Air monitoring and clearance

Air monitoring tells you whether controls are working, whether workers have the right respirators, whether fibres are escaping the work area, and whether an area is clean enough to be reoccupied. This lesson explains the purposes and types of asbestos air sampling, how phase contrast microscopy counts fibres and what its results mean, why transmission electron microscopy is sometimes needed, and how clearance testing works. It is written to help supervisors and workers understand monitoring results, not to train air sampling technicians.

Why monitor

Requirements for monitoring vary. Ontario's regulation requires clearance air testing after Type 3 removal of friable material inside a building and specifies how it is done, and many project specifications also require daily sampling outside the enclosure with stop-work triggers. BC requires air monitoring as part of the exposure control plan where needed to assess exposure and the effectiveness of controls. Results must be shared: in Ontario, clearance results must be posted at the workplace within 24 hours and given to the joint health and safety committee or representative.

Phase contrast microscopy (PCM)

Most asbestos air samples are analysed by phase contrast microscopy using NIOSH Method 7400. Air is drawn by a battery-powered pump through a 25-millimetre mixed cellulose ester filter held in a cassette with a conductive extension cowl. In the laboratory or a site lab, a wedge of the filter is made transparent and examined under a phase contrast microscope at about 400 times magnification. The analyst counts fibres in a set of graticule fields according to counting rules.

Under the A counting rules of NIOSH 7400, a fibre is counted if it is longer than 5 micrometres and has a length-to-width ratio of at least 3 to 1. A fibre lying entirely inside the graticule counts as one; a fibre with one end inside counts as a half; fibres crossing the graticule entirely, or with both ends outside, do not count. The analyst counts until 100 fibres have been counted or 100 fields examined, with a minimum of 20 fields.

PCM has an important limitation: it counts all fibres that meet the size rules, not only asbestos. Gypsum, cellulose, fibreglass, mineral wool and other fibres are counted too. PCM also cannot see the thinnest asbestos fibres, below about 0.25 micrometres wide. PCM results are therefore an index of fibre exposure that works well for comparing against the limits, which were set using PCM, but they cannot prove whether fibres are asbestos.

Calculating a PCM result

The laboratory converts the fibre count into a concentration. The fibre density on the filter (fibres per square millimetre) is the count per field, corrected for the blank, divided by the area of one graticule field. Multiplying by the effective area of the filter gives the total fibres collected, and dividing by the air volume gives the concentration.

Here \(C\) is the concentration in fibres per cubic centimetre, \(E\) the fibre density in fibres per square millimetre, \(A_c\) the effective collection area of the filter (about 385 mm² for a 25-mm filter), \(V\) the air volume in litres, \(F\) and \(n_f\) the fibre count and number of fields on the sample, \(B\) and \(n_b\) the count and fields on the blank, and \(A_f\) the field area (about 0.00785 mm² for a standard graticule).

A_C = 385.0      # mm2, effective collection area of a 25 mm filter
A_F = 0.00785    # mm2, area of one Walton-Beckett graticule field

def pcm_concentration(count, fields, volume_l, blank_count=0.0, blank_fields=100):
    density = (count / fields - blank_count / blank_fields) / A_F   # f/mm2
    return density, density * A_C / (volume_l * 1000)

# Personal sample during wet removal: 2.0 L/min for 120 min
d, c = pcm_concentration(count=48.5, fields=40, volume_l=2.0 * 120, blank_count=1, blank_fields=100)
print(f"personal sample: density {d:6.1f} f/mm2, concentration {c:.3f} f/cc")

# Clearance sample: 10 L/min for 240 min = 2,400 L
d, c = pcm_concentration(count=9.5, fields=100, volume_l=2400, blank_count=1, blank_fields=100)
print(f"clearance sample: density {d:5.1f} f/mm2, concentration {c:.4f} f/cc -> "
      f"{'pass' if c <= 0.01 else 'fail'} at 0.01 f/cc")

# Approximate limit of detection: about 7 f/mm2 by NIOSH 7400
lod_density = 7
for v in (240, 1200, 2400):
    print(f"volume {v:5d} L: limit of detection about {lod_density * A_C / (v * 1000):.4f} f/cc")

personal sample: density  153.2 f/mm2, concentration 0.246 f/cc
clearance sample: density  10.8 f/mm2, concentration 0.0017 f/cc -> pass at 0.01 f/cc
volume   240 L: limit of detection about 0.0112 f/cc
volume  1200 L: limit of detection about 0.0022 f/cc
volume  2400 L: limit of detection about 0.0011 f/cc

The output shows why clearance samples need large volumes. With only 240 litres, the method cannot reliably measure concentrations as low as 0.01 f/cc; with 2,400 litres, it can. Personal samples during work are smaller because filters overload when dust is heavy, and an overloaded filter cannot be counted.

