Air monitoring and clearance

Air monitoring tells you whether controls are working, whether workers have the right respirators, whether fibers 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 fibers 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

OSHA requires employers to monitor asbestos exposure: initial monitoring at the start of each job unless a valid negative exposure assessment exists, daily monitoring for Class I and II work in regulated areas unless the employer has an NEA and other conditions are met, and periodic monitoring for other operations representative of each worker's exposure. Employees must be notified of results within 5 working days, in writing or by posting. OSHA does not set clearance criteria; EPA's AHERA rule does for schools, and most project specifications for other buildings adopt similar criteria.

Phase contrast microscopy (PCM)

Most asbestos air samples are analyzed by phase contrast microscopy using NIOSH Method 7400. Air is drawn by a battery-powered pump through a 25-millimeter 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 fibers in a set of graticule fields according to counting rules.

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

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

Calculating a PCM result

The laboratory converts the fiber count into a concentration. The fiber density on the filter (fibers per square millimeter) 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 fibers collected, and dividing by the air volume gives the concentration.

Here \(C\) is the concentration in fibers per cubic centimeter, \(E\) the fiber density in fibers per square millimeter, \(A_c\) the effective collection area of the filter (about 385 mm² for a 25-mm filter), \(V\) the air volume in liters, \(F\) and \(n_f\) the fiber 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 liters, the method cannot reliably measure concentrations as low as 0.01 f/cc; with 2,400 liters, 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 fibers far thinner than PCM can detect, and it identifies asbestos by its crystal structure and chemical composition. TEM can therefore tell asbestos fibers from other fibers 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 fibers counted by PCM are asbestos, and the AHERA TEM method, which counts asbestos structures per square millimeter of filter and is used for clearance in schools.

TEM is useful when PCM results are high and it is suspected that non-asbestos fibers, 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 fibers.

• Aggressive air sampling. Air is stirred with fans and leaf blowers directed at surfaces before and during sampling so that any settled fibers become airborne. AHERA requires aggressive sampling for clearance.

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

AHERA sets clearance criteria for school abatement projects. For most projects, five samples inside the work area are analyzed by TEM; the area passes if the average is no more than 70 structures per square millimeter of filter, or, using five inside and five outside samples plus blanks, if a statistical test shows inside concentrations are not significantly higher than outside. For small projects, up to 160 square feet or 260 linear feet, PCM may be used and each sample must be 0.01 f/cc or less. Outside schools, project specifications usually adopt AHERA-style criteria, and some states set their own.

Interpreting results as a supervisor

Supervisors do not need to analyze 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 fibers?

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 analyzed. 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 have completed the NIOSH 582 course or an equivalent, and laboratories should participate in proficiency testing such as the AIHA programs; TEM laboratories doing AHERA analysis must be NVLAP accredited. Project specifications often require on-site analysis 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. OSHA requires written notification of personal monitoring results within 5 working days, by individual notice or by posting in an accessible place, and employees and their representatives have the right to observe monitoring. Explain what results mean in plain language. 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.