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doses produced pulmonary oedema and bronchopneu-monia. In view of idiosyncrasy, it is never possible toestablish a safe concentration for man, but it is usuallyassumed that 70 parts per million would be the uppersafety limit for short exposures and 50 parts per millionfor exposures lasting over eight hours. Once poisoninghas occurred, treatment, apart from removing the

patient from exposure, is of little value ; thereforeattention should be directed to preventive measures.Methyl bromide is capable of passing a Service gasmask in a quarter of an hour, and Clarke, Roworth, andHolling 11 therefore suggested that a pungent substanceshould be added to draw attention to its presence ;Viner 8 mentioned that it can be mixed with 20% carbontetrachloride, but phosgene may then be formed whenthe mixture is put on a fire. Fire-fighters should bewarned against spilling the substance on their clothingand instructed that clothing wetted with it must beremoved immediately. Ships, storehouses, and barnsmust be thoroughly ventilated after it has been used forfumigation. Prevention of leaks, adequate dilution ofescaping gas, and exhaust ventilation through the floorof the workshop are the most important preventivemeasures in industry. Bathing the hands in a weaksolution of sodium carbonate may help to prevent skinlesions by decomposing the methyl bromide. Periodicmedical examination will not only detect early symptomsbut draw attention to dangerous mechanical parts andlead to their repair before serious trouble has arisen.

STATISTICS AND EPIDEMIOLOGY

AT a meeting of the Royal Statistical Society onFeb. 27, Dr. Major Greenwood, F.R.S., emeritus professor ofvital statistics and epidemiology at the London Schoolof Hygiene and Tropical Medicine, discussed the partwhich statistical methods could play in the study ofepidemic disease. After referring to the history of ideason the nature and mode of spread of epidemics, he gavea neat example of the fitting of an algebraic structureto the hypothesis of the patient-to-patient transmissionof measles. A cynic, as Professor Greenwood noted,might observe that the mathematical ingenuity of hisproof was rather wasted on the verification of a resultwhich should be self-evident to any mother with morethan two children. On the other hand, the epidemicnature of influenza was not self-evident even to so

distinguished a medical historian as Hirsch writinglittle more than 60 years ago. Truth, as Pontius Pilateinferred, is relative, and proof in the strict sense is seldompossible in the biological sciences. The value of statisticalmethods seems to lie in the rigorous testing of a hypo-thesis-it deals not with absolute proofs but with relativelikelihoods. In,practice, Professor Greenwood suggested,the methods which seemed almost superfluous whenapplied to measles can be profitably used in the study ofsuch baftling problems as the spread of epidemic polio-myelitis. Essentially, these methods consist in an

analysis of the time intervals between successive casesand a study of the departures of these intervalsfrom a purely chance variability-a problem of somemathematical complexity.Much has been done by medical statisticians in an

effort to uncover the " laws " of the waxing and waningof epidemics. The explosive outburst of a food-bornedisease is evident in the skewed shape of the epidemiccurve which results from the sudden initial rise in thenumber of cases. Conversely, the more even rise andfall of an influenza epidemic is indicative of its airbornespread from person to person. Farr, Brownlee, Ross,and McKendrick have all tried to generalise in mathe-matical form the rise and fall of epidemic disease. Rossand McKendrick have stressed the effects of the influxof non-immunes into the herd and the development ofherd immunity. Brownlee, from the statistical character-

istics of the epidemic curve, suggested a possible waningof the invasive power of the infecting organism itselffrom the very start of the epidemic. Epidemics, in fact,did not decline only because all the susceptible had eithersuccumbed or acquired immunity, but also because theorganism itself had become less invasive. This view wasat least partly vindicated by the experimental studiesof Greenwood and Topley on epidemics in mice.

Professor Greenwood ended with an appeal to mathe-matical statisticians. The medical and biological com-plexities of the study of epidemics had added freshvariables which must be accounted for in the algebraicformulations of epidemic laws. Such formulations calledfor a mathematical sophistication which few in the medicalprofession could hope to attain, but in cooperation betweenstatisticians and field epidemiologists lay the future

promise.FOLIC ACID AGAIN

LAST year chemists in the United States synthesiseda substance with similar properties to the folic acid, or L. cosei," factor, which had been isolated from liver,and Spies and his colleagues claimed that this materialeffectively relieved nutritional macrocytic anaemia.These claims have now been considerably extended.Darby, Jones, and Johnson 2 gave synthetic folic acidin a dose of 15 mg. intramuscularly daily to 3 cases ofsprue, with no other treatment. The cases presented allthe classical features-glossitis, loss of weight, diarrheeawith increased fat in the stools, flat glucose-tolerancecurve, macrocytic anaemia, and sternal marrow like thatof untreated pernicious anaemia. In all 3 cases there wasa rapid remission of all symptoms with a notable reticu-locyte response, and the sternal marrow showed dis-

appearance of the more primitive erythroblasts. Spies 3has claimed a satisfactory response to folic acid in 5 casesof Addisonian pernicious anaemia, 8 cases of sprue, and3 of macrocytic anaemia of pregnancy, and others inaddition; but he gives no details.These findings are important and surprising. It was

to be expected that, sooner or later, the vitamin-B factorthat relieved nutritional macrocytic anaemia would beisolated ; but that the same factor should be effectivein true pernicious anaemia and relieve all the variedsymptoms of sprue is unexpected. Workers on nutri-tional anaemia elsewhere than America, such as Wills inIndia and Trowell in Uganda, have found that thepurified liver extracts which are effective in perniciousanaemia had to be given in very large doses to produceany effect in nutritional macrocytic anaemia-in dosesmuch larger than those of crude liver concentrates, whichsuggested that the factors responsible were different. Sofar only preliminary reports have appeared, but fullerinformation should soon be available. We need to knowthe chemical constitution of synthetic folic acid, and weneed fully illustrated clinical records that will show clearlythat folic acid brings about a change from megaloblasticto normoblastic marrow in cases of true perniciousanaemia and the megaloblastic nutritional anaemias ; for,if these claims are confirmed, all present liver extractsare out of date.

THE Rockefeller Foundation has made to the TavistockClinic in London a grant of £22,000 for practical workin such problems as industrial relations, resettlement ofreturning Servicemen, and the current difficulties offamilies and communities in the reconstruction period.Two anonymous donors have added £3500 to the fund.This work will form the main task of a new organisation,the Tavistock Institute of Human Relations.

1. See Lancet, Dec. 29, 1945, p. 855.2. Darby, W. J., Jones, E., Johnson, H. C. Science, Jan. 25, 1946,

p. 108.3. Spies, T. D. Lancet, Feb. 16, 1946, p. 225.