Chlorine was used in an attack on a rebel-held area in Syria in 2016 in which at least 20 people suffered breathing difficulties, the world’s chemical weapons watchdog concluded Tuesday. The incident near a field hospital outside the town of Kafr Zeita came shortly after witnesses reported a helicopter dropping at least one object, the Organisation for the Prohibition of Chemical Weapons (OPCW) said. Inspectors obtained an industrial chlorine cylinder retrieved from the site and were able to “positively link” it to the October 1, 2016 attack, helped by digital evidence and witness interviews, …