No "main" one. You see ten possible positions at which Cl can be substituted for H; mono-, di-, tri-, ... , decachlorobiphenyl are all included in the term "polychlorobiphenyl." There are three monochloros, ortho-, meta-, and para-, or 2-, 3-, 4-, doesn't matter which phenyl group, since they are equivalent. Dichloro? 2,3-, 2,4-, 2,5-, 2,6-, 2,2'-, 2,3'-, 2,4'-, 3,3'-, 3,4'-, 4,4'-, where unprimed indicates one phenyl group, and the prime indicates substitution on the second phenyl group. The rest of the patterns "are left to the reader as an exercise."
Environmental effects? Rats on PCBs (by gavage) lived on an average 6 mos. longer than control animals; there was an increased incidence of liver tumors observed, but it was not clear whether this was caused by the physical abuse associated with gavage feeding, or the activity of PCBs on rat metabolism.
There is a bioaccumulation observed in fatty tissues as you move up the foodchain.
