This study investigated the effect of upstream flow conditions on the efficiency of a reversible, two-layer filter composed of one layer of coarse and one layer of fine fibres. Efficiency was higher when the leading layer of the filter contained fine fibres, but the drop was the same regardless of which fibres led. Efficiency was higher and the fractional efficiency curve was shallower when the upstream velocity profile was non-uniform and very turbulent. Three phenomena were identified that account for these observations: (1) non-uniformity of the upstream velocity profile, (2) mean velocities tangential to the filter's surface and (3) fluctuating components of velocity, inherent to a turbulent airflow. Air entering a filter becomes generally perpendicular to the filter face as fibres dissipate non-uniform and tangential velocity components; this dissipation is greatest in the leading layer of the filter. Fine fibres have a higher efficiency that coarse fibres, so efficiency was higher when the region of greatest dissipation contained fine fibres. Innovative housing designs, based on these observations, could be made to promote upstream airflow conditions that maximize filter performance.