ful solutions may be present in the mine water or toxic dusts may be formed. Blasting, shoveling, hauling, timbering, dry drilling, chute loading, and other activities in dry mines are likely to be dust producers. Breathing certain kinds of dusts has harmful effects on the health, and much has been written about dust disease, which is the greatest menace to the health of miners.

Although the attention of those interested in diseases peculiar to mining has been focused recently on one disease, silicosis, other respiratory diseases may cause more suffering and economic loss. The increased occurrence of these diseases usually is attributed to exposure to abnormal air conditions underground, such as sudden changes from high to low temperatures, excessive dust, and noxious gases. The principal respiratory diseases to which miners are subject are bronchitis, influenza, pneumonia, pulmonary tuberculosis, anthracosilicosis, and silicosis."

7

The Medical Research Council of Great Britain reported that: " Workers who are most often ill tend most often to have accidents, indicating that the prevention of accidents may depend to a considerable degree on the prevention of sickness and emphasizing the need for attention to industrial health programs.

DUST

EFFECTS OF INHALING DUST

Literature on the effects of breathing dust abounds in various experimental, theoretical, and factual data but lacks conclusiveness in almost every phase, except possibly that harm to health can be expected from prolonged inhalation of excessive amounts. There appears to be good reason to believe that this applies virtually to any or all dusts or combinations of dusts, organic or inorganic.

8

It is well established that exposure to certain kinds of dusts, such as those containing considerable quantities of free silica, has increased the morbidity and mortality rate from respiratory diseases, and some metallic dusts, such as lead and its compounds, have been associated with general system poisoning. According to Drinker, four reactions are produced in man by inhalation of dust. The first (and most important) reaction is produced by the pneumoconioses, such as silicosis and asbestosis, which cause specific lung pathology and often are followed by pulmonary tuberculosis. The second reaction is caused by toxic dusts like lead, cadmium, and radium. A third reaction follows inhalation of finely divided, metallic, fume particles (such as zinc oxide) and is known as metal-fume fever. The fourth reaction, allergic in character, is caused by inhaling organic dusts, such as pollen and certain types of pulverized wood and flour. all four the sole cause of the disability may be dust inhalation, but reactions from toxic dusts do result from swallowing dust as well as inhalation."

In

Sayers, R. R., Pulmonary Diseases in the Mining Industry: Bureau of Mines Inf. Circ. 7146, 1941, 26 pp. American Medical Association, Proneness to Accidents: Jour. Am. Med. Assoc., Nov. 14, 1942, p. 841.

8 Forbes, J. J., Davenport, Sara J., and Morgis, Genevieve C., Review of Literature on Dusts Bureau of Mines Bull. 478, 1950, 333 pp.

Drinker, Philip, The Causation of Pneumoconiosis: Jour, Ind. Hyg. and Toxicol., October 1936, p. 524.

Silicosis is the term now applied to the lung trouble of miners (in the past it was referred to as asthma, lung consumption, or miner's disease) that is due, it is agreed rather generally, to some form of the element silicon. A good proportion of the present-day authorities on dust disease think that silica dust is harmful chiefly because, in the finely divided form in which it enters the lungs, it goes into solution and causes attendant chemical reactions, impairing or destroying the lung tissue.10 Formerly, it was believed that fibrosis (the formation of fibrous tissue in the lungs) was produced in response to the irritation caused by the hard, sharp, quartz particles, as well as by filling or partial filling of the lungs with dust; that is, the dust was supposed to act mechanically. A rational view seems to be that the harmful action is both chemical and mechanical.11

The numerous ill effects on human organs other than the lungs, owing to breathing more or less insoluble dust, need to be considered, as well as the harm to other organs (stomach, liver, kidneys, etc.) or to the nose, throat, and bronchial tubes from breathing more or less soluble or so-called poisonous dusts. Moreover, numerous dusts (both organic and inorganic) have harmful effects of various kinds on the eyes and ears and other parts of the body and on the skin from external contact; indeed, some dusts cause harm to health by absorption through the skin. Hence, although pulmonary diseases due to the breathing of airborne dusts are unquestionably of great present-day importance, their various ramifications (pneumoconiosis, silicosis, anthracosis, or these combined with tuberculosis), although vitally important, by no means constitute the only detrimental effect to human beings who come in contact with dust, externally or internally.1

