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462 and falls with the general death-rate for all causes, but more rapidly. Thus, while the mortality from phthisis among males in the past 50 years has fallen 50%, the general mortality has fallen only 25%. Consideration of phthisis for this reason is of especial value. If the mortality from the disease among males during the period 1851-60 be taken at a comparative figure of too, then that of females was 106 at that date; in 1901-10 the comparative figure for males had fallen to 50, and that of females to 35. These data show that females are as liable to phthisis as are males: indeed up to the age of 15 years they continue to experience a heavier mortality than males; and that whatever influences have contributed to the fall which has taken place, they have clearly not been so powerful in relation to males.

The distribution of phthisis from the age of 15 years and upwards, i.e. the period of industrial life, discloses that the disease is more prevalent in urban than in rural districts, and that it attacks males more than females. This increased incidence among males is less marked in rural districts than in urban districts. Since housing conditions are the same for the two sexes, these figures point to the occupation of the male exerting a powerful influence. War-time experience showed that industrial occupation can exert such an influence, for in Great Britain the mortality from phthisis among females rose during 1915, 1916, 1917, and 1918, in proportion to their employment in the munitions industry. It rose most for towns, like Birmingham, Coventry, Manchester, Newcastle, and Sheffield, much affected by war industries; less in towns less affected, like Ipswich, Norwich, Stoke-on-Trent, York, and Worcester; and did not rise or even fall in unaffected towns, like Bournemouth, Brigh- ton, Oxford, and Great Yarmouth. Since the war, women in indus- try have given place to men, and their mortality from phthisis has fallen again, even below pre-war figures.

When the data for phthisis mortality according to occupation are closely examined, four broad groups can be distinguished. One in which the phthisis mortality is below normal; this group includes agriculture and coal-mining, industries in which opportunity for infection through close personal contact is at a minimum. A second in which the phthisis mortality is above normal, but is the only cause of mortality which is high ; this group includes printers, tailors, and boot-makers, who work sufficiently close together to enable infection to pass from person to person (and in rooms where the atmospheric conditions are physiologically adverse). A third in which a high mortality from phthisis is associated with a high mortality from other respiratory diseases; this group includes sandstone-masons, tin-miners, grinders of metal, and others exposed to the inhalation of fine particles of silica. Silica-dust when inhaled appears to possess the power (not yet fully understood) of sensitizing the lung tissue to attacks of tuberculosis. And a fourth in which high mortality from phthisis is associated with a high mortality from all other causes of death ; this group includes general labourers and publicans. The publican owes his high general mortality to the power possessed by alcohol of sensitizing the general capacity for succumbing to disease. The general labourer who belongs to the poorest trade of the industrial world is an instance of the claim (mainly supported by evidence other than occupational) that phthisis is a pestis pau- perum. A possible explanation of this claim is forthcoming from recent investigations into food, which have revealed that the energy contained in food cannot be exploited unless certain food accessory factors (vitamines) are present; vitamines occur most plentifully in expensive forms of food, such as butter, meat and fresh vegetables. These forms of food, especially in the form of animal fats like cod- liver oil, have long been accepted to be of value in relation to phthisis. Further, the supply of vitamines is required to be increased if the output of energy is to be increased. Poverty connotes a low standard of food supply. Industry calls for expenditure of physical energy, which must be obtained from food. The combination of low wages, with a call for expenditure of physical energy, such as is found in the general labouring class, should be, and indeed is, found to be associated with a high mortality rate due to phthisis and to other causes of death as well.

Discovery of a high mortality from phthisis combined, or not

combined, with a high mortality from other respiratory diseases alone or from all other causes of death, enables the industry in which it occurs to be placed in its proper group, when the prevalent factor influencing the mortality is indicated. The method of procedure may be seen from the data embodied in the table at foot of page.

Statistical investigation may also provide further evidence. Reference to the age distribution of any phthisis death-rate brings out that the disease is more prevalent at different ages in different groups, as may be seen from the diagram. The median age at death from phthisis for shoe-makers lies between 38 and 39 years of age, which is the same as that for occupied and retired males. And their curve, although at a higher level throughout life, closely resembles that for occupied and retired males. The conclusion follows that if increased infection is the paramount influence determining the in- fluence of phthisis among shoe-makers, it is also the main influence affecting its incidence among occupied and retired males.

Mortality from Phthisis

Death Rate

Per 1000

Living

The curve for the general labourer is different in shape ; it diverges from the standard continuously up to the age of 50, and although the difference remains considerable in later years it does not maintain the position attained in middle life. The median age at death from phthisis for this group lies between 40 and 41 years of age. Here, apparently, the influence which leads to the increased mortality takes some years exerting its full effect. In the case of the tin-miner the median age at death lies between 43 and 44 years of age, and the curve shows in early adult life in an exaggerated form the same ten- dency to diverge from the standard. Apparently, the adverse in- fluence once established maintains its power throughout life.

Investigation of phthisis in industry indicates that the disease is to be combated in the general community by maintaining a gen- eral resistance against disease, especially through a food supply sufficient for energy needs, by preventing infection from individual to individual through avoiding over-crowding, and in special indus- tries by ensuring the absence of harmful silica-dust.

Injury or trauma, of which pulmonary silicosis may be taken as an example, has long been recognized to sensitize to tuberculosis, whether of bones in childhood or of the lungs in adult life. Tuberculosis here resembles miners' nystagmus, pneuqjonia and delirium tremens, all of which may be precipitated by an accident. But silicosis further provides a reverse influence ; for if a patient who appears to be only slightly affected with silicosis becomes infected with tubercle, the silicotic condition may advance rapidly, almost as though it were precipitated by the infection. The similarity between such a pre- cipitation and the formation of tophi in gout is significant.

The only form of phthisis legally recognized to have a definite industrial origin is tubercular silicosis in which the disease is pre- disposed to by, or super-imposed upon, a condition of pulmonary silicosis caused by inhaling silica-dust.

COMPARATIVE MORTALITY OF CERTAIN CLASSES AGED 15 YEARS AND UPWARDS, 1900-20

Cause of Death

Standard

Group i. Low General Mortality

Group 2. Phthi- sis alone in Excess

Group 3. Phthi- sis & Respira- tory Diseases in Excess

Group 4. High General Mortality

(Occupied & Retired Males)

(Agricultural Class)

(Coal-miner)

(Shoe-maker)

(Tin-miner)

(Publican Inn Servant)

(Dock La- bourer)

Influenza.

IOO

IOO

88

67

104

/-/

79

Alcoholism & Liver

.

Diseases

IOO

47

Si

77

28

670

167

Cancer

IOO

74

78

103

IOI

no

112

Phthisis

IOO

45

48

J 45

436

173

jfic

Other Lung Diseases Nervous Diseases

IOO IOO

49 60

in 84

84

IOI

419 84

148 178

206 109

Circulatory Diseases

IOO

66

92

IOO

144

129

Bright's Disease

IOO

51

66

86

143

243

117

Accident.

IOO

69

208

38

92

88

180

Suicide

IOO

89.

58

IOO

32

216

63