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9
PART II.—REPORT BY THE MEDICAL OFFICER OF HEALTH.
I present the report prepared by Dr. Andrewes in pursuance of his instructions to investigate
twelve samples of air from different points in the Central London Railway, and to compare them
bacteriologically with twelve similar samples taken at the same times from the fresh air outside.
This investigation has been spread over four months, and has necessitated the detailed study
microscopically and in culture of nearly 600 colonies arising in the plate cultures. Each
of the twelve observations involved the making of four plate cultures, the organisms in the sample
taken and in the control sample from the external air undergoing incubation in gelatine at
20° C., and also in agar-agar at 37° C. Two additional experiments were made with a view
to determining whether anaerobic organisms were present in the air of the tube; these two
experiments yielded negative results.
As regards the total number of micro-organisms found, the average, i.e., average of twelve
examinations of five litres of air, in the tube air was 44.1, while the control samples of fresh air
gave an average of 33.9—the range in the tube air being from 14 to 103, and in the fresh air
samples from 2 to 93. The highest averages, Dr. Andrewes notes, were found in the carriages and
lifts, i.e., in the most crowded places examined, while the platforms and passages came out actually
better than the fresh air, and the tunnels were only a little worse. There was thus a direct
correspondence between the amount and concentration of the human traffic and the number of
organisms in the air. Dr. Andrewes concludes that, so far as the total number of micro-organisms
is concerned, the Central London Railway does not compare unfavourably with conditions known
to exist in inhabited rooms generally.
On turning to the relative proportion of organisms growing at 20° C. and 37° C. in the tube
air and fresh air respectively, a difference is apparent. The tube air contains rather more than
twice as many “mesophil organisms," i.e., organisms growing at the higher temperature, as the
fresh air, and the average” mesophil ratio," i.e., the proportion of organisms growing at the
higher, to the organisms (other than moulds) growing at the lower temperature, was in the case
of fresh air 10.6 per cent., and in that of tube air 17.9 per cent. The difference is not a great one,
and the number of observations not large enough to justify much stress being laid upon it; moreover,
it appears that it was due, in the main, to the fact that sarcinse, which are almost invariably
mesophil organisms, were more abundant in the tube air than in fresh air. Sarcinae are common
air organisms, they are non-pathogenic, and there is no sufficient proof that the species found in
the tube air had a human origin.
Dr. Andrewes compares his bacteriological finding in each instance with the chemical
results obtained by Dr. Clowes. On the whole the number of bacteria was in correspondence with
the degree of chemical impurity testified to by the samples ; there were, however, occasional
discrepancies, and it is clear that chemical examination alone cannot be taken as a trustworthy
guide to the bacterial contamination of the air.
In all, 58 species of bacteria, streptothrices and yeasts were identified in the colonies
examined ; they included 26 micrococci, 24 bacilli, 3 streptothrices, and 5 yeasts. There were
in addition some 10 or 12 species which could not be identified and several moulds. None of the
bacteria identified were species which possess pathogenic property. As noted already, the most
striking differences between the tube air and fresh air was the relative preponderance in the former
of sarcince; again, the genus streptothrix occurred in tube air twice or thrice as frequently as in
fresh air. The species of micro-organisms found in the tube air were in the main identical with
those occurring in the fresh air.
In summarising his results, Dr. Andrewes concludes that while micro-organisms are
present in tube air in a somewhat greater proportion (13 to 10) than in fresh air, the excess is not
so considerable as to cause the tube air to compare unfavourably with the conditions known to exist
in inhabited rooms generally, and although there was considerable excess of organisms capable
of growing at the temperature of the body, this excess was due to non-pathogenic sarcinae and
allied species, and no pathogenic organisms were found in the tube air.
Dr. Andrewes adds a word of caution with regard to the observed absence of pathogenic
species from the 600 or so colonies which he examined, pointing out that it may be safely
inferred that the distribution of such organisms in air must be extremely sparse, and that while
the method which he employed in his investigation was excellently adapted to the study of the
general bacterial flora of the air, it was ill-adapted to the recognition of given pathogenic
organisms on account of the small volume of the samples examined.
He observes that what is needful is, in the first instance, that special methods for the
recognition of individual species should be devised, and that these methods should be then
employed upon very large volumes of air.
Shirley F. Murphy,
Medical Officer of Health.
Public Health Department,
8, St. Martin's-place, W.C.,
November 20th, 1902.
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