Survivors reported symptoms of a feeling of loss of control in their limbs, foaming at the mouth, coughing, vomiting and intense headaches.
Masa and her family were among 75 neighbours who had taken shelter in a basement while bombs were dropped on their homes.
Those who were not so lucky – including men, women and children – were found crammed together on the floor with limbs spread and mouths foaming. Volunteers said their skin peeled off at a touch – and anyone who touched them became sick.
Masa’s mother Amani, 34, said she grabbed her daughter and made her way up the stairs as white gas and dust poured down past her and people fell around them.
While her family survived everyone in the neighbouring basement died.
Her husband Diaa, who was weak from diabetes, could not carry Masa’s twin sister Malaz so his brother took her. As Amani climbed up to escape, she saw gas and dust streaming down past her and poured water over Masa’s mouth.
Amani said: “The gas was spicy. Spicy in my throat like chilli. I couldn’t control my body. I was just shaking the whole time. There wasn’t oxygen.”
Amani told how she collapsed unconscious before coming to with Masa lying next to her foaming at the corners of her mouth.
Amani, now in a refugee camp in northern Syria with her family, said: “There are three basements in our street. Only three people died from ours, because we were warned. But the basement next door didn’t hear the gas. They all died where they were.”
Choking and staggering, Amani and her family headed to a clinic which was “overwhelmed” with dead and dying. The worst affected were put on respirators but most were just sprayed down with water in the hope it would help. The twins, who could barely breathe, were given injections.
Another survivor Ibrahim, 50, said: “I saw a doctor. She started crying because she had 40 patients but she only had medicine for three.”