Method for Determination of Airborne Asbestos Fibre Concentration in Work Environment by Light Microscopy (Membrane Filter Method)
2006 Edition

Per IS 11450, recommended sampling durations and schemes include:

• Single sample duration is calculated using:

  [ t = \frac{4 \times A \times L}{C_{exp} \times r} ]

  where (A) = effective filter area (mm²), (L) = filter loading (fibres per graticule area), (C_{exp}) = expected fibre concentration (fibres/cm³), and (r) = flow rate (cm³/min).

• Table 1 provides duration guidelines based on expected fibre concentrations, recommending minimum, optimal, and maximum sampling times.

• Sampling schemes include full-shift consecutive (Type A, B), partial-shift consecutive (Type C, D), random samples (Type E), and systematic samples (Type F), with durations typically exceeding one hour.

• Short sampling periods below 10 minutes are generally discouraged to maintain accuracy.

Preparation involves collecting airborne fibres on membrane filters made of mixed cellulose esters or cellulose nitrate, typically 25 mm diameter with 0.8–1.2 µm pores.

For microscopic analysis:

1. Place the filter slide on the microscope stage.
2. Open the illuminator and condenser diaphragms without inserting phase annuli.
3. Position the condenser close to the slide and focus using a 10x objective.
4. Adjust illumination and center the light source using the Bertrand lens or by removing the eyepiece to observe the bulb filament.
5. Switch to the 40x objective, close the field diaphragm, refocus, and re-center.
6. Insert the appropriate phase annulus and center it.
7. Revert to normal viewing mode to perform fibre counting.

Strict adherence to this protocol minimizes variability and ensures accurate fibre detection.

IS 11450 specifies the microscope should have:

• Kohler or equivalent illumination with adjustable intensity.
• An Abbe or achromatic phase contrast condenser with centering and focusing mechanisms.
• A mechanical stage with slide clamps and x-y movement.
• Objectives including 10x and 40x parfocal phase contrast achromatic lenses; the 40x objective must have NA = 0.65 and phase ring absorption between 65% and 85%.
• Binocular eyepieces providing total magnification between 400x and 600x, with at least one eyepiece accommodating a graticule.
• A Walton-Beckett graticule with a circle diameter of 100 ± 2 µm at 40x magnification.

Calibration involves using a stage micrometer (with 2 µm or 10 µm divisions) to measure the eyepiece graticule scale accurately. The process includes focusing on the micrometer, aligning the graticule, counting divisions, estimating partial divisions, and calculating the actual graticule size. Recalibration is necessary whenever microscope components or interpupillary distance change.

Fibres are collected on membrane filters and examined under phase contrast microscopy. Counting criteria include:

• Selecting random, non-overlapping fields free from grid lines.
• Rejecting fields with more than one-eighth area covered by fibre clusters.
• Counting at least 100 fibres over a minimum of 20 fields.
• Considering fibres with diameter less than 3 µm, length greater than 5 µm, and length-to-diameter ratio exceeding 3:1.
• Excluding fibres touching particles larger than 3 µm.

Counting rules specify that fibres fully within the field count as one, while fibres partially inside count as half. Bundles are counted as individual fibres if distinguishable; otherwise, they count as one fibre if meeting the criteria. This approach prevents undercounting and ensures accurate exposure assessment.

To ensure reliability, IS 11450 recommends:

• Maintaining a controlled laboratory environment to reduce observer fatigue and systematic errors.
• Performing regular checks of microscope and observer performance using the HSE/NPL Test Slide Mark II, targeting detection at block 5.
• Engaging in inter-laboratory slide exchanges to validate and harmonize results.
• Assessing intra- and inter-observer variability statistically to standardize counting accuracy.
• Applying strict, reproducible sampling and analytical procedures as detailed in Annex K to minimize errors.

Combining these measures ensures consistent, accurate asbestos fibre quantification compliant with the standard.