The Toxicity of Screening Smokes

SCREENING smokes, properly used, are perfectly safe. Some irritation of the throat and chest may be caused, but a properly designed smoke screen will rarely produce ill-effects from any but very prolonged or frequently repeated exposures. These chemical smokes may, however, be harmful if high concentrations are inhaled for mor'e than brief periods, so that dangers can arise from their misuse or the accidental ignition in enclosed spaces, such as dug-outs, buildings, or tunnels. Hardly a year goes by without such an accident occurring, and the diagnosis and treatment of the casualties always seem to present difficulties to the medical officers concerned. After a recent series of cases about which advice was sought it seemed both opportune and desirable to summarise, for the guidance of medical officers, the relevant experimental and clinical facts on the toxicity of the common screening smokes. All chemical smoke mixtures can be respiratory irritants and therefore their main toxic effect is one of damage to the respiratory system. However, they do vary in their irritancy and therefore in the danger they present. ' HCE-Smoke mixture contains equal quantities of hexachlorethane (HCE) and zinc oxide with 10 per cent calcium silicide. On ignition, a dense white smoke is produced and the possible products of combustion are zinc chloride, carbon monoxide, carbon' dioxide,' phosgene, hydrocarbons and chlorinated hydrocarbons. Chemical analysis of the smoke cloud, produced in an enclosed gas chamber, has shown that the carbon monoxide concentration does not exceed 0.04 per cent, that only traces of phosgene (less than 15 mg./m3 ) are present, and that the only constituent of the smoke likely to be harmful is zinc chloride. On an average, the efficiency of the smoke generators is such that for every 100 g. mixture used, a little over 40 g. of zinc chloride is released. When inhaled, zinc chloride behaves as a corrosive irritant and animal experiments have shown that it will produce a severe tracheobronchitis and intense pulmonary congestion and redema. Guinea-pigs, which are susceptible to bronchospasm, may die rapidly from this and at autopsy the lungs are markedly emphysematous. The dosage of zinc chloride required to produce death in , animals is, however, fairly large. Thus_ the Let 50 for mice (i.e. the dosage, expressed as the product of the concentration and time of exposure to kill 50 per cent of the mice) has been estimated as 11,800 mg. min./m3• However, the dosage had to be reduced to 2,000 mg. min./m3 or less before no macroscopic or histological sign of lung damage was seen .