Mainstream American media outlets have willingly and openly embraced a narrative provided by Al Qaeda affiliates whose record of using chemical weapons in Syria and distorting and manufacturing “evidence” to promote anti-Assad policies in the west, including regime change, is well documented.
Al Nusra has a long history of manufacturing and employing crude chemical weapons; the 2013 chemical attack on Ghouta made use of low-grade Sarin nerve agent locally synthesized, while attacks in and around Aleppo in 2016 made use of a chlorine/white phosphorous blend. If the Russians are correct, and the building bombed in Khan Sheikhoun on the morning of April 4, 2017 was producing and/or storing chemical weapons, the probability that viable agent and other toxic contaminants were dispersed into the surrounding neighborhood, and further disseminated by the prevailing wind, is high.
The counter-narrative offered by the Russians and Syrians, however, has been minimized, mocked and ignored by both the American media and the Trump administration. So, too, has the very illogic of the premise being put forward to answer the question of why President Assad would risk everything by using chemical weapons against a target of zero military value, at a time when the strategic balance of power had shifted strongly in his favor.