12

There is no satisfactory medical answer at present to this dust problem, but an engineer is making a bad mistake if he lets men breathe heavy dust concentrations of any material. If no other reason for dust control can be found, then we should read transcripts of some of the recent suits at common law in which large damages were awarded for alleged silicosis to men who breathed dust containing little or no silica. The courts and compensation boards are not impressed with subtle distinctions between dusts with 10- and 40percent quartz, especially when medical experts are reluctant to make definite statements as to the comparative significance of such differences.

It would be well to realize that men working in dusty atmospheres suffer more from respiratory troubles than do men who work in clean air-evidence that excessive dustiness of any kind is harmful is beyond argument.13

10 Harrington, D., Silicosis as Affecting Mining Workmen and Operations: Bureau of Mines Inf. Čirc. 6867, 1936, 14 pp.

11 See work cited in footnote 8.

12 See work cited in footnote 8.

13 Harrington, D., Methods of Protection Against Silicosis and When They Are Justified: Bureau of Mines Inf. Circ. 6989, 1938, 9 pp.

The Federal Bureau of Mines 14 made a careful study of industrial lead poisoning a number of years ago. The conclusions were that most cases of lead poisoning are contracted by inhaling air laden with lead carbonate dust and that the opportunity to contract lead poisoning varies in proportion to the solubility of the ore mined, the quantity of lead dust thrown into the atmosphere, and the ventilation afforded. Other metallic dusts may have harmful effects on the human system when breathed; soluble forms of certain metals may have toxic properties, and insoluble forms are likely to be irritating to the respiratory tract. Considerable is known regarding the hazards of metallic poisoning by inhaling fine particles as dust, swallowing dust or solutions, or absorbing dust through the skin. However, the particulars of these hazards are of interest chiefly to operations where such metals or compounds are encountered; references describing the hazards and their treatment are given in the suggested reading at the end of this publication.

ALLOWABLE CONCENTRATIONS OF DUST IN MINING

The quantity of dust taken into the respiratory organs is a controlling factor in harmfulness of dust to the respiratory system, and common sense indicates that the quantity of dust breathed is one of the factors determining possible harmfulness from any kind of airborne dust.15 Investigations over a period of years show that exposure to certain concentrations of harmful dusts for a known time causes

CUBIC FOOT OF AIR)

PARNCENTRATION

FIGURE 3.-Relation of Average Dust Concentration and Duration of Dust Exposure to Workers Having Pneumoconiosis.

varying degrees of respiratory disability and that below these concentrations disabling pulmonary diseases do not occur within a working lifetime.1 The effects of concentrations of dust, as indicated by medical studies of some mine and quarry workers, are shown in figure 3.

14 Murray, A. L., Lead Poisoning in the Mining of Lead in Utah: Bureau of Mines Tech. Paper 389, 1926, 40 pp. 1 See work cited in footnote 8.

10 Sayers, R. R., Pulmonary Diseases in the Mining Industry: Bureau of Mines Inf. Circ. 7146, 1941, 26 pp.

From the viewpoint of harmfulness to the respiratory tract, it is generally stated (though actual knowledge on this point is by no means certain) that the dust particles (such as free silica) that affect health adversely upon entering the lungs are less than 10 microns in size; different persons vary in their opinions as to what the maximum may be-10, 8, or 6 microns and some say that the harm can be done by 2-micron particles or smaller.

