There is much ambiguity surrounding the alleged chemical weapons attack in Douma, said to have taken place late Saturday, but there are a few matters that are clear:
First, the reports are “unverified”, according to The Wall Street Journal  and British Foreign Office  and are unconfirmed, according to the US State Department . What’s more, The New York Times noted that it “was not possible to independently verify the reports,”  while The Associated Press added that “the reports could not be independently verified.” 
Second, according to The Wall Street Journal, it isn’t “clear who carried out the attack”  assuming even that one was carried out.
Third, the “unverified photos and videos”  which form the body of (unverified) evidence, were produced by two groups which have an interest in fabricating atrocities to draw the United States more deeply into the Syrian conflict. Both groups, the White Helmets and Syrian American Medical Society, are funded by Western governments , which openly seek regime change in Syria and therefore have an interest in producing a humanitarian pretext to justify stepping up their intervention in the country. The Western government-funded White Helmets and Syrian American Medical Society are allied with anti-government jihadists and are active only “in opposition-controlled areas.”  They, too, are clearly interested parties.
Fourth, The New York Times indirectly revealed a possible motivation for the two groups to bring forward fabricated atrocity stories. “A new confirmed chemical attack in Syria,” the newspaper noted, “would pose a dilemma for President Trump, who … recently said he wants to get the United States out of Syria.” 
Trump’s recent musings about ending the US military occupation of nearly one-third of Syrian territory, including the country’s richest oil fields, was swiftly met by Pentagon opposition, led by US Defense Secretary Jim Mattis. The US president reluctantly accepted a continued occupation, so long as it ends in a matter of months rather than years.
Fabricating an atrocity would pressure Trump to maintain the US occupation indefinitely and possibly escalate US military intervention in Syria, much to the pleasure of Islamist insurgents, their White Helmet and Syrian American Medical Society allies, and US war planners.
If that is the intention, the maneuver appears to have met with success. Trump reacted on Twitter to the unverified (and unverifiable) reports, by dehumanizing Syrian president Bashar al-Assad as an “animal,” who the US president said was responsible for a “humanitarian disaster for no reason whatsoever.” That the US State Department acknowledged that the reports were unconfirmed failed to restrain the “shoot-from-the-hip” Trump.
Fifth, a chemical attack by the Syrian government would be manifestly self-defeating, and therefore would seem to be highly unlikely. The Syrian Arab Army is on the cusp of an all but inevitable victory in Eastern Ghouta. Why would it cancel its gains by handing the United States a pretext to continue its military intervention in Syria, in the aftermath of Trump signalling his intention to withdraw US troops?
Sixth, it is difficult to conceive of any military benefit to the Syrian Arab Army of deploying chemical weapons. The Syrian military has more lethal conventional ways of killing than using chemical agents, whose effects are unpredictable and typically small scale. In all the alleged chemical attack incidents in Syria, the claimed number of victims is always smaller than that which could easily be produced by air strikes and artillery. Why, then, would the Syrian government use relatively ineffective chemical weapons, creating a pretext for continued US intervention, when it could use more deadly conventional weapons, without a crossing a red line?
Seventh, much of the discourse about chemical weapons in Syria implicitly assumes the Syrian government has them, despite the country cooperating with the Organization for the Prevention of Chemical Weapons to eliminate them years ago.
Finally, allegations of chemical weapons use are routinely made against the Syrian government, and while, through repetition, have been transfigured into received truths, have all proved to be unverified. Jim Mattis acknowledged this at a February 2 news conference.
Q: Just make sure I heard you correctly, you’re saying you think it’s likely they have used it and you’re looking for the evidence? Is that what you said?
SEC. MATTIS: … We do not have evidence of it…we’re looking for evidence of it….
Q: So the likelihood was not what your — you’re not characterizing it as a likelihood? I thought I used — you used that word; I guess I misunderstood you.
SEC. MATTIS: Well, there’s certainly groups that say they’ve used it. And so they think there’s a likelihood, so we’re looking for the evidence.
Q: So there’s credible evidence out there that both sarin and chlorine —
SEC. MATTIS: No, I have not got the evidence, not specifically. I don’t have the evidence.
What I’m saying is that other — that groups on the ground, NGOs, fighters on the ground have said that sarin has been used. So we are looking for evidence. I don’t have evidence, credible or uncredible. 
Absence of evidence is not evidence of absence, but neither is it evidence of guilt. The complete lack of evidence, along with a political context that favors the production of spurious allegations, suggests that the latest chemical weapons claims are—like all that have preceded them—dubious at best.
1. Raja Abdulrahim, “Dozens killed in alleged chemical-weapons attack in Syria,” The Wall Street Journal, April 8, 2018.
2. Ben Hubbard, “Dozens suffocate in Syria as government is accused of chemical attack,” The New York Times, April 8, 2018.
5. Zeina Karam and Philip Issa, “Syrian rescuers say at least 40 people killed in eastern Ghouta has attack,” The Associated Press, April 8, 2018.
6. April 8.
7. Abdulrahim, April 8.
8. Raja Abdulrahim, “Syria airstrikes hit hospitals in rebel territory,” The Wall Street Journal, February 5, 2018; Louisa Loveluck and Erin Cunningham, “Dozens killed in apparent chemical weapons attack on civilians in Syria, rescue workers say,” The Washington Post, April 8, 2018.
9. Abdulrahim, April 8; Abdulrahim, February 5.
11. Media Availability by Secretary Mattis at the Pentagon, Secretary of Defense James N. Mattis, Feb. 2, 2018, https://www.defense.gov/News/Transcripts/Transcript-View/Article/1431844/media-availability-by-secretary-mattis-at-the-pentagon/