Page 220 - Advanced Mine Ventilation

200 Advanced Mine Ventilation

place in the Federal Mine Safety and Health Act of 1977 was changed to mean any
place in a coal mine where miners are normally required to work or travel.
Previously a working place was defined as the area of a coal mine inbye the last
open crosscut in a working section. The standard is applied to designated occupations
in mechanized mining units in underground sections, designated areas in nonface
areas, and designated surface work locations. Once a reduced standard is applied to
a mechanized mining unit, the standard remains with the unit regardless of locations.
To have the standard reevaluated, the operator must ask MSHA to return to the mining
unit to collect samples for analysis. Reduced standards applied to designated areas or
designated work places remain with the area or work place until the quartz level at the
location is reduced (again, MSHA must be asked to return for evaluation) or until the
area or work place is abandoned.
In summary, the current MSHA procedure for the determination and enforcement of
quartz standard suffers from the following drawbacks:
1. Compared with world standards, the US quartz standard appears to be arbitrarily severe.
2. The number of samples (one) taken for quartz analysis appears to be inadequate.
3. The analytical technique used for silica analysis needs to be standardized.

12.5.1 The Quartz Standards

In the United States, and other countries, exposure limits for dust containing free silica
have been based on the concept that the toxicity of the dust is directly proportional to
the concentration of free silica in the dust. The exposure limits were developed from
epidemiologic studies relating incidence of silicosis to the concentration of dust in the
atmosphere. The studies of silica exposure were primarily in the granite industry, metal
mines, and foundries. However, some researchers [14,15] do suggest that the presence
of quartz in coal dust contributes to the progression of CWP.
The United States has taken a fundamentally different approach to the development
of a quartz exposure standard compared with the European countries. In the United
States, the standard was set at a value low enough to prevent the occurrence of silicosis
in any worker, regardless of technical feasibility, the workers’ personal susceptibility
to the disease, and his personal habits. The European countries recognize that even at
very low quartz concentrations, some individuals may contract silicosis and that the
present state of the art in dust control may not be sufficient to achieve excessively
low dust standards. They set their standards at a technically achievable value and care-
fully monitor the health of the workers. If any signs of silicosis or susceptibility to sili-
cosis appear, the worker is transferred to a less dusty work area.
3
In Great Britain, the dust standard, when no quartz is present, was 7 mg/m for a
3
longwall face and 5 mg/m for heading machines. These concentrations are measured
in the returns, 210 ft from the face. If quartz is present in concentrations greater than
3
3
0.45 mg/m , then the respirable dust standard is reduced to 3.0 mg/m [16].
In West Germany, dust exposure limits were divided into five categories for quartz
concentrations less than 5% and five categories for quartz concentrations greater than
5% as shown in Table 12.2.