Depends on the type of impurity. What kind of starting material are you using?

I am assuming this is a research question and not a homework question (I've seen a lot of homework questions submitted by students from K L University).

hunins and rna can look like dna if poorly purified away and lead to lower pcr yield due to low amount of statring material. Protein can interfere with magnesium concentartion as well as enzyme binding leading to failed or low pcr yield.

Many recombinant modern polymerases can amplify in the presence of pcr inhibitors or the purity of the dna can be improved using proteinase K digestion and phenol chloroform purification. Column purification of the dna will help dna purity and in exceptional cases PhiX enzymes and whole genome amplification folloed by pcr can work as phi is much less fussy about amplification than normal polymerases