GUARDING INDUSTRIAL WORKERS AGAINST Demon of Dust (Jun, 1936)
GUARDING INDUSTRIAL WORKERS AGAINST Demon of Dust
Scientific Sleuths Give an Invisible Public Enemy the Third Degree with Odd Instruments
By Walter E. Burton
N AN amazing laboratory at Pittsburgh, Pa., a group of scientific sleuths are waging a never-ending war to protect American workers everywhere from the insidious and deadly menace of industrial dusts.
There, equipped with a microprojector, an amazing mechanical lung, and photoelectric eyes, Dr. Carlton E. Brown and his assistants at the U. S. Bureau of Mines Experiment Station look upon dust as a real public enemy. Like police dealing with a hardened criminal, they set elaborate traps for it and, when it is captured, put it through a rigorous “third degree.” It is quizzed under the glare of blinding lights, made to stand in the “line-up” before a ruled screen so that it can be observed and measured, and—believe it or not— it is sometimes electrocuted!
Although the world’s attention has been attracted lately to the silicosis cases among tunnel workers at Gauley Bridge, W. Va., this is but one outstanding instance of how dust can endanger life. In hundreds of factories, mines, shops, and other places where men work and live, constant guard must be kept against harmful dust, mist, and fumes.
Suppose we go with Dr. Brown on one of his dust cases. An industrial concern has sent out an appeal for assistance in determining whether its workmen are being subjected to undue danger because of dust-laden air. The industry may be a mining company, and the particular place a copper mine; or it may be a manufacturing concern with a grinding department where dust is a nuisance if not an actual danger.
The chief weapon this dust sleuth uses during his visit to the scene of the suspected “crime” is a dust collector. One type frequently employed looks like a small hand pump and captures dust by forcing it against a glass microscope slide. The dust, usually slightly moist, adheres to the glass in a layer. The moisture soon evaporates, leaving the dry dust on the glass, the particles forming a narrow line.
Back to the laboratory Dr. Brown goes, with his evidence. Although, from a survey of plant conditions, he knows in a general way what kinds of dust specks he has captured, and their probable amounts, he must employ scientifically accurate methods of identifying and counting the dust particles before steps can be recommended for eliminating the hazard. What kind of dust is it? How big are the particles? How are they distributed as to size? These are some of the vitally important questions that must be answered.
There are a number of ways of identifying dust. In one, the dust samples are tested chemically to determine the materials present. Thus a sample may be tested for lead, arsenic, copper, silica, or other probable substances by standard chemical methods. Because of the tiny amounts of dust available, the extremely small-scale chemical reactions are often watched with the aid of a microscope.
Another method of identification enlists the aid of a petrographical microscope—an instrument used in the study of minerals. Polarized light, which vibrates in only one plane, is directed on the dust particles. Because certain substances always look the same in polarized light, no matter what the shape of the specimen, identification may be made. Another way is to measure the refractive index of the particles—their ability to bend light rays—by immersing them in various liquids whose refractive indices are known. When the liquid is found in which a given particle becomes invisible, its index is then known to be the same as that of the test liquid.
Two other ways of identifying dust are being put into use at the laboratory. One is to examine the dust with a spectroscope by measuring the wave length of the light given off by its white-hot vapor. The other is to study the molecular structure of the dust particles with X rays.
SETTLING the question of the size and number of the particles is done in a darkened room, with an ingenious micro-projector devised by the Bureau of Mines. The usual way of measuring tiny specks of dust is too look at them through a microscope equipped to superimpose a measuring scale on the image. A piece of accurately ruled glass, known as a micrometer scale, usually is employed. It takes an expert about three hours of steady peering down the microscope tube to count and measure some 200 dust particles. And the work is fatiguing and hard on the eyes.
To speed up the counting process, and at the same time to make the job easier and less eye-tiring. the microprojector was arranged. In one corner of Dr. Brown’s laboratory office is a booth made of composition board, with one side closed by a dark curtain. This contains the projector, which itself consists of an arc light, a water cell for cooling the light beam, and a microscope equipped with a projecting eyepiece and prism. Diagonally across the room, in another corner, is the screen. This is no ordinary piece of white cloth, but a square of lacquered tracing cloth carefully ruled so that it is divided into squares, some of them one centimeter on a side and others half that size. The screen is supported by an iron frame. Running from the frame to the microscope are three pairs of overhead wires supported by grooved wood pulleys. These wires are connected to grooved knobs on the microscope. Two pairs control the movements of the mechanical slide carrier, and the third the fine-adjustment focusing knob.
To count and measure the particles in a dust sample, the dust expert projects the image on the tracing-cloth screen, where it is seen at 10,000 times its actual linear size. By turning the control knobs at his side, he moves the overhead wires and focuses the microscope or moves the slide one way or the other. Size values and the number of particles in a given size group are recorded on a counting machine. The ruled squares of the screen make measurement easy, each centimeter representing a distance of one micron on the slide. A micron is a thousandth of a millimeter, which is equal to about one-twenty-five thousandth of an inch.
With the microprojector, ruled screen, and ingenious system of remote control, a man can do in fifteen minutes a dust-counting job that formerly required three hours, and do it much easier and better.
And so these scientific sleuths carefully work out the criminal records of various dusts. When these records are complete, they answer a number of pressing questions. They reveal whether a dust hazard really exists—whether the men working in the factory where the samples were collected are in danger of contracting some lung disease. If a danger does exist, knowledge of the facts enables the Bureau of Mines investigators to recommend remedies. Perhaps changes can be made in the way the work that causes the dust is done. Maybe the ventilation can be improved so as to remove the danger. Or it may be possible to use a water spray to settle the dust. Finally, it no other way can be found, the Bureau of Mines recommends the use of approved breathing masks.
