Phosgene is the colorless poison gas, which was first used during World War I by the Germans, which has the odor of newly-mowed hay.
Phosgene gas is highly toxic; when it is inhaled it reacts with water in the lungs to create hydrochloric acid and carbon monoxide.
Symptoms may take from 2 to 24 hours to appear after exposure; the hydrochloric acid in the lungs causes pulmonary edema and can also cause bronchial pneumonia and lung abscesses.
If enough of the gas is inhaled death may result within 36 hours!
You can find plenty of tips for dealing with a chemical weapon incident online including the Department of Transportation's Guidelines for Responding to a Chemical Weapons Incident, the FBI's Chemical Weapons Awareness page, and Wilderness-survival's Chemical Weapon Survival: Take Protective Measures page.
In "trying to make the world just a little bit safer" news, dismantling of Syria’s chemical weapons facilities is expected to begin this month, and the first of the 12 facilities should be destroyed by the end of November.