6.9 - OTHER NATURAL AND MAN-MADE FIBRES
€ Gravimetric Methods
At present, in most countries, standards for most natural and man-made fibres are based on gravimetric methods (mg/m3). As reported by Krantz et al. (1987) the most frequently used way of regulating a dust level is by introducing a limit value for total dust, very often combined with a limit value for respirable dust, as measured by customary gravimetric methods. Accurate measurement methods for such determinations can be found in the Manual of Analytical Methods produced by the National Institute of Occupational Safety and Health, NIOSH (1984) for the measurement of nuisance dust.
- Nuisance Dust, Total.
NIOSH Method 0500. Issued 2/15/84.
- Nuisance Dust, Respirable.
NIOSH Method 0600. Issued 2/15/84.
Fibre Counting Methods
Some countries have already introduced regulations with fibre number concentrations which apply to some natural and man-made fibre (MMF). For the determination of airborne concentrations in the workplace, the most widely recommended membrane filter methods are:
€ «Reference Methods for Measuring Airborne Man-Made Mineral Fibres» prepared by the WHO/EURO (1985).
€ «Determination of the Number Concentration of Airborne Inorganic Fibres by Phase Contrast Microscopy ‹ Membrane Filter Method» a third draft international standard of the International Standard Organization, ISO (1986).
-- Size distribution
To assess the size distribution of the airborne man-made mineral fibres (MMMF) in the workplace, the WHO/EURO (1985) developed a Scanning Electron Microscope (SEM) method:
«Reference Method Using a Scanning Electron Microscope to Determine Size of Airborne MMMF in the Workplace».
With increasing use of MMMF, many situations are developing where workers may be exposed to more than one variety of fibre, either man-made or natural. In such circumstances, it is important to be able to characterize the fibre types present. Unfortunately, no standardized technique exists yet for the identification of man-made fibres. The use of the analytical transmission electron microscopy appears the best available technique for airborne fibre identification but it is a research tool which needs an extensive expertise and is too costly to be used on a routine basis.
As no standardized routine sampling techniques exist for the identification of airborne fibres it is sometimes convenient to characterize bulk or settled dust in the workplace even though the components of the settled dust may quantitatively differ from those of the airborne dust. The cheapest technique to identify fibrous mineral dust in settled dust is using the polarized light microscope. Analytical transmission electron microscopy is recommended when the particles are smaller than 1.0 µm.
€ Open source emissions;
The open source emissions are generally less accurately measured with existing techniques than the point source emissions which can be monitored with relatively accurate measurement methods.
€ Quasi-stack sampling technique;
The upwind-downwind sampling technique is universally used to measure the fugitive dust, but it is the least reliable of the three techniques, being affected by so many variables including weather conditions, the wind speed and direction, the precipitation, the soil type, the vegetation cover, the surface moisture and the traffic activity as discussed by Jutze and Axetell (1976).
€ Mass determination method;
In practice, such a complete approach cannot be reasonably implemented for obvious reasons among which are the excessive cost of such a program and the unavailability of routine sampling and analytical methods for reliable measurements.
A more practical approach will emphasize the monitoring of the point source emissions which are usually the most important sources of dust (usually more than 80% of all dust emissions). By monitoring the point source emissions, we are monitoring the performance of the industrial ventilation system, which if working properly, will limit the amount of fibre emitted into the environment.
A good monitoring program of point source emissions will usually lessen, if not eliminate, the need for costly and inaccurate surveys of airborne concentrations of fibres in the ambient air outside factories and surrounding communities.
Special Considerations for Airborne Man-Made Mineral Fibres
-- Fibre identification
-- Fibre type identification technique
6.10 - ENVIRONMENTAL MONITORING
Introduction
Two categories of emission sources into the environment are generally generated from the industrial activities.
€ Point source emissions.
Regulations
Asbestos emission standards are from no visible emissions to the outside air in the United States to 2 fibres per cubic centimeter of the gases in Canada and in the European Economic Communities where for the latter, the emission standard can also be of 0.1 mg/m3.
Open Source Sampling
The three well known techniques of measurement of open source emissions were described by Kolnsberg (1976):
€ Roof monitor sampling technique;
€ Upwind-downwind sampling technique.
Point Source Sampling
For point source sampling, there exists conventional and well established methods which rely on manual techniques such as stack sampling, aimed at collecting samples by filtration to measure the mass and fibre concentration of particulate emissions.
Sample Evaluation
The commonly used methods of evaluation can be briefly classified:
€ Fibre counting method.
The mass determination is simpler to carry out with good accuracy. However the fibre counting method provides specific information on the fibre count which is usually preferred for correlation with health risks.
Recommendations
A complete program for the environmental surveillance of industrial activities will include the monitoring of diffused and point source emissions, and fibre concentrations in ambient air outside factories and air surrounding communities (e.g. the general environment).
A list of suitable equipments for the membrane filter method with suppliers is given in Appendix-IV.
Training
The person assigned to the fibre monitoring should receive adequate training to ensure the proper use of the reference methods for the determination of airborne fibre concentrations.
Overview of the Membrane Filter Method
To collect a sample, a volume of air is drawn through a membrane filter. The filter is later changed from an opaque membrane into a homogeneous optically transparent specimen. The fibres are then sized and counted using a phase contrast optical microscope. The result is expressed as fibres per ml of air, calculated from the number of fibres on the filter and the measured volume of air sampled.
Quality Control
Optically-visible fibre concentrations can only be defined in terms of the results obtained with a given measurement method. Uniformity of results between laboratories can only be ensured by a satisfactory quality control program. Because the membrane filter method is operator dependent, it is essential to ensure that the results are comparable between laboratories to ensure that details of the method are applied completely as specified.
Variations in method are therefore permitted, provided it is demonstrated that these have no significant effect on the results obtained. The quality control assurance is regarded as being part of the membrane filter method.
Record Keeping
The guidelines listed in the booklet Safety in the Use of Asbestos by ILO (1984) should be followed:
Records should be kept by the employer on all aspects of dust exposure. Such records should be clearly marked by date, work area and plant location.
Records regarding all aspects of dust exposure should be maintained, as far as it is practicable, for at least a 30-year period following termination of employment. Records dealing with dust sources, product composition and content, and environmental concentrations in the workplace may be reduced to micro-film for storage.
Conclusion
The measurement of airborne fibre in workplace is crucial:
€ To preserve the health and safety of workers.
€ To comply with regulations.
€ To improve the working conditions of employees.
They constitute the three main measurement objectives of a good monitoring plan that need to be endorsed by the managers of every industry as part of any management process.