THE testing of masks to see if they meet standards set up by the laboratory involves equipment every bit as fascinating as the microprojector. Masks are tested as to the materials they will exclude from the air breathed, the rate at which they clog up and make breathing difficult, the snugness with which they fit about the face, their comfort, and other properties essential to good performance.
A mask also may be used for protection against fumes or mist. Fumes are products of various metals, usually such compounds as oxides or carbonates, and often are highly poisonous. Mists are composed of liquid particles, and are met with in spray painting, chromium plating, and similar operations. Frequently, combinations of dust, fumes, and mists are encountered.
The most satisfactory respirators are essentially mechanical filters having screens of one kind or another through which the air to be breathed is drawn. Particles in the air become enmeshed in the filtering material. There are, also, respirators which have individual air supplies, an example being the hoods used by sand blasters.
The dust investigators in Pittsburgh, when testing a dust mask, are interested principally in what gets through the filters. If the quantity of dust, mist, or fumes passing through is sufficient to cause trouble, the device is, of course, unsafe. By various methods, such as subjecting animals to known concentrations of dusts, safe limits have been established; all masks approved must conform to these limits.
Most of the mask tests are performed by a sort of robot that breathes air through the filter pad and traps dust particles in a glass “lung” or respiratory system. This machine also measures the resistance to air flow caused by the filter unit. Incidentally, if you use a respirator, you will be interested to know that the filter pad is least efficient when it is new, and most efficient when partially clogged by trapped particles.
If you were to drop into the Bureau of Mines dust laboratory while a dust test on a respirator was being run, you would find that finely divided silica, the same material that causes silicosis when breathed in sufficient quantity, is employed in the trial.
SILICOSIS, incidentally, is not a new disease. It was known to the early Greeks, and has been giving trouble ever since. Silica, or silicon dioxide, is a very hard, flinty substance which can be divided into extremely small particles. When breathed into the lungs, it lodges in the tissues and poisons them. This poisoning is believed to be mainly chemical. In an effort to test this assumption, finely powdered aluminum oxide and other dusts have been breathed by experimental animals. Although these dusts lodged in the lungs just like silica, and are practically as hard and sharp as silica, they did not cause the disease known as silicosis.
But to get back to the dust-breathing robot : Silica dust is used in it because the particles can be made extremely small, the majority of them less than a micron, or one twenty-five thousandth of an inch, in diameter. This dust, made by mechanical grinding and screening, is placed in a long, glass tube. Projecting down into this tube is a smaller one, at the lower end of which is a spirally grooved fitting and a series of prongs supporting a ring-shaped knife that breaks up the silica column. The smaller tube is connected to the intake of an atomizing chamber or aspirator. A suction fan, like that in a vacuum cleaner, draws air through an air cleaner and through the atomizing chamber where it picks up silica dust, and exhausts it into a large glass-walled room. Paper baffles in the room control the flow of dust-laden air currents. From this mixing chamber, pipes lead to a similar room below.
By the time the air gets into the second
chamber, it is uniformly laden with silica dust. A rough check on the concentration is made by a beam of light that shoots across the chamber and strikes a photo-electric cell connected to instruments that indicate the intensity of the light beam.
In one side of the lower dust chamber is a little door with a hole in it. Through this hole projects one end of the glass “lung.” This lung is an L-shaped glass tube a yard or so long. One end has a cone-shaped opening into which fits a glass support carrying the mask to be tested. The mask or filter unit is sealed to the glass with wax. The other end of the lung, the short leg of the L, is connected to a vacuum pump. When air is sucked through it by the vacuum pump, the filter removes most of the silica dust. The amount that passes through determines the efficiency of the filter.
The dust that does struggle through the filter is trapped electrically. Immediately behind the mask holder connection, inside the glass lung, is a smaller glass tube that has, extending along its center, a nichrome wire. Surrounding this tube is a cylindrical metal screen. The wire is connected to one terminal of a 30,000-volt, sixty-cycle transformer, similar to those used for operating neon signs; the screen is connected to the other terminal. Dust particles are unable to pass through this strong electrostatic field, and are deposited on the small tube. This tube is carefully cleaned and weighed before being inserted into the device; and is again weighed at the end of the test, the weight difference representing the amount of dust that passed through the filter.
FOR testing the resistance of a mask to fumes, poisonous lead vapor is generated by burning gas to which has been added a small quantity of liquid tetraethyl lead—the same stuff that is put into gasoline to keep your cars motor from knocking. Mist tests are made with three different materials—chromic acid mist produced by standard chromium-plating equipment; wet silica dust that simulates the solutions sprayed on bathtubs and the like in vitreous enameling, and lead-paint spray like that encountered by painters who use spray guns. Sand-blasting helmets are tested under actual sandblasting conditions.
Finally, masks are checked to see whether they fit the face snugly. Volunteers with different types of faces put on sample masks, and then remain for a time in an atmosphere heavily laden with coal dust; or else stand in front of a jet of coal-dust-laden air. Where the masks fit snugly, the covered portions of the face remain unblackened; where it does not, the coal dust is plainly visible. Inspection of nose and mouth discharges and of the nasal passages of persons employed in such tests is considered a reliable index of the quantity of coal dust breathed.
As these Bureau of Mines sleuths solve each new dust mystery, more and more scientific information is added to the criminal records of dust, mists, and fumes, and the lives of industrial workers in countless fields of work are made safer. Their research is taking the dust out of industry.