speaking, all they need to show is that some kind of noxious chemical was released with the intention of harming people.