According to R’ Akiva (Yoma 14a) a tahor person who purifies a tamei person (by sprinkling him with the water of purification) is himself rendered tamei. However, thehalachah follows the dissenting opinion of the Sages (ibid.) that purifying another person does not make you tamei (Rambam Hil. Parah Adumah 15:1)
How then can we explain Bemidbar 19: 21: “Someone who sprinkles the water-of-purification must immerse his clothes (a high degree of tumah affecting both him and his clothes), and someone who touches the water-of-purification shall be tamei (a lower degree of tumah affecting only his person) until nightfall”? Doