Question

what is the effect of decreases of pressure on the boiling point

Answer 1

At higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. The boiling point increases with increased pressure up to the critical point, where the gas and liquid properties become identical.

Answer 2

on decreasing the pressure the boiling point also decreases...
and on increasing boiling point also increases..

i.e, P is directly proportional to B.P.