With intense dusting all the dusts used produced much the same effects, both immediate and remote.

With moderate exposures some dusts are much more readily eliminated than others. While coal dust and shale dust enter the lung with great readiness they do not produce, under these conditions, permanent lesions; and the lung might pass for normal after a twelvemonth.

Flue dust and crystalline silica are not eliminated with such readiness.

Two methods are available for distinguishing the dusts—perhaps three.

1. Coal and shale are taken up by cells which are quickly shed and consequently do not set up processes which block lymphatics. These cells frequently break down, and masses of dust are to be seen on the surface of the alveolar walls.

Flue dust and the crystalline silica are taken up by cells which tend to remain in situ and form plaques, which appear early and persist. They are the only site of fibrosis made out in this investigation.

Dusts which form plaques are not readily eliminated.

2. Rate of elimination is more important than rate of invasion. Dusts which are not readily eliminated after moderate exposures are to be avoided.

3. There are two ways out of the lung: direct via the bronchi in the form of plugs of mucin, dust cells and dust; and indirectly via the lymphatics. These ultimately bring some of the dust to the large bronchi at the root of the lung, and the dust can be traced through the connective tissue, muscular, and mucous coats into the epithelium, whence it is excreted. Colourless dusts become pigmented, and to judge from the data as yet available, the more marked the pigmentation the readier the elimination. It is possible that pigment plays a part in this bronchial excretory process, on the analogy of the lipochromes and the deposit of particles in the skin. The different rates at which dusts reach the mucous coats of large bronchi illustrates their relation to lymphatic drainage.

It is of importance to make out whether dusts observe an “all or none rule” or whether they exhibit gradation.

Flue dust for instance in all these experiments produced permanent lesions; but a great deal entered the lung and a great deal was eliminated. Specimens taken from animals immediately after exposure to flue dust, Transvaal quartz, and flint, show more flue dust than crystalline silica; but in cases taken later, owing to flue dust elimination being more rapid, the dust remaining is nearly the same in quantity in all three.

Experiments were carried out (series 10) to determine whether these three dusts can be eliminated like coal and shale if the amount of dust in the air be slight and the invasion of the lung moderate. It is worth making great efforts to diminish the dust cloud if there are reasonable grounds for thinking that under these circumstances rate of elimination might approach rate of invasion. If, on the other hand, a considerable proportion of any dust that enters remains, then other methods must be sought.

The result of these experiments was not very encouraging although it was fairly clear that dosage was an important factor. While it is certainly not the case that concentration of dust in the air × duration of exposure is a constant; still with flue and silica the dose must be very small if accumulation is to be avoided.

More encouraging results were obtained by adding coal. The vigorous catarrhal reaction produced results in the relatively inert dust being carried out in the plugs along with the coal; and these experiments have certainly left the impression that under the conditions described the lungs would practically free themselves from flue or crystalline silica dust if these dusts enter in small quantities only and with coal. The fact that crystalline silica—even when present in considerable quantities—does not always tend to produce grave pulmonary disease, may perhaps be accounted for by assuming that when silica is relatively benevolent there is inhaled together with it some substance that provokes a reaction in the lung with shedding of epithelium and expectoration of débris. If flue dust has been used for stone dusting with impunity it may be due to the presence of coal, since, experimentally, it behaves like the silica group when used pure.

Under the conditions of these experiments:

Dusts that make mischief are dusts that accumulate.

Dusts that are eliminated are dusts that produce a marked initial reaction with much shedding of epithelium.

Dusts that accumulate do not produce so marked an initial reaction, there being much less shedding of epithelium.

Dusts that produce an initial reaction tend to carry out with them the more inert.