1) It is noted that as temperature increases the effective impedance of the ferrite material decreases.

(formula)

2) DC Bias and 50 or 60 Hz alternating current also results in a lower impedance of the ferrite wire. The impedance change in this case is most pronounced at frequencies where reactive components are high. This can be compensated by using a small air gap in the split cores. Another way of reducing this effect is to wind the ferrite core with 2 windings connected in series opposing each other, also know as common mode winding.

Effect of DC bias for 43 Material
(Shield bead FB43-801)