Recently there have been several reports on PFC emissions (CF4 + C2F6) during normal operation. Laboratory recordings of the so-called critical current density for the initiation of anode effect are useful to understand this phenomenon. The current density then increases until the alumina concentration at the anode surface approaches zero, whereby the current abruptly drops back to a much lower value, and PFC is being evolved. In an aluminum cell operating at low alumina concentration, alumina depletion may occur at one or more anodes, whereby PFC emission is initiated by this mechanism. Due to the low current density the local ohmic drop decreases, allowing the higher anode polarization needed for PFC formation. The remaining anodes evolving CO2, may sustain normal electrolysis close to normal cell voltage. PFC emission at close to normal cell voltage has also been observed in electrowinning of neodymium in a fluoride melt.