6 - FIBRE MONITORING
The occurrence of airborne fibrous dust in the occupational environment is well known and documented. Possible health effects are associated with the exposure to respirable fibrous dust. Airborne fibre levels need to be monitored with reliable measurement techniques such as the membrane filter method commonly used for inorganic fibres monitoring. In the membrane filter method, the sample is collected by drawing a measured volume of air through a filter. The filter is cleared using a drop of mounting solution with the proper refractive index and covered with a clean cover slip. The fibres are counted using a phase contrast optical microscope. Countable fibres are defined as having length (l) greater than or equal to 5 µm, diameter (d) smaller than 3 µm and aspect ratio (l/d) greater than or equal to 3:1.
The main objective of an effective dust measurement program is to provide accurate information on airborne fibre concentrations in order to ensure the health and safety of workers. Other objectives include:
€ Ensure safe working habits;
€ Minimize worker exposure;
€ Ascertain efficiency of engineering dust control measures;
€ Check compliance with regulations;
€ Assist medical surveillance of workers;
€ Provide exposure measurements for health research.
Maximum level of protection is afforded by minimizing fibre related exposure. Maximum protection requires personal monitoring, notification of the exposed workers, adherence to practices to minimize mineral dust release, and special attention to the design of worker protection.
Key factors to be taken into consideration when developing an effective fibre monitoring program include:
€ Type of dust;
€ Dust regulations;
€ Sampling strategy;
€ Measurement technique;
€ Quality assurance;
€ Implementation.
6.2 - DEFINITION OF ASBESTOS AND MMMF
Asbestos
The term "asbestos" is used for certain hydrated silicates when these silicates crystallize into the asbestiform variety. There are six recognized varieties of asbestos. One fibrous serpentine (chrysotile) and five fibrous amphiboles (amosite, anthophyllite, crocidolite, tremolite and actinolite).
MAN MADE MINERAL FIBRES (MMMF)
Man-made mineral fibres include several types of fibres and can be classified as mainly (see also Annex III of Appendix-I in Section 3):
- Rock wool / slag wool;
- Glass wool;
- Continuous filament glass fibre;
- Ceramic fibre;
- Carbon fibre;
- Modified inorganic fibres;
- Refractory fibres (tobermorite (CaSiO3), ...).
From the epidemiological and animal studies, the most important factors for the biological activity of a fibre are:
€ The respirability as defined by the dimensions and the density of the fibres;
€ The dose or dose-response;
€ And the durability in the biological system.
Other factors, such as the surface chemistry, the susceptibility of workers, etc., are also important
Phase contrast microscopy counting rules include respirable fibres of at least 5 micrometers in length. That will constitute an index of exposure that should never be exceeded over a certain period of time.
Roughly speaking, respirable dust includes those unit density particles with a diameter less than 7 micrometers (µm) according to the criteria of the British Medical Research Council (BMRC) or a diameter less than 10 µm according to the criteria of the United States Atomic Energy Commission (AEC) or the American Conference of Governmental Industrial Hygienists (ACGIH). Fibres and dust particles satisfying these criteria are capable of reaching and being deposited in the nonciliated portion of the lung where gas-exchange takes place.
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