More than 5,000 people died in only 20 minutes and another 10,000 were severely injured in the attack, according to the U.S. State Department.
One reason for the high toll was the way the Iraqi forces deployed their arms, said Patricia Lewis, a fellow at the UK-based think tank Chatham House.
They first used conventional weapons to blow out the windows and doors of the homes where civilians were taking shelter, often in cellars. When they then fired chemical weapons, the toxic gases seeped in and often pooled in the cellars, proving even more deadly, she said.
In the years since the attack, civilians who survived have suffered much higher rates of serious diseases because of the toxic chemicals in the weapons, the UNODA said.
Iraq used unconventional weapons despite it being party to the 1925 Geneva Protocol.
Currently, 188 nations, representing the vast bulk of the world's population, have signed up to the Convention on Chemical Weapons, which is overseen by the Organization for the Prohibition of Chemical Weapons.
Syria is not one of them.
What impact could chemical weapons have if used now?
According to Esfandiary, chemical weapons' utility is "quite limited," as they are more of a deterrent than a real battlefield or tactical weapon.
"If you shoot a missile at a a population center, you can be fairly certain that anyone it hits will die," she said. "Chemical weapons use is not as clear cut as that -- it depends on topography, weather, how you deliver the chemical weapons, and you can't always be clear it will cause maximum casualty."
Their real value is in their psychological power, she said. "It's a fantastic weapon of fear."
They can cause economic damage too if their use contaminates agricultural areas or water, making them also "a good weapon of disruption," she said.
Martin, of King's College London, points out that the threat of chemical weapons being deployed against foreign forces by Hussein did not stop international intervention in Iraq in 1991 or 2003.
The Iraqi army had experience in targeting and using such weapons in war, she added, making it a greater threat in this respect than Syrian forces today.
But Esfandiary argues that although Syrian forces are not believed to have used chemical weapons, they are "very much present in Syria's military doctrine."
She added: "The Syrian military has been very well trained in their use and deployment, so if anyone was to use them and use them successfully, it's Syria."
Modern armies are equipped to cope with such threats. Civilians, or ill-equipped rebel forces, are not.
If such weapons are fired in confined spaces, such as buildings, their effects are far more deadly than in the open air, said Lewis, of Chatham House.
An added danger is that chemical weapons have a long shelf life. Even if not usable as munitions, the chemicals can still present a threat decades on.
"One of the issues which is still being dealt with is munitions that are left over from the Second World War," said Martin.
How can you tell if chemical weapons have been used?
It's difficult to determine if chemical weapons have been deployed, unless you can recover a munition that still has traces of agent on it, said Martin.
Some conventional weapons or legal crowd control can also release smoke that causes respiratory problems, a common symptom of chemical weapons exposure.
This seems to have been the case in Homs last December, where the Syrian government was accused of using chemical weapons against civilians.
A U.S. State Department investigation subsequently concluded that the Syrian army did not use chemical munitions but apparently misused a riot-control gas in the attack, which Syrian doctors and activists said killed six people and left dozens suffering from respiratory, nerve and gastrointestinal ailments.