HSE guidance · HSG248
Asbestos: The Analysts' Guide (HSG248)
A four-stage clearance signed off under HSG248 is what turns a stripped enclosure back into a habitable room. Every stage — preliminary inspection, thorough visual, air test, final decontamination — is prescribed and auditable.
Health and Safety Executive · Second edition, 2021 · Status: Current
Scope
What it covers.
Covers air sampling design and interpretation, phase-contrast microscopy (PCM), transmission electron microscopy (TEM), fibre counting rules, personal exposure monitoring, background reassurance sampling, licensed-removal enclosure inspection, four-stage clearance and reoccupation certification.
Who it applies to
- UKAS 17025 accredited testing laboratories and analysts
- Licensed asbestos removal contractors coordinating with analysts
- Dutyholders commissioning air testing before reoccupation
- Employers arranging personal exposure monitoring
Key provisions
The duties, provision by provision.
Defines the four stages of enclosure clearance
Stage 1 preliminary check of site condition and paperwork. Stage 2 thorough visual inspection inside the enclosure. Stage 3 air monitoring inside the enclosure with disturbance. Stage 4 final assessment post-dismantling. All four must pass before reoccupation.
Prescribes the design of every air sample type
Personal exposure, static background, reassurance, leak, and clearance samples — each with defined flow rate, duration, filter type and reporting units. Samples are analysed to WHO fibre-counting rules under PCM.
Requires analytical independence from the removal contractor
The analyst must be a separate legal entity from the removal contractor to avoid conflict of interest — a client protection that HSE audits.
Sets the trigger for TEM confirmation
Where the PCM count is inconclusive or exceptionally high, TEM is used to distinguish asbestos from other fibres. TEM is not a routine substitute for PCM.
In practice
How we apply it.
- Every four-stage clearance we conduct is independent of the removal contractor; we never work as an in-house analyst for a removals company.
- Personal exposure monitoring is offered as a standalone service for construction clients running non-licensed work.
- Reassurance sampling after suspected disturbance can be same-day for London and Home Counties.
FAQs
Common follow-up questions.
What changed in the 2021 second edition?
The most significant changes were a strengthened emphasis on analyst independence, updated guidance on TEM triggers, revised reassurance sampling scenarios and updated interpretation of the WHO fibre-counting rules for low counts.
Can the removal contractor's own analyst sign clearance?
No — HSG248 requires analytical independence. The analyst must be a separate legal entity, and the certificate cannot be relied on if that separation is absent.
What's the clearance limit for a stage 3 test?
Below 0.01 fibres/cm³, calculated as a rolling average over the sample period. Above this the enclosure fails and must be re-cleaned before retesting.
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