Filters produced from uncharged glass fibre and also from electrostatically charged polyolefin fibres were installed in adjacent, virtually identical heating, ventilated, and air-conditioning ( HVAC ) systems which were operated continuously for a period of more than 19 weeks, during which time the filtration efficiency and pressure drop were measured. The filters made from charged fibres exhibited efficiency reductions as large as 40%, whilst those of uncharged fibre showed little change. Efficiency measurements of clean filters were also made, using accelerated dust loading according to ASHRAE standard 52.2-1999. Both filter types exhibited increasing efficiency, which was contrary to their behaviour in service. The observed differences between the results of the two tests may have being caused by differences in loading and in the compositions of the particles which were used. 10 refs.