Phosgene (CG) (carbonyl chloride) is produced as a liquid or liquefied gas, which is colorless to light yellow. At ordinary temperatures and pressures, CG is a colorless gas. Odor range from strong and stifling, when concentrated, to the smell of freshly mowed hay in lower concentrations. Phosgene has a boiling point of 8.2ºC (45.6ºF) and is noncombustible. The vapor density is 3.4, which is heavier than air, and the specific gravity is 1.37 at 68ºF (20ºC), which is heavier than water. The primary route of exposure is through inhalation, by which it is highly toxic. It is also a strong eye irritant and has a TLV of 0.1 ppm or 0.40 mg/m³ in air. General population limits are 0.0025 mg/m³. In addition to mild conjunctival irritation, direct effects of exposure to phosgene result in damage to the lungs. The primary effect of exposure is pulmonary edema (fluid in the lungs). Death can occur within several hours after an exposure to a high concentration. Most fatalities, however, reach a maximum effect from the pulmonary edema in about 12 h, and death occurs within 24-48 h after exposure. Symptoms include coughing, choking (thus the term choking agent), tightness in the chest, nausea, and possibly vomiting, headache, and lacrimation. There is no real relationship between the symptoms and the prognosis. The structure and molecular formula for phosgene are shown in Figure 8.37. The DOT lists phosgene as a 2.3 poison gas. The NFPA 704 designation for CG is health 4, flammability 0, reactivity 1, and special 0. Vapors of phosgene are highly toxic. It has a four-digit UN identification number of 1076.