The efficiency of contamination of the core plasma arising from different impurity generation mechanisms at the edge has been examined using the two-dimensional impurity transport code LIM. The specific example of the JET belt shape limiters, made of carbon or beryllium, has been analysed. If self-sputtering is neglected, then primary generation mechanisms which produce slow neutrals such as sublimation including carbon Radiation Enhanced Sublimation (RES), are found to be relatively benign compared with D+ physical sputtering when compared on the basis of equal influxes. For the same influx of neutral impurities, the core contamination level can be smaller by up to an order of magnitude. When self-sputtering is included in the analysis, the differences in the overall efficiency of core contamination associated with the different primary generation mechanisms are much reduced. It is thus found that with regard to the assessment of the permissible range of operating edge plasma conditions, specifically the edge temperature, the critical process to take into consideration is self-sputtering.