photophobia (extreme sensitivity to light), severe pain, corneal ulceration, and blindness. Inhalation of chlormethine damages the upper and lower airways sequentially, with more severe exposures causing faster damage that afflicts lower parts of the respiratory tract. Early symptoms include rhinorrhea (runny nose), epistaxis (nosebleed), toneless voice, sneezing, barking cough, and dyspnea (in smokers and asthmatics). Later symptoms include pain in the nose/sinuses and inflammation of the airway. In severe cases, there may be epithelial necrosis throughout the respiratory tract, causing pseudomembrane formation, which can obstruct the airway. Pneumonia may develop and prove fatal. Skin exposure mainly causes erythema (redness) and vesication (blistering) at first, but absorption through the skin causes systemic toxicity. In cases where more than 25% of the skin is affected, fatal exposure is likely to have occurred. Though ingestion is uncommon, if mechlorethamine is swallowed it causes severe chemical burns to the gastrointestinal tract and concomitant nausea, vomiting, diarrhea, abdominal pain, and hemorrhage. Long-term effects of acute or chronic chlormethine exposure are caused by damage to the immune system. White blood cell counts drop, increasing the risk of infection, and red blood cell and platelet counts may also drop due to bone marrow damage. Chronic eye infections may result from exposure, but blindness is temporary. Long-term effects on the respiratory system include anosmia (inability to smell), ageusia (inability to taste), inflammation, chronic infections, fibrosis, and cancer. Skin that has been damaged by HN2 can change pigmentation or become scarred, and may eventually develop cancer. History The effect of vesicant (blister) agents in the form of mustard gas (sulfur mustard, Bis(2-chloroethyl) sulfide) on bone marrow and white blood cells had been known since the First World War. In 1935 several lines of chemical and biological research yielded results that would be explored after the start of the Second World War. The vesicant action of a family of chemicals related to the sulfur mustards, but with nitrogen substituting for sulfur was discovered—the "nitrogen mustards" were born. The particular nitrogen mustard chlormethine (mechlorethamine) was first synthesized. And the action of sulfur mustard on tumors in laboratory animals was investigated for the first time. After the U.S. entry into the Second World War the nitrogen mustards were candidate chemical warfare agents and research on them was initiated by the Office of Scientific Research and Development (OSRD). The OSRD let contracts to study them to two universities—Yale University and the University of