Quick answer: Magnesian limestone — the stone used to build York Minster, the Bar Walls and the majority of York's historic core — contains a measurable free crystalline silica fraction that generates RCS above the COSHH WEL of 0.1 mg/m³ during cutting, grinding and abrasive cleaning. H-Class extraction (H14 HEPA, 99.995%) is the legal minimum under COSHH 2002 for all high-energy operations on this material — M-Class is not adequate. This applies to all masonry work on York's historic building stock, not just the Minster.
York is a city where the question of dust extraction class is answered by the geology. The stone that defines the city — magnesian limestone, quarried from the Permian belt that runs through Yorkshire, used to build York Minster, the Bar Walls, the medieval gatehouses and the majority of the historic core — is not the same material as the generic 'limestone' that appears in general COSHH guidance. Its crystalline silica content and the particle size distribution it generates on cutting, grinding and abrasive cleaning make it one of the higher-risk masonry materials in the UK built environment.
For heritage contractors, conservation specialists, stonemasons and maintenance teams working on York's historic building stock, this is not a theoretical compliance point. It is the practical reality that shapes the COSHH assessment, the extraction specification, and the RAMS for every project that involves working the stone.
Magnesian limestone from the Yorkshire Permian belt consistently generates RCS above the levels that M-Class extraction can reduce to below the COSHH WEL of 0.1 mg/m³ on high-energy cutting and grinding operations. Every cutting, shaping, grinding and abrasive cleaning operation on this stone generates RCS at levels that require H-Class extraction as the legal minimum under COSHH 2002. COSHH Regulations 2002 and CDM 2015 both apply. Planning conditions and Historic England requirements for method statements are increasingly specific about extraction standards — a COSHH assessment that correctly identifies H-Class for magnesian limestone is more defensible and more likely to be accepted by the conservation authority than a generic M-Class approach. H-Class extraction on magnesian limestone means H-Class at every dust-generating operation, with on-tool extraction at source as the primary engineering control and FFP3 RPE as a final control layer. For contractors on York heritage frameworks, this documentation is reviewed as part of contract performance management.
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