An employer or self-employed person must ensure that the exposure of a person to asbestos is either prevented or, where this is not reasonably practicable, adequately controlled: Provided that the control of exposure is regarded as adequate if the measured airborne concentration of regulated fibres is-
(a) at or below the OEL for asbestos; or
(b) above the OEL for asbestos, but the reason has been identified and action is taken, as soon as is reasonably practicable, to lower airborne concentrations to a level as low as reasonably practicable below the OEL for asbestos.