According to the principles of contemporary quantum electrodynamics, the existence of a charged particle field of any kind results in a vacuum polarization contribution to the Lamb-Retherford line shift of an amount proportional to the square of the charge and inversely proportional to the square of the mass of the field particle. On the basis of the present agreement between theory and experiment with respect to the line shift in hydrogen, one may conclude that no singly charged particles of spin- 1/2 with mass less than four electron masses, other than the electron and positron, can exist without spoiling this agreement. Similar reasoning argues against the existence of singly charged particles of spin-0 with mass less than twice the electron mass. For doubly charged particles these limits are quadrupled. The assumptions involved in these conclusions as well as some experimental evidence are briefly discussed.