Asbestos fibers

Asbestos fibers are released from asbestos containing materials (ACMs). Friable asbestos containing materials release fibers more readily than encapsulated asbestos containing materials.

Determining airborne asbestos fiber levels

The standard methodology(MDHS100 in the UK) for determining airborne asbestos fiber concentration (expressed as fibers per millitre (fiber/ml) or cubic centimetre (fiber/cm³))is via the following method:
1. Air pumps are used to suck air through a filter at 8 litres per minute for 60 minutes.
2. The filter is mounted on a slide using acetone
3. The slide is viewing using either Phase Contrast Microscopy(PCM), Scanning Electron Microscopy(SEM), or Transmission Electron Microscopy(TEM). Usually PCM due to cost. A PCM microscope costs approx £2500 GBP.
4. The analyst looks through the microscope and counts the number of objects which meed the MDHS criteria for counting as potential asbestos fibers. A crowd counter may be used to count fibers per field.
5. The fibers per microscope field are extrapolated to calculate a concentration value (fiber/ml). Unfortunately this is only a "transient index of concentration" rather than an absolute value as the smallest fibers are not visible via PCM.

Comparative exposure levels to airborne asbestos fibers by activity (Fibers per millilitre)

• Environmental Exposure 0.0001 fiber/ml
• Rural environmental exposure 0.0001 fiber/ml
• Urban environmental exposure 0.0001 to 0.001 fiber/ml
• Environmental exposure downwind from an asbestos cement plant at 300 m: 0.0022 fiber/ml
• Exposure in buildings with friable asbestos concentrations 0.001 to 0.01 fiber/ml(counted with an optical microscope)
• Controlled removal of "Artex" (3% to 5% Chrysotile) textured decorative coating: 0.08 fiber/ml
• Vigorous dry scraping for 60 minutes to remove textured decorative plaster from a wall: 0.09 fiber/ml personal sample, estimated PCME
• Throwing asbestos cement sheets into lorry 0.161 fiber/ml personal
• "Representative" Asbestos cement roof Removal: Not detected to 0.2 fiber/ml personal
• "Representative" Asbestos cement roof Repair: Not detected to 0.3 fiber/ml personal
• Asbestos cement sheets stacked in confined space 0.30 to 0.53 fiber/ml personal
• Hand sawing asbestos cement sheets and pipes < 1 fiber/ml
• Careful removal of whole asbestos insulation boards by unscrewing with shadow vacuuming and spray application of a wetting agent on unsealed surfaces: Up to 3 fiber/ml
• Jig sawing asbestos insulation board with LEV 1 - 5 fiber/ml
• Opening raw material Chrysotile from Chongqing, China, asbestos plant 5.8–7.5 fiber/ml
• Drilling overhead asbestos insulation boards (No LEV): 5 to 10 fiber/ml
• Hand sawing asbestos insulation board (No LEV): 5 to 10 fiber/ml
• Jig sawing asbestos insulation board (No LEV): 5 to 20 fiber/ml
• Breaking and ripping out asbestos insulation boards dry with no unscrewing: 5 to 20 fiber/ml
• Abrasive disc cutting of asbestos cement sheets and pipes without LEV 15 to 25 fiber/ml
• Bagging raw material Chrysotile in Chongqing, China, asbestos plant (1999) 5.2–58.4 fiber/ml
• Sweeping asbestos insulation board debris up to 100 fiber/ml
• Dry removal of asbestos lagging up to 100 fiber/ml
• Dry removal of sprayed (limpet) coating up to 1000 fiber/ml

Rate of decline of asbestos fiber concentration in room air

Airborne asbestos fibers settle very slowly and in relation to their diameter.
A 3 micrometre diameter fiber is estimated to settle at 150 millimetres per minute.
However a 0.5 micrometre diameter fiber is estimated to settle at 4 millimetres per minute.
Thus the time for a fiber to fall 2 metres is:

• 3 micrometre fiber: 13 minutes
• 0.5 micrometre: 8 hours

References

Further reading

• AIRBORNE ASBESTOS CONCENTRATIONS IN BUILDINGS G. J. BURDETT and S. A. M. T. JAFFREY Occupational Medicine and Hygiene Laboratory, Health and Safety Executive, 403 Edgeware Road, London NW2 6LN, U.K
  
• G.Burdett and G.Revell (2006), "Summary Report On Additional Work Carried Out On The Monitoring Of Chrysotile Containing Textured Decorative Coatings" UK HSL/2006/19
  
• G.Burdett (2007), "Investigation of the chrysotile fibres in an asbestos cement sample" UK HSL/2007/11