It is no longer reasonable to maintain that all kinds of dust are equally dangerous to the lungs. A very convincing array of clinical, statistical and experimental observations has demonstrated that dusts composed, in whole or in part, of silica are capable of exciting a characteristic, progressive, nodular fibrosis of the lungs and that at the same time these organs become abnormally. susceptible to the tubercle bacillus. All the other types of dust that have thus far been investigated can apparently be inhaled almost with impunity for long periods of time.
It was originally claimed that only uncombined or "free" silica in the form of quartz was capable of producing this effect, but in recent years there has been a growing tendency to look with suspicion on some of the silicates (combinations of silica with bases). One of them, asbestos, a silicate of magnesium, is now a well recognized cause