The efficiency and pressure drop of filters made from synthetic fibres carrying electrostatic charges and filters made from uncharged glass fibres were measured repeatedly for more than 19 weeks in operating HVAC systems. Results showed efficiency reductions as large as 40% for the filters made from charged fibres whereas the efficiency of filters made from uncharged fibres changed little. Clean filters were sent to a testing laboratory for efficiency measurements according to ASHRAE Standard 52.2-1999. When the filters experienced accelerated dust loading according to the ASHRAE procedure, the efficiency of both the glass and synthetic fibre filters increased. Efficiency reductions like those observed for the charged filters during actual use were not replicated by this standard test.