Transmission electron microscopy (TEM)

Transmission electron microscopy uses an electron beam to see fibres far thinner than PCM can detect, and it identifies asbestos by its crystal structure and chemical composition. TEM can therefore tell asbestos fibres from other fibres and count structures that PCM misses. It is slower and more expensive than PCM. TEM methods include NIOSH 7402, which is used to determine what fraction of the fibres counted by PCM are asbestos, and the AHERA TEM method, which counts asbestos structures per square millimetre of filter and is used for clearance in schools.

TEM is useful when PCM results are high and it is suspected that non-asbestos fibres, such as gypsum or cellulose from demolition, are responsible, and when the strongest possible assurance of cleanliness is needed.

Clearance testing

Clearance testing is the final check before an enclosure is removed and the area is returned to use. It happens in a fixed sequence.

• Final cleaning. All visible debris is removed, and surfaces are HEPA vacuumed and wet wiped, often more than once.

• Visual inspection. A competent person or consultant inspects the area for any remaining debris or residue. Visible residue means the area fails before any air test.

• Drying and, in many specifications, lockdown. Surfaces are allowed to dry, and a lockdown encapsulant may be sprayed on surfaces from which asbestos was removed to bind any remaining fibres.

• Aggressive air sampling. Air is stirred with fans and leaf blowers directed at surfaces before and during sampling so that any settled fibres become airborne. Ontario's regulation requires forced air before and during PCM clearance sampling.

• Analysis and decision. Samples are analysed and compared with the clearance criteria. If the area fails, it is recleaned and retested.

Ontario's regulation sets the clearance criteria for Type 3 removal of friable material inside a building. Samples may be analysed by PCM (NIOSH 7400) or TEM (NIOSH 7402). For PCM, at least 2,400 litres of air must be drawn through each filter, and the area passes only if every sample is 0.01 f/cc or less; samples that fail by PCM may be re-analysed by TEM against the same limit. Alternatively, at least five samples inside and five outside the enclosure can be analysed by TEM, and the area passes if the inside average is statistically lower than, or not different from, the outside average. The number of samples depends on the enclosure size, from two for small enclosures to five for large ones, under Table 3 of the regulation.

Interpreting results as a supervisor

Supervisors do not need to analyse filters, but they should be able to read a monitoring report and act on it. Useful questions include:

• Was the sample personal or area, and what task was under way?

• What was the sampling time and volume, and was the result above the limit of detection?

• Is the result compared with the right limit: 8-hour limit, excursion limit, respirator MUC or clearance criterion?

• Do outside-enclosure results show any rise, which would suggest a leak?

• Were PCM results high enough that TEM analysis is needed to tell asbestos from other fibres?

A rising trend outside the enclosure, or personal results approaching the respirator MUC, should lead to action: stopping work, checking barriers and negative air, improving wetting, or upgrading respirators.

Quality in air sampling

Air sampling results are only as good as the way samples are collected and analysed. Sampling pumps must be calibrated with the sampling cassette in line before and after each sample, and samples whose flow changed by more than the method allows are invalid. Field blanks, cassettes taken to the site and handled like samples but with no air drawn through them, detect contamination of filters. Each sample needs a record of the location, worker or area, start and stop times, flow rate and task, and a chain of custody that follows the sample to the laboratory.

PCM analysts should be trained, for example through the NIOSH 582 course or an equivalent, and laboratories should take part in proficiency testing programs such as those run by the American Industrial Hygiene Association. Project specifications often require the consultant's analysts to be on site so that results are available the same day, which allows problems to be fixed during the work rather than discovered afterwards.

Sharing results with workers

Workers have a right to know the results of air monitoring that concerns them. Post or distribute personal and area results promptly, explain what they mean in plain language, and give copies to the joint health and safety committee or representative. Results that show controls are not working should lead to visible action, so workers can see that monitoring is used to protect them and not only to fill a file.