Unlike ordinary filters, the initial high collection efficiency of the electret filter (E) might briefly fall as dust load increases. Here, the electrical enhancement factor of the electret fiber (lambda(E)) under dust-loaded conditions for uncharged and charged particles is obtained from published experimental results. It is found that the collection efficiency of the electret fiber (n) can be approximated as the sum of the electrical and mechanical collection efficiency. The predicted values of E are shown to agree well with reported experimental observations for both uncharged and charged particles. (c) 2005 Published by Elsevier Ltd.