The efficiency of fibrous filter by combined mechanism is given by(where ηimp,ηint,ηdif are the efficiencies by impaction, interception and diffusion respectively)

• 1 - ηimpηintηdif
• 1 - (1 - ηimp)(1 - ηint)( 1 - ηdif)
• 1 - ( ηimpηint/ηdif)
• None of the above