This method describes a standardized measurement procedure for determining the number concentration of inorganic, fibrous particles, which also include asbestos fibres and artificial mineral fibres.
According to VDI 3492 “Measuring inorganic fibrous particles – Scanning electron microscopic method”, the following measuring tasks are intended:
- Measurements for establishing the status quo: Determination of the fibre concentration during operation in order to assess whether preliminary measures are necessary to reduce the fibre concentration or whether the room can continue to be used.
- Monitoring the success of provisional measures: Have the provisional measures reduced the fibre concentrations below the permissible level or is the success of the measures continuing?
- Control measurement before lifting the protective measures: Is the fibre load so low that the protective measures can be lifted?
- Success control measurement to assess the decontamination: Is the fibre concentration below the reference values?
- Control measurement for the protection of third parties: Are partitions and negative pressure sufficient to prevent fibre releases from the work areas into other rooms, or have fibres been carried away?
For sampling, gold-vapourised core pore filters with a diameter of 25 mm and a pore size of 0.8 µm are used in a filter housing and suitable measuring equipment. In our company, the SG12 and APC measuring devices are used for this purpose.
The sampled filters are glued onto an aluminium sample carrier in the laboratory and treated with oxygen plasma, whereby organic material on the filter is removed and the visibility of fine fibres is greatly improved.
The evaluation is carried out in a scanning electron microscope with an EDX (energy dispersive X-ray analysis) detector. The filters are searched according to the guideline and the fibres found are counted and assigned to certain classes (asbestos, other inorganic fibres, gypsum). Specially developed counting software and experienced personnel guarantee a fast and reliable count.
If desired, fibres can be assigned to the respective product fibres by EDX analysis. This only requires a reference sample of the material used to clearly assign the fibres.
There is currently no occupational exposure limit for artificial mineral fibres (AMF). Until the new Hazardous Substances Ordinance came into force on 01.01.2005, a limit value of 250.000 F/m³ applied. This limit value can still be used for orientation. For asbestos in demolition, renovation or maintenance work without protective measures, an occupational exposure limit of 10.000 F/m³ applies. According to the Ordinance on Hazardous Substances § 23, an air limit value of <7br> 1.000 F/m³ applies to asbestos at workplaces that are not demolition, renovation or maintenance work.
In the case of measurements in connection with the performance of demolition, renovation and maintenance work on materials containing asbestos (but not measurements in areas during work on materials containing fibres), the following conditions must be met in accordance with the asbestos guidelines of the federal states when checking the success of decontamination work:
- The asbestos fibre concentrations with fibre lengths L > 5µm shall be calculated from the number of fibres observed on the filter. Each reading shall be less than 500 fibres/m3.
- The upper limit of the asbestos fibre concentration calculated from the number of asbestos fibres with a fibre length L > 5µm according to the Poisson distribution (95 % confidence interval) must be below 1000 fibres/m3.
Measurements carried out under the standard conditions specified in the guideline result in a detection limit of about 300 fibres/m³. If sampling deviates from the standard conditions (for example shortened sampling time), the detection limit can be improved with increased counting effort.