The empirical analysis of the electron density evolution during ECRH is given. It is shown that when recycling from the walls is low and the neutral gas injection rate is kept constant, during ECW injection the line density drops mainly as a consequence of a net decrease in the total particle number, whereas profile reshaping occurs only for some conditions. It is also shown that the phenomenon is of resonant nature but not a by-product of plasma heating. By considering the fast time scale on which the particle balance changes and by comparing the profile density evolution during ECRH to that following a step-wise reduction of the gas injection rate, it is inferred that particle confinement is degraded in the presence of EC waves. The consistency of this conclusion with all the experimental evidence is positively verified.