One becomes pure by removing the impurity :)
I called the period blood filth not women, never the women.
Soma gave them purification,the Gandharva, sweet speech,Agni, perfect purity, therefore verily women are always pure. — Yajnavalkaya smriti 71.+

Women (possess) an unequalled means of purification; they never become (entirely) foul. For month by month their temporary uncleanness removes their sins. (Vashistha Dharmasutra 28.4)
Keep reading +

Women belong first to three gods, Soma (the moon), the Gandharva, and Fire, and come afterwards into the possession of men; according to the law they cannot be contaminated. (Vashistha Dharmasutra 28.5)+

Soma gave them cleanliness, the Gandharva their melodious voice, and Fire purity of all (limbs); therefore women are free from stains (Vashistha Dharmasutra 28.6)+

Pure is the mouth of a goat and of a horse, pure is the back of a cow, pure are the feet of a Brâhmana, but women are pure in all (limbs). (Vashistha Dharmasutra 28.9).
Will explain via God’s own words someday :) I never said “WOMEN are filth”, the period blood is!