Asbestos · Mandatory · Construction
Air Monitoring Record — Post-Removal
The Air Monitoring Record documents fibre concentration levels measured during and after asbestos removal works. It must be carried out by an independent UKAS-accredited analyst to verify that airborne asbestos fibre levels are below the clearance indicator of 0.01 fibres/ml. These records form a critical part of the four-stage clearance process before an area is reoccupied.
Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. CAR 2012 remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.
| When required | Air monitoring is mandatory during all licensed asbestos removal work (CAR 2012 Regulation 20). It is also required during the 4-stage clearance process — specifically Stage 3 (the clearance air test) — and may be carried out as reassurance monitoring during NNLW or non-licensed work where the effectiveness of controls needs to be verified. |
| Control limit | 0.1 fibres per cm3 measured as a 4-hour time-weighted average (TWA). For clearance air testing (Stage 3), the clearance indicator level is 0.01 fibres per cm3 — one-tenth of the control limit. This lower threshold must be achieved before the area can be cleared for reoccupation. |
| UKAS requirement | CAR 2012 Regulation 20 requires air monitoring during licensed work to be carried out by an analyst accredited by the United Kingdom Accreditation Service (UKAS) to ISO 17025 for asbestos fibre counting by phase contrast microscopy (PCM). Non-UKAS analysts cannot carry out statutory air monitoring for licensed work. |
| Independence | HSG248 (Asbestos: The Analysts' Guide, Edition 2) requires that the analyst carrying out air monitoring and clearance testing must be independent of the licensed asbestos removal contractor (LARC). The analyst must not be employed by, sub-contracted by, or financially dependent on the LARC. This independence requirement exists to prevent conflicts of interest in clearance decisions. |
| Records | Air monitoring records must be retained for 40 years from the date of the last entry. This reflects the long latency period of asbestos-related diseases and ensures that exposure records are available for future medical investigations, compensation claims, and regulatory enquiries. |
1. Air Monitoring During Licensed Removal
Air monitoring during licensed asbestos removal serves two distinct purposes. First, it provides ongoing verification that the controls specified in the plan of work — enclosure integrity, negative pressure, wet suppression, RPE — are effectively preventing fibre release beyond the enclosure boundary. This is achieved through background monitoring at the enclosure perimeter and at other strategic locations around the site, particularly near occupied areas or air intakes. Second, it provides the data needed for the Stage 3 clearance air test, which determines whether the area inside the enclosure has been adequately decontaminated and is safe for reoccupation.
The monitoring must be carried out by an analyst holding UKAS accreditation to ISO 17025 for asbestos fibre counting by phase contrast microscopy (PCM). UKAS accreditation is a statutory requirement under CAR 2012 Regulation 20 for air monitoring during licensed work. The analyst must be independent of the LARC — this means they must not be employed by, sub-contracted to, or financially dependent on the removal contractor. The independence requirement is set out in HSG248 (The Analysts' Guide, Edition 2) and exists to prevent the conflict of interest that would arise if the LARC were responsible for both carrying out the removal and certifying that the area is safe for reoccupation.
During the removal itself, the analyst will typically carry out personal monitoring (sampling the air within the breathing zone of operatives inside the enclosure) and static background monitoring (sampling the air at the enclosure boundary and at locations around the site). Personal monitoring verifies that operatives' exposure does not exceed the control limit and that the specified RPE provides adequate protection. Background monitoring verifies that fibre release is not occurring beyond the enclosure. If background monitoring detects elevated fibre levels, the LARC must stop work and investigate the cause — typically a breach in the enclosure or inadequate NPU performance.
The analyst must be independent of the LARC
The independence of the analyst is a fundamental safeguard in the asbestos removal process. If the LARC appoints and pays the analyst, there is an inherent conflict of interest: the analyst's commercial relationship with the LARC could influence their judgement on whether a clearance air test has passed or failed. HSG248 requires the analyst to be appointed by, and report to, a party other than the LARC — typically the client, the Principal Contractor, or an independent project manager. The analyst's report must be issued directly to the appointing party, not routed through the LARC. On construction projects, the Principal Contractor should appoint the analyst independently and should verify their UKAS accreditation before appointment.
