Eyes watering, struggling to breathe, Abd al-Mouin, 22, dragged his nephews from a house reeking of noxious fumes, then briefly blacked out. Even fresh air, he recalled, was “burning my lungs.” The chaos unfolded in the Syrian town of Sarmeen one night this spring as walkie-talkies warned of helicopters flying from a nearby army base, a signal for residents to take cover. Soon, residents said, there were sounds of aircraft, a smell of bleach and gasping victims streaming to a clinic.