With this method, fibres in the air can be counted with a phase contrast optical microscope by applying the NIOSH 7400 method. It allows for the measurement of different fibres such as asbestos and any other fibres for which the refractive index is compatible with the mounting solution (e.g., artificial vitreous mineral fibres, natural mineral fibres, etc.). The sampling device used in this method is a filtering membrane inserted into a cartridge. The method has been updated to adapt it to the new regulations that come into force on October 28, 2022, which entail a change in the definition of respirable asbestos fibre and an adjustment of the reference values.