2. The Stage 3 Clearance Air Test
The Stage 3 clearance air test is the critical analytical step in the 4-stage clearance process for licensed asbestos removal. It can only be carried out after Stage 2 (visual inspection) has been passed — that is, after the analyst has confirmed that all visible ACM debris, dust, and contamination have been removed from the enclosure. The Stage 3 test involves deliberate disturbance of surfaces within the enclosure (using leaf blowers, brushing, or other mechanical agitation) followed by air sampling to determine whether residual asbestos fibres are released. The test simulates the conditions that would exist if the enclosure were dismantled and normal activities resumed in the area.
| Element | Detail |
|---|---|
| Disturbance | Before air sampling begins, the analyst must carry out a systematic disturbance of all surfaces within the enclosure. This typically involves using a leaf blower directed at walls, ceilings, floors, and any remaining fixtures. The purpose is to dislodge any residual asbestos fibres that may have settled on surfaces after the final clean. Without adequate disturbance, the air sample may not be representative of the conditions that would exist during normal use of the area. The disturbance must be carried out methodically and must cover all surfaces within the enclosure. |
| PCM analysis | Air samples are collected using membrane filter samplers positioned within the enclosure at locations specified in HSG248. The samples are analysed by phase contrast microscopy (PCM), which counts the number of fibres meeting the WHO (World Health Organisation) fibre counting criteria on each filter. The fibre count is converted to a concentration expressed in fibres per cm3 of air. The analyst must be UKAS-accredited for PCM fibre counting to ISO 17025. |
| Clearance level | The clearance indicator level is 0.01 fibres per cm3 — one-tenth of the workplace control limit of 0.1 fibres per cm3. All samples taken during the Stage 3 test must fall below this level for the test to pass. If any sample exceeds 0.01 fibres per cm3, the test has failed and the area cannot be cleared for reoccupation. The 0.01 level is set deliberately low to provide a margin of safety and to demonstrate that effective decontamination has been achieved. |
| Fail procedure | If the Stage 3 air test fails, the LARC must carry out additional cleaning within the enclosure, addressing the likely source of residual contamination. After additional cleaning, the Stage 2 visual inspection must be repeated, followed by a fresh Stage 3 air test. This cycle continues until the clearance level is achieved on all samples. Each failed attempt must be documented in the air monitoring record, including the fibre concentrations recorded, the cleaning actions taken, and the date and results of the subsequent re-test. |
| Time-stamped record | The air monitoring record must include the date and time of each sample, the sample location (identified on a site plan), the sampling duration, the flow rate, the fibre count, the calculated concentration, and the name of the analyst who read the sample. All entries must be time-stamped and sequential. The record must also note the disturbance method used, any observations made during the test, and the overall pass/fail determination. This record forms part of the permanent project documentation and must be retained for 40 years. |
3. Common Mistakes
Using a non-UKAS accredited analyst for licensed work
CAR 2012 Regulation 20 requires air monitoring during licensed asbestos work to be carried out by an analyst accredited by UKAS to ISO 17025 for asbestos fibre counting. Using a non-accredited analyst — regardless of their experience or qualifications — means the monitoring results have no legal standing. The clearance air test results will not be accepted by the HSE, and any Certificate for Reoccupation based on non-UKAS monitoring will be invalid. The client and Principal Contractor must verify the analyst's UKAS accreditation before appointment, not simply accept a claim of accreditation.
The LARC appointing and paying the analyst
HSG248 requires the analyst to be independent of the LARC. If the LARC appoints and pays the analyst, the independence requirement is compromised. The analyst should be appointed by the client, Principal Contractor, or an independent third party. The commercial relationship must be structured so that the analyst's fee is not contingent on the clearance test passing. In practice, this means the analyst's contract and payment should be managed entirely separately from the LARC's contract. The HSE will investigate the appointment arrangements during any enquiry and will treat LARC-appointed analysts as a significant compliance failure.
Proceeding to Stage 4 before Stage 3 results are confirmed
The 4-stage clearance process is sequential. Stage 4 (final inspection and enclosure dismantling) cannot begin until Stage 3 (clearance air test) has been completed and the results confirmed as below the clearance indicator level of 0.01 fibres per cm3. Proceeding to dismantle the enclosure before the Stage 3 results are available — even where the analyst expects the results to pass — is a breach of the clearance procedure. If the results subsequently show a failure, the enclosure has already been dismantled and the contaminated area may have been accessed by unprotected personnel. Stage 3 results must be formally confirmed in writing before Stage 4 begins.
Not retaining air monitoring records for 40 years
Asbestos-related diseases have latency periods of 15 to 60 years. Air monitoring records may be needed decades after the work was carried out to support medical investigations, compensation claims, or regulatory enquiries. CAR 2012 requires these records to be retained for 40 years from the date of the last entry. In practice, this means the client and the LARC must have robust arrangements for long-term record storage. Records stored only on a single computer or in a single office location are at risk of loss. The records should be stored in multiple formats and locations, and responsibility for their retention should be clearly assigned.
Carrying out Stage 3 without adequate disturbance of surfaces
The Stage 3 clearance air test is only valid if adequate disturbance of surfaces has been carried out before sampling. The purpose of the disturbance is to simulate normal activity in the area and to dislodge any residual fibres that have settled on surfaces. If the disturbance is perfunctory — a brief sweep rather than systematic agitation of all surfaces using a leaf blower — the air sample may not be representative. A test that passes without adequate disturbance provides false assurance. The analyst must document the disturbance method used, and the HSE will examine whether the disturbance was adequate if the clearance is subsequently challenged.
5. Frequently Asked Questions
What is the difference between background monitoring and Stage 3 clearance air testing?▾
Background monitoring is carried out during the removal work itself, at the perimeter of the enclosure and at strategic locations around the site (near occupied areas, air intakes, site boundaries). Its purpose is to verify that fibre release is not occurring beyond the enclosure — that is, that the enclosure integrity and NPU performance are maintaining containment. If background monitoring detects elevated fibre levels, work must stop and the cause must be investigated. Stage 3 clearance air testing is carried out after the removal and final clean are complete. It involves deliberate disturbance of surfaces inside the enclosure followed by air sampling to confirm that residual fibre concentrations are below the clearance indicator level of 0.01 fibres per cm3. The two types of monitoring serve different purposes and occur at different stages of the process.
How long does a Stage 3 clearance air test take?▾
The sampling duration for a Stage 3 clearance air test depends on the volume of the enclosure and the number of sampling positions required by HSG248. For a typical single-room enclosure, the minimum sampling period is usually 4 hours to achieve the analytical sensitivity needed to detect fibres at the clearance indicator level of 0.01 fibres per cm3. However, the total elapsed time from disturbance to confirmed results is typically 24 to 48 hours, because the membrane filters must be transported to a laboratory for PCM analysis (unless the analyst has on-site microscopy capability). Some UKAS-accredited analysts offer on-site analysis, which can reduce the turnaround to the same working day. The programme should allow for the possibility of a failed test requiring additional cleaning and re-testing.
What happens if the Stage 3 clearance air test fails?▾
If any sample exceeds the clearance indicator level of 0.01 fibres per cm3, the Stage 3 test has failed. The LARC must carry out additional cleaning within the enclosure, targeting the likely source of residual contamination. This may involve further wet wiping of all surfaces, HEPA vacuuming, and inspection for debris or dust deposits that were missed during the initial clean. After additional cleaning, the Stage 2 visual inspection must be repeated — the analyst must re-inspect the enclosure and confirm that it is visually free of ACM debris and dust. Only then can a fresh Stage 3 air test be carried out. Each cycle of cleaning, visual inspection, and re-test must be documented in the air monitoring record, including the fibre concentrations from the failed test, the cleaning actions taken, and the results of the subsequent re-test. There is no limit on the number of attempts, but persistent failures indicate a fundamental problem with the decontamination that must be investigated.
Does the UKAS-accredited analyst need to be on site throughout the removal?▾
The analyst does not need to be on site continuously throughout the entire removal. However, they must attend site at specific points: to set up and collect background monitoring samples at the start of each working day, to carry out the Stage 2 visual inspection, to carry out the Stage 3 disturbance and air sampling, and to carry out the Stage 4 final inspection. Between these visits, the LARC supervisor is responsible for maintaining controls and monitoring enclosure integrity. The analyst must be available to attend site at short notice if background monitoring results indicate a problem. The frequency and timing of the analyst's site visits should be agreed in the plan of work and should be sufficient to provide meaningful oversight of the removal process.
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