Page 118 - Advanced Mine Ventilation

Health Hazards of Respirable Dusts 99

Figure 7.2 Definition of respirable dust as collected by sampling instruments.

2
equivalent diameter as respirable. This has been agreed upon and adopted by the
Pneumoconiosis Conference in Johannesburg [9]. The Atomic Energy Commission,
United States, regards particles below 10 equivalent diameter as respirable. Fig. 7.2
shows these hypothetical depositional curves [10].
From a health point of view, the particle size distribution determines (discussed in
Chapter 8) the following:
1. The overall retention of the particles in the respiratory system.
2. Locations within the respiratory system where the particle would be deposited.
3. The rate of growth of disease, especially in the case of physiologically and chemically active
dust.
Fig. 7.3 shows a real lung deposition efficiency curve [11]. The full line shows the
efficiency obtained for the particle size range 1e6 mm equivalent diameter experimen-
tally. Because it is generally agreed that 10 mm (unit density) is the upper limit of respi-
rable size, the curve is extrapolated to zero efficiency at 10 mm equivalent diameter. As

2 1/2
Equivalent diameter ¼ Stoke’s diameter x (density) . It is equivalent to particle with unit (1.0) density
that has the same settling velocity.