Human Rights Watch analyzed reports of chemical weapons attacks in Syria: https://www.hrw.org/news/2018/04/04/syria-year-chemical-weapons-attacks-persist
The data used in this analysis is available here for download.
Human Rights Watch collated the data in this chart from seven sources: Human Rights Watch, the Organization for the Prohibition of Chemical Weapons−United Nations Joint Investigative Mechanism, the United Nations Commission of Inquiry, the OPCW Fact−finding Mission in Syria, the United Nations Mission to Investigate Allegations of the Use of Chemical Weapons in Syria, Amnesty International, and Bellingcat, an open source investigative organization. Human Rights Watch has not independently verified the attacks, except where indicated. The sources differ in their methodologies and the manner in which they present their evidence. Human Rights Watch has included only attacks that the sources confirmed. Where a variable, such as type of chemical or perpetrator was unconfirmed, this is indicated in the chart. Many attacks were documented by multiple sources. When sources differed on the number of estimated injuries, the Human Rights Watch confirmed estimate or the lowest estimate was used.
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