One of the chief objections to this view, apart from the experimental evidence, is that in those trades where metallic lead is handled, particularly lead rolling, very few hygienic precautions have ever been taken in regard to washing before meals, smoking, etc. Although in these trades the hands become coated with a lead compound (oleate), and the workers frequently eat their food with unwashed hands, thus affording every opportunity for the ingestion of lead, the incidence of poisoning is by no means as high or so pronounced in these occupations as in those giving rise to lead dust, such as the white lead industry, where special precautions are taken, and where the incidence of poisoning is always related to the dust breathed.

Respiratory Tract.

—In a report on the incidence of lead poisoning in the manufacture of paints and colours, one of us [T. M. L.[2]] in 1902 laid stress on the marked incidence of poisoning in the specially dusty lead processes. Following on that report special attention was given to the removal of dust by means of exhaust ventilation. With the introduction of precautionary measures, the incidence of poisoning underwent a marked decrease, this decrease being most definite in those industries where efficient exhaust ventilation could be maintained (see [p. 47]). Experience shows that cases of poisoning in any given trade or manufacturing process are always referable to the operations which cause the greatest amount of dust, and where, therefore, the opportunity of inhaling lead dust is greatest.

The investigations of Duckering[3], referred to on [p. 203], show the amount of dust present in the air in certain dangerous processes. His results clinch the deductions made from general observation, that dusty processes are those especially related to incidence of industrial poisoning. Ætiologically, therefore, the relationship of dust-contaminated air and poisoning is undeniable, and in not a few instances on record persons residing at a distance from a lead factory have developed poisoning, although not employed in any occupation involving contact with lead, aerial infection through dust remaining the only explanation. The actual channel through which the lead dust suspended in the air gains entrance to the body is, therefore, of especial importance; one of two channels is open—gastro-intestinal and respiratory.

The investigations of one of us (K. W. G.) on the experimental production of lead poisoning in animals has shown conclusively that the dust inhaled was far more dangerous, and produced symptoms far earlier than did the direct ingestion of a very much larger quantity of the same compound by way of the mouth and gastro-intestinal canal. There is no doubt whatever that the chief agent in causing lead poisoning is dust or fume suspended in the air. That a certain amount finds its way into the stomach direct is not denied, but from experimental evidence we consider the lung rather than the stomach to be the chief channel through which absorption takes place (see [p. 81]).

The following table gives a specific instance of the incidence of lead poisoning in a white lead factory, and demonstrates clearly the ætiological importance of dust. The increase in reported cases, as well as in symptoms of lead absorption not sufficiently severe to prevent the individual from following his usual occupation, was associated with the rebuilding of a portion of the factory in which the packing of dry white lead had been carried on for a large number of years. The alterations necessitated the removal of several floors, all of which were thoroughly impregnated with lead dust. Before the alterations were undertaken it was recognized that considerable danger would arise; stringent precautions were therefore taken, and the hands engaged in the alterations kept under special observation. Notwithstanding this there was an increase in the number of reported cases, which were all mild cases of colic; all recovered, and were able to return to their work in a short time.

Table I.—Lead Poisoning in a White Lead Factory.

The figures refer to the weekly examination of the whole of the men. For example, if a man was returned as suffering from anæmia on three occasions, he appears as three cases in Column 7.

YearTotal
Number
of
Exami-
nations		Total
Cases of
Poisoning		Cases
in Dusty
Processes		Cases
in Other
Processes		Cases of
Suspen-
sion		Cases of
Anæmia		Cases of
Tremor		Blue Line
(1)(2)(3)(4)(5)(6)(7)(8)(9)
19055,464 9 812078[B]249[B]311[B]
1906[A]5,09618162 9256 215 532
19074,303 4 31 6628138
19083,965 4 31 5402511

[A] Structural alterations in progress, including cutting up “lead floor,” saturated with white lead dust.