At least 106 chemical attacks have taken place in Syria since September 2013, when the president signed the international Chemical Weapons Convention (CWC) and agreed to destroy the country's chemical weapons stockpile.
Syria ratified the CWC a month after a chemical weapons attack on several suburbs of the capital, Damascus that involved the nerve agent Sarin and left hundreds of people dead.
The horrific pictures of victims convulsing in agony shocked the world. Western powers said the attack could only have been carried out by the government, but Assad blamed the opposition.
The US threatened military action in retaliation but relented when Assad's key ally, Russia, persuaded him to agree to the elimination of Syria's chemical arsenal. Despite the Organisation for the Prohibition of Chemical Weapons (OPCW) and the United Nations destroying all 1,300 tonnes of chemicals that the Syrian government declared, chemical weapons attacks in the country have continued.
The OPCW, the global watchdog that oversees implementation of the Chemical Weapons Convention, says a chemical weapon is a chemical used to cause intentional death or harm through its toxic properties.
The use of chemical weapons is prohibited under international humanitarian law regardless of the presence of a valid military target, as the effects of such weapons are indiscriminate by nature and designed to cause superfluous injury and unnecessary suffering.
Since 2014, the OPCW's Fact-Finding Mission (FFM) in Syria and the now-disbanded OPCW-UN Joint Investigative Mechanism (JIM) have investigated allegations of the use of toxic chemicals for hostile purposes in Syria.
They have determined that 37 incidents have involved or are likely to have involved the use of chemicals as weapons between September 2013 and April 2018.
The UN Human Rights Council's Independent International Commission of Inquiry (COI) on Syria and other UN-affiliated bodies have meanwhile concluded that there are reasonable grounds to believe that chemical weapons have been used in 18 other cases
Panorama and BBC Arabic examined 164 reports of chemical attacks alleged to have happened since Syria signed up to the CWC just over five years ago.
The BBC team determined that there was credible evidence to be confident a chemical weapon was used in 106 of those 164 incidents.
While only a few of these attacks have made headlines, the data suggests a pattern of repeated and sustained use of chemical weapons.
"The use of chemical weapons has delivered some outcomes for [government forces] that they believe are worth the risk, and [chemical weapons] have subsequently been shown to be worth the risk because they keep using them, repeatedly," said Julian Tangaere, former head of the OPCW mission to Syria.
The highest number of reported attacks took place in the north-western province of Idlib. There were also many incidents in the neighbouring provinces of Hama and Aleppo, and in the Eastern Ghouta region near Damascus, according to the BBC's data.
All of these areas have been opposition strongholds at various times during the war.
According to the reports, the deadliest single incident took place in the town of Khan Sheikhoun, in Idlib province, on 4 April 2017. Opposition health authorities say more than 80 people died that day.
Although chemical weapons are deadly, UN human rights experts have noted that most incidents in which civilians are killed and maimed have involved the unlawful use of conventional weapons. Such as cluster munitions and explosive weapons in civilian populated areas.