The objective of the present study is to investigate the relationship between ultrafine particle concentrations and removal efficiencies for an electrostatic fibrous filter in a laboratory environment. Electrostatic fibrous filters capture particles efficiently, with a low pressure drop. Therefore they have applications in building ventilation systems. The relationship between particle removal efficiency and particle concentration has not been widely investigated in ventilation systems and indoor environments. In order to achieve the objective of this study, experiments were performed in a controlled laboratory environment using two different particle counters: a Scanning Mobility Particle Sizer and a NanoTracer. Particles were generated at different concentrations by burning a pure wax candle in a test room. The set-up consisted of a test room, a fan a duct and the particle filter. The results show that the efficiency of the electrostatic fibrous filter increased with increasing exposure levels. The filter efficiency varies from 45% to 80% depending on the particle concentrations and particle sizes including ultrafine particles. The results are consistent with a mechanism in which the particles deposit on the fibers and form chain-like agglomerates known as dendrites. The dendrites themselves contribute in capturing the other particles. Increasing exposure will result in increasing the number of the dendrites because of the static charging and consequently increasing the efficiency. Static electrical charging of dendrites will spread out the branches, increasing capture.