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INVESTIGATION AS TO POSSIBLE DANGER FROM INHALING
PHOSGENE GAS THROUGH THE USE OF FIRE-EXTINGUISHERS
OF THE CARBON-TETRACHLORIDE TYPE.
By E. T. Shelbourn, F.I.C., Assistant Chemist, London County Council,
and
C. L. Bennett, B.Sc.(Lond.), A.I.C.
The question of possible danger from inhaling phosgene gas through the use of
fire-extinguishers of the carbon-tetrachloride type having been raised by the chief
officer, London Fire Brigade, it was decided to make experiments with some halogen
compounds, which may be or are used for the purpose of extinguishing fire. These
were carbon-tetrachloride, pentachlorethane, methyl bromide, trichlorethylene,
chloroform, and the liquid from a fire extinguisher (containing pentachlorethane and
methyl bromide).
These compounds were in turn subjected to the following treatment to simulate
as far as possible what actually takes place in treating a fire, the products being
examined for the presence of phosgene gas (COCl2) and the amount determined
(а) when mixed with burning petrol, (b) when the vapour, mixed with air, was
passed through a red-hot silica tube; (c) when passed through a heated iron tube;
(d) when passed through a burning cigarette.
It was found that in the first experiment the amount of phosgene produced
ranged from 6 pts. per million in the case of chloroform to 14 pts. per million with
pentachlorethane. This amount is not great, being considerably less than it is
considered fatal to inhale. It is, however, conceivable that this amount might
cause unpleasant symptoms. These conditions are similar to those which might
occur in putting out a small petrol fire, except that if the fire were in the open, the
amount of phosgene would be very considerably reduced, owing to rapid diffusion
into the surrounding air.
When however the vapours mixed with air came in contact with red hot silica
(b), or hot iron (c), the results were very different, except that, in contact with hot
silica, carbon tetrachloride gave only 5 pts. per million and pentachlorethane only
11 pts. per million of phosgene, whereas the other compounds gave amounts ranging
from 136 pts. per million (methyl bromide) and 1,830 pts. per million (chloroform).
The test with red-hot silica was intended to simulate what actually happens when
the vapours come in contact with any hot material, other than metals, in an
ordinary fire.
In the final experiment (d), which was undertaken to prove how far it is
possible to produce dangerous conditions due to the production of phosgene (as has
been alleged) through smoking a cigarette in proximity with the vapour of these
compounds, a concentration of 10 per cent, by volume of the vapour was used. In
the first experiment using carbon tetrachloride, the vapour was passed through a
glowing cigarette, but no trace of phosgene was found. Using trichlorethylene,
however, and drawing two litres of vapour and air through the cigarette so that it
was completely burned in five minutes, a very slight trace of phosgene was found
amounting to not more than 5 pts. per million. It appears therefore improbable
that any harmful effects due to phosgene would accrue from smoking in the vicinity
of these vapours.
In the case of the bromine compounds used in these experiments ((a), (b) and (c)),
there was the formation of the bromine compound (COBr2) analogous to phosgene,
which is very much less toxic than phosgene, and this rather suggests that it is safer
to use fire extinguishers containing methyl bromide in all cases where the fire is in
an enclosed space.
In all the experiments, the method used for' the determination of phosgene gas
was one that is specific for that compound.
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