stolen from wikipedia for the 'flux' reference :Y:
Due to the symmetry of a toroid, little magnetic flux escapes from the core (leakage flux). Thus a toroidal inductor/transformer, radiates less electromagnetic interference (EMI) to adjacent circuits and is an ideal choice for highly concentrated environments.[3] Manufacturers have adopted toroidal coils in recent years to comply with increasingly strict international standards limiting the amount of electromagnetic field consumer electronics can produce.
Due to the symmetry of a toroid, little magnetic flux escapes from the core (leakage flux). Thus a toroidal inductor/transformer, radiates less electromagnetic interference (EMI) to adjacent circuits and is an ideal choice for highly concentrated environments.[3] Manufacturers have adopted toroidal coils in recent years to comply with increasingly strict international standards limiting the amount of electromagnetic field consumer electronics can produce.