The available evidence, including video, photographs and eyewitness testimony, suggests that at least 51 of the 106 reported attacks were launched from the air. The BBC believes all the air-launched attacks were carried out by Syrian government forces.
Although Russian aircraft have conducted thousands of strikes in support of Mr Assad since 2015, UN human rights experts on the Commission of Inquiry have said there are no indications that Russian forces have ever used chemical weapons in the Syria.
The OPCW has likewise found no evidence that opposition armed groups had the capability to mount air attacks in the cases it has investigated.
Tobias Schneider of the Global Public Policy Institute has also investigated whether the opposition could have staged any air-launched chemical attacks and concluded that they could not. "The Assad regime is the only actor deploying chemical weapons by air," he said.
Dr Lina Khatib, head of the Middle East and North Africa programme at Chatham House, said: "The majority of chemical weapons attacks that we have seen in Syria seem to follow a pattern that indicates that they were the work of the regime and its allies, and not other groups in Syria."
"Sometimes the regime uses chemical weapons when it doesn't have the military capacity to take an area back using conventional weapons," she added.
Sarin was used in the deadliest of the 106 reported attacks - at Khan Sheikhoun - but the evidence suggests that the most commonly used toxic chemical was chlorine.
Chlorine is what is known as a "dual-use" chemical.
It has many legitimate peaceful civilian uses, but its use as a weapon is banned by the CWC.
Chlorine is thought to have been used in 79 of the 106 reported attacks, according to the BBC's data. The OPCW and JIM have determined that chlorine is likely to have been used as a weapon in 15 of the cases they have investigated.