Phosgene - Assassinator of alveoli and butcher of bronchioles

• also called carbonyl chloride (it's an acyl halide) or CG (I'm partial to phosgene, myself)
• a colourless gas with a musty odour that was developed and employed as a chemical warfare agent during WWI, first being used by the German army in December of 1915
• was the most effective lethal chemical agent used in WWI, accounting for about 80% of the approximately 100,000 gas-induced casualties
  
• is used in the manufacture of plastics and is produced as a by-product of welding and from the combustion of chlorine-containing household products
• following inhalation it is transported deep into the gas-exchanging region of the lungs, where it slowly reacts with water to produce hydrochloric acid and carbon dioxide - the acid causes tissue damage, leading to the development of pulmonary edema (accumulation of fluid in the lungs, a potentially life-threatening condition)
  
• is an alkylating agent and so is carcinogenic
  
• acute exposure presents as eye, skin, pulmonary, and gastrointestinal irritation, with pulmonary edema often developing as a delayed symptom preceded by a relatively asymptomatic (no symptoms, you appear to be getting better) period
• can progress to adult respiratory distress syndrome (ARDS) and death
• there is no specific and effective antidote for its toxic effect

http://en.wikipedia.org/wiki/Phosgene