A report by the Health and Safety Executive's Mines Inspectorate also concludes that there is no satisfactory explanation for certain critical discrepancies in official readings.
Relatively high dust concentrations were found in spot checks by British Coal's own research establishment and lower readings in routine samples at the collieries.
Union officials believe that incidents of malpractice in the Selby coalfield in North Yorkshire, revealed in the Independent last October and investigated by the inspectorate in the report, are probably the 'tip of the iceberg'. The Independent reported cases of pit personnel placing two filters, rather than one, in monitoring devices so lower concentrations were recorded.
British Coal said the report found no evidence to support allegations of widespread interference in dust- sampling devices. But Peter McNestry, general secretary of the pit supervisors' union Nacods, believes that some managers tamper with equipment. Consistently high dust readings can halt output and affect judgements about the viability of the mine.