As against this, it is stated that fibers of asbestos as long as 200 microns have been found in the lungs of men and that, undoubtedly, these long asbestos fibers exerted some detrimental effect. Since a micron is only about 0.00004 of an inch, or much smaller than the naked eye can see, it is usually assumed that the larger dust particles, which can be seen floating in air and settle relatively quickly, are not harmful to health. Certainly, this is questionable, as the larger particles suspended in the air (some of them 100 or more microns in size), if present in considerable quantities, clog the air passages leading to the lungs. Some of these air passages are so constructed that, under ordinary circumstances, they intercept larger dust particles before they can reach the lungs, and this clogging then allows the smaller and probably most dangerous dust particles to enter the lungs unimpeded. A commonsense opinion as to what constitutes an atmosphere so dusty that dust-prevention action should be taken is that any atmosphere in which dust can be seen by the naked eye is too dusty, not only for health but also, at least in many cases, for safety and efficiency. If visible dust is eliminated, much of the invisible or probably most dangerous dust will unquestionably have been removed also, and very likely the health hazard, as well as other hazards, will be much minimized or removed. After visible dust is removed, it may be necessary, under some conditions or with some types of dust, to make more intricate investigations as to the occurrence of invisible dust, using the impinger, konimeter, or other instruments.1

Analyses of the data collected by the Federal Public Health Service to determine safe limits of dust exposure indicate that employment in an atmosphere containing less than 50 million dust particles per cubic foot produces a negligible number of cases of silicosis when the quartz content of the dust is less than 5 percent; when the silica content is about 13 percent, 10 to 15 million particles appear to be a safe limit. The position of the threshold value below which no silicosis cases were found, the magnitude of the trends between percentages affected, and time and intensity of exposure vary in different sets of data, depending to a large extent on the chemical nature of the dust to which workers were exposed and possibly on the regularity of employment. It does not seem advisable to set up any mathematical guide, because only the survivors have been studied. There is no adequate way of determining whether or not the men who dropped out of industry were in a better or worse state of health as far as silicosis

17 See work cited in footnote 13.

is concerned than the men who remained in the industry; thus, there is no way of introducing a correction in this element of selection by a mathematical calculation.18

Moreover, air with a certain number of dust particles per cubic foot, with a silica content of 1 percent (or any other percentage), could be far more harmful to one person than another, depending on individual susceptibility, exertion, etc. Dusty air would be more harmful to a person working in an atmosphere of 85° or 90° F., relative humidity 90 or 95 percent, than to one working in an atmosphere of 60° F., relative humidity 60 percent. Numerous other more or less similar contributing factors make it futile to establish regulations as to the allowable number of dust particles on a sliding scale in proportion to the silica content of the dust. In some industries, such as metal mining and tunneling, there may be as many different percentages of silica in the mine air as there are working places, and in many metal mines it is very unlikely that the percentage of silica in the air of a working place today will be at all close to the percentage of silica in that same place tomorrow.19

Regarding health, the outstanding and principal source of information is the pamphlet on threshold-limit values adopted from year to year by the American Conference of Governmental Industrial Hygienists.20 Each value expresses the average concentration of a substance in the air to which an industrial worker can be exposed for 8 hours daily for an indefinite period without injury or occupational disease. With some differences, these values are used by all the State governments that have accepted values along these lines.21 Table 1 gives these threshold-limit values (1956) for gases and vapors; toxic dusts, fumes, and mists; and mineral dusts. Many, but not all, of these substances are encountered in metal and nonmetallic mining, processing, and refining.

TYPES OF MINE DUST INJURIOUS TO HEALTH

As a result of investigations in South Africa and other mining districts of the world, so much attention was focused on one particular dust (silica) as the most harmful encountered in industry that most investigators had accepted other dusts as harmless or of negligible importance as health hazards in industry and considered the disease silicosis resulting from breathing silica dust as the only important dust disease.22 Other dusts, such as silicates (asbestos for example), have been found almost as harmful as silica dust; but the effect on the lungs is somewhat different from that of silica, and the hazard is not so widespread.

18 See work cited in footnote 16.

19 Forbes, J. J., Davenport, Sara J., and Morgis, Genevieve G., Review of Literature on Dusts: Bureau of Mines Bull. 478, 1950, 333 pp.

20 Reprinted and copyrighted by the American Medical Association, 535 Dearborn Street, Chicago 10, Ill.

21 Barkley, J. F., Accepted Limit Values of Air Pollutants: Bureau of Mines Inf. Circ. 7682, 1954, 6 pp.

22 See work cited in footnote 19.