An internal paper, sent to pit managers in the five-pit Selby coalfield in North Yorkshire, reveals that British Coal is anxious about 'discrepancies' between the figures recorded by mine employees and those found in spot checks.
The memo, dated 28 August, was issued two months before the Independent disclosed that dust detecting equipment at Whitemoor Colliery, part of the Selby complex, had been sabotaged to give low readings so that production would not be interrupted.
British Coal implied at the time that the Whitemoor case was an isolated incident, but the memo, from the assistant director of the Selby area to pit managers, betrays a wider concern about misuse of the safety equipment.
It concerns dust levels in 'drivages' - new tunnels being cut to exploit virgin deposits. The document also says that, while routine sampling found an average of three milligrams per cubic metre, checks by British Coal's safety auditors found an average figure of 6mg/m3 .