Commento su Levitico 15:16

ואיש כי תצא ממנו שכבת זרע, “and if a flow of seminal emission originated from a man;” the word איש in this verse exempts a minor from this legislation, as long as that minor is not yet nine years of age and one day.

כי תצא ממנו, as long as this semen has not left his body it does not confer ritual impurity. Seeing that this sounds somewhat superfluous, the Rabbis interpreted this to mean that both a priest and an ordinary person who experience an erection can or should (as the case may be) take hold of their member to prevent the emission until it is not likely to come into contact with something subject to contamination. (Sifra)

ורחץ במים, “he may even immerse himself in a ritual bath;” [does not require spring water. Ed.]

את כל בשרו, the bath must contain enough water to cover his entire body at one time when he immerses himself in it. According to halachah this is 3 cubits high, 1 cubit wide and 1 cubit deep, or approx .530000 cubic centimeters.

That is that He commanded us that semen be impure. And this commandment includes the regulations of [the impurity of] semen. (See Parashat Metzora; Mishneh Torah, Other Sources of Defilement 5.)