Monitoring Asbestos Removal

The removal of asbestos has the potential for people to be exposed to asbestos fibres. Asbestos is only a risk if there is the potential for people to be exposed to fibres of a specific geometry that may lodge within the lining of the lungs.

The workers at risk can be comprised of two Similar Exposure Groups (SEG's) for air monitoring purposes:

• People removing the asbestos within the removal area; and
• People outside the work area within the work site.

Air monitoring for the workers removing bonded asbestos may not be necessary as they will generally be aware of the risks and control measures required such as respirators. It would be of benefit to undertake exposure monitoring through the use of personal monitors within the workers breathing zone during removal or different types and forms of asbestos. This data may then be useful to determine the level of protection required of the respirator and determine if the removal work is "under control". Other areas may be required to be monitored and is dependent on the extent and nature of the removal works, including whether the material is bonded or friable. Monitoring may be required within decontamination units and around enclosures.

Placing static monitors to the boundary of the removal area will assist to determine if there are effective control measures in place to limit potential exposure to people outside the removal area.  There may be instances where exposure monitoring by sampling the air within a person's breathing zone may be conducted. This is generally done to alleviate an individual's concern.

The method of asbestos air sampling is commonly conducted by the process outlines within the guidance note on the membrane filter method for estimating airborne asbestos fibres by the National Occupational Health & Safety Commission. The method involves sampling a known quantity of air though a filter, fibres impact on the filter, whereby analysis of the filter can determine the number of fibres identified, hence a concentration of fibres per milliltre can be determined and compared against the exposure standard.

The air pumps will required used must be pulsation free and be able to maintain a constant flow when the filter is loaded with particulates such as dust. The flow rate should be set to achieve a total of between 500 - 1000 litres over the sample duration. Generally flow rates of 2 l/min are used. If extremely high fibre concentrations are anticipated, a lower volume of 100 litres may be necessary to avoid overloading the filter with excessive fibres and dust.

The sampling head consists of a filter holder which is of a metal or conductive cowl (to reduce potential static electricity) with length of 50 mm. The filter comprises a 25 mm diameter gridded membrane filer usually of a cellulose material with a 0.8 micrometer pore size. The filter traps the particles and fibres as it impacts the surface.

The pumps with sampling train attached is then set at the required flow with a rotameter, to read within 5 percent of true) and maintain the flow within 10 per cent. At the completion of the monitoring the pumps with sampling train attached are then checked to ensure that a drop of no more than 10 percent has occurred.

The filters are then transported to the lab with the end caps in place and mounted on a microscope slide. The filer is removed from the cowl and placed on the microscope slide. A stream of acetone vapour is applied to dissolve the cellulose filer in which then becomes transparent. Glycerol triacetate is then applied and then a cover slip protects the filter. The slide is then warmed at about 50 degrees for a two or minutes to clear the filter in which analysis can then take place.

The filter is analysed by a phase contrast microscope using Koehler illumination with a total magnification of approximately 400x (times). The filters are then viewed using a Walton-Beckett circular eyepiece, known as the graticule. The eyepiece provides the field of view and provides fibre templates to assist in the specific fibre counting process.  Fibres with a specific geometry or counting criteria are recorded. As a generalisation a countable fibre is a fibre greater than 5 µm in length, less than 3 µm thick and has an aspect ratio of greater than 3:1 which is observed within the graticule area. 100 difference areas of the slide are viewed to determine the total number of fibres per 100 fields of graticule view.

The concentration is fibres within the air is expressed in Fibres per milliliter of air (fibres/mL) and calculated by the formula:

C = A/a. N/n. 1/r. 1/t

Where:

C = Concentration (fibres/mL)

A = area of the filter (mm²)

a = area of the field of graticule view (mm2)

N = number of countable fibres

N = number of fields of graticule view

R = flowrate

T = sampling time

When assessing asbestos control monitoring levels at the boundary below 0.01 fibres/mL the removal work may continue with control measures, between 0.01 - 0.02, the control measures shall be reviewed and for concentrations above 0.02 the removal work shall stop until the cause for the high concentration are identified and controlled.

In consideration to exposure monitoring Safe Work Australia, Hazardous Substances Information Search lists the time weighted exposure (TWA) standard for all asbestos types to be 0.1 fibres/mL.

For more information on asbestos control monitoring in NSW, Sydney, ACT & Canberra please email info@AsbestosTesting.com.au or call:

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