Updated at 4:35 p.m. ET
December 23, 2012. Bayada, Homs.
July 29, 2014. Old City, Aleppo.
July 4, 2018. Douma, outside Damascus.
A study released on Sunday tallies the chemical weapons attacks over the course of the Syrian civil war, which has left hundreds of thousands dead. At least 336 have occurred, according to authors Tobias Schneider and Theresa Lütkefend of the Berlin-based Global Public Policy Institute.