The control limit refers to the concentration of asbestos fibres in any localised atmosphere, measured and averaged over a continuous period of four hours, in accordance with the 1997 World Health Organisation’s recommended method. At the moment, the control limit is 0.1 asbestos fibres per cubic centimetre of air (0.1 f/cm3). The control limit is not a ‘safe’ level and work activities involving asbestos should be designed to be as far below the control limit as possible. HSE has progressively tightened the control limit since 1987.
Work involving some lower risk asbestos-containing materials (eg asbestos cement products and textured decorative coatings, among others) can be carried out if a risk assessment demonstrates that the control limit will not be exceeded and that any exposure to asbestos is expected to be ‘sporadic and of low intensity’. The control limit for this type of non-licensed work is 0.6 asbestos fibres per cubic centimetre of air (0.6 f/cm3), measured over a ten-minute period. Any work which is likely to result in exposures at or above this level cannot be considered to be sporadic and of low intensity and should therefore only be carried out by a licensed contractor.