Air testing
The four HSG248 stages that must pass before an enclosure is released.
The four-stage clearance is the only mechanism by which a licensed asbestos enclosure can be handed back to normal use. Skipping or shortening a stage is one of the most common findings in HSE prosecutions.
Overview
Why this exists.
The four stages are sequential. Each is a distinct decision point with its own pass criteria. Any failure resets the process — you do not skip forward.
The analyst issuing the certificate of reoccupation is regulated by ISO/IEC 17025. Their signature carries the same legal weight as the licensed removal contractor's — they are independent and must be free to fail the site.
The explainer
4-stage clearance
- Stage 1
Preliminary check
Visual and documentary review — permits, plan of work, waste routes, enclosure integrity.
~ 30 min
- Stage 2
Thorough visual
Full internal inspection under strong light — no visible debris, dust or ACM residues.
~ 1–2 hr
- Stage 3
Air monitoring
Air sampled under agitation. Result must be below 0.01 f/ml. If not, re-clean and retest.
~ 2 hr
- Stage 4
Reoccupation certificate
Enclosure dismantled under controlled conditions. Certificate of Reoccupation issued to the client.
~ 1 hr
How to read this
- Read left to right. Each block is a discrete phase with a documented outcome.
- The final Certificate of Reoccupation is issued by an independent UKAS 17025 analyst — never by the removal contractor.
Key takeaways
Independence
The analyst cannot be an employee or subcontractor of the removal contractor — this is a regulatory boundary.
Air clearance limit
0.01 f/ml is the clearance indicator, sampled inside the enclosure at working conditions.
Certificate retained
The Certificate of Reoccupation must be retained on file for 40 years.
FAQs
Common follow-up questions.
Can the removal contractor also do the clearance?
No. HSG248 requires the analyst issuing the certificate to be independent of the removal work. Using the same firm for both undermines the entire audit trail.
What if stage 3 fails?
The enclosure is re-cleaned and the whole air test is repeated. The certificate cannot be issued until a clean stage 3 result is obtained.
Explore further
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HSG248 (HSG248)
The 2021 second edition of HSG248 — air sampling, four-stage clearance, PCM and TEM analysis, enclosure inspection and clearance certification.
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