Rebels said the attack – which reportedly left four people dead and many injured – was carried out by government forces using chlorine gas.
The UN special envoy for Syria said a chlorine attack, if confirmed, would amount to a “war crime”.
Footage obtained by the BBC shows people with breathing difficulties receiving treatment at a hospital.
Men, women and children are shown being fitted with oxygen masks by medical staff.
The gas is thought to have been chlorine dropped in a barrel bomb, said the Syrian Civil Defence – volunteer emergency response workers who operate in opposition-held areas.
UN special envoy Staffan de Mistura confirmed the global body’s experts were investigating reports of a gas, believed to be chlorine, being dropped on Aleppo.
“There is a lot of evidence that it actually did take place,” he told reporters. “If it did take place, it is a war crime and as much it would require everyone… to address it immediately.”
A man receiving treatment in hospital said he had been in the Zebdieh area of the city, where he lives, when two missiles landed near him and a group of friends.
“And a few minutes later, the smell of gas started spreading… and I felt my eyes burning and difficulties in breathing,” he said. “The smell was very strong – beyond any description.”
A medic at the hospital said they had received a lot of casualties, who were “all ages” including children and elderly people.
“When we examined these casualties, we realised it was due to chlorine,” he added.
Chlorine is a common industrial chemical, but its use in weapons is banned by the Chemical Weapons Convention.
In 2013, the BBC found strong evidence suggesting residents of Saraqeb had been subject to a chemical attack by government helicopters, something denied by the Syrian authorities.