Where water can be applied freely in the ways that have been mentioned, air dustiness can be greatly reduced and, where these measures are combined with suitable ventilation, the dust hazard is well-controlled.

Ventilation.-Circulation of clean air through working places in sufficient volume to dilute dust that remains in suspension, despite other means of control, has been found an effective method of keeping the dust concentration within safe limits. Air currents taken into working places, including air circulated from one active section to another, should be relatively free from dust.

Water sprays may be used to trap dust carried by intake air, or filtering units of other types may be devised.

Split systems of ventilation may be designed to remove dust from working faces and take it directly to return airways without passing through other active places. Blower fans and tubing are usually needed

to bring the ventilating current of air close to the face at a velocity sufficient to sweep away the dust as it forms.

Fixed installations, such as crushers or screens, where an unusual amount of dust is made, may be provided with exhaust systems that remove the major part of the dust directly from the unit, so that it does not spread into the surrounding air. Dust-collecting systems have been successfully applied to surface plants at many mines; for example, the installation shown in figure 11 removes the dust from over a conveyor belt in a shaft house.

Respiratory protection.-In controlling exposure to dust, the primary consideration should be to prevent harmful contamination of air that is breathed. Where such measures cannot be applied or are ineffective, for any one of a number of reasons, respirators will be required for protection, either as a primary method or as a means of supplementing other methods. Respirators are classed by the Federal Bureau of Mines as: Supplied-air respirators, hose type; supplied-oxygen respirators, self-contained oxygen breathing apparatus; air-purifying respirators; chemical-filter and mechanical-filter types of gas masks; and dust and fume respirators.

Devices most commonly used for protection against dust hazards are the air-line or hose-type supplying pure air from an outside source, and the mechanical-filter type that remove dust particles as air is breathed through a filter. These devices are efficient when properly maintained, but they are inconvenient and often uncomfortable to wear. Air-line respirators supplying air at reduced pressure from the compressed-air line have been used to protect drillers in mines; in most cases use of the equipment has later been discarded in favor of dust control by water and ventilation. Mechanical filter-type respirators are widely used

both underground and on the surface where, for various reasons, other dust-control methods are not adequate. Use of a filter-type respirator by an attendant at an underground shaft pocket is shown in figure 12. The Bureau of Mines has established standards of approval for respiratory protective devices of all types to aid manufacturers in the development and marketing of safe and suitable equipment and to aid the purchaser in securing the type of equipment needed. A current list of respiratory protective equipment which has been tested and approved may be obtained from the Bureau of Mines.24 Additional

24 Schrenk, H. H., List of Respiratory Protective Devices Approved by the Bureau of Mines: Bureau of Mines Inf. Circ. 7030R, 1941, 11 pp.

information on the subject may be obtained from Information Circular 7236, Selection, Use, and Maintenance of Respiratory Devices, by H. H. Schrenk and S. J. Pearce, issued by the Bureau in 1943.

MEDICAL CONTROL OF DUST HAZARD

Physical examinations.-The effect of exposure to dusty atmospheres may be pronounced on some persons, while others will show no signs after an even longer time. Every person employed under possibly hazardous dust conditions should undergo a real physical examination before entering upon duty and at least yearly thereafter. Persons found to have developed symptoms of respiratory trouble should be removed from dusty air if possible to prevent aggravation of their condition. A. J. Lanza in his book, Silicosis and Asbestosis, observes that, It is not practicable to lay down definite rules. Obviously no one with active or recently active tuberculosis should be exposed to silica dust, nor anyone suffering from other pulmonary disease, especially if of an infective nature, which would tend to increase the hazard. On the other hand, there is often no good reason for rejecting an applicant who already has silicosis if the silica hazard in the work place involved is under control and if the workman's physical condition is properly supervised thereafter. This is an important consideration where the industry is a dominant one in its locality and other opportunities for work are limited.

Possible uses of inert dusts as preventives.-For many years some investigators considered certain dusts-coal, hematite, shale, limestone, cement, and clay-not only as harmless in themselves but as exercising a favorable effect when inhaled with more dangerous dusts. More thorough methods of investigation gradually revealed the fallacy of using the so-called protective dusts to prevent or cure diseases caused by the dangerous dusts.

As data accumulated tending to show that all dusts may be harmful if breathed in large amounts over long periods, the announcement in 1939 by Canadian investigators that inhalation of aluminum dust would prevent silicosis was received with skepticism by many persons familiar with the effects of exposure to dusts. By subsequent experimental study and the practical application of the treatment to men afflicted with silicosis, it is claimed to have been definitely established that aluminum is a harmless means of preventing silicosis in human beings and ameliorating the symptoms where this dust disease has gained a start.

A number of authorities on silicosis agree, however, that the only certainty about the use of aluminum is that it will inhibit the action of silica in experimental animals. In man, silicosis develops slowly in most instances, and only long-time, carefully controlled administration of the treatment will tell whether the results obtained with animals are applicable to men. It is admitted that aluminum powder cannot be considered a cure for silicosis as regards restoring to normal lung tissue that already has undergone fibrotic change. The statement is made, however, that its use is followed by beneficial results in a significant number of cases, chiefly in the amelioration of symptoms and in increased capacity for work. All investigators have not obtained the same results with application of the treatment as have the men who developed the method.

British investigators have raised the point that, as 1 part of aluminum dust is required to render inactive 99 parts of quartz, increased amounts of aluminum would be needed to counteract the toxic effects of air containing heavy concentrations of siliceous dust. The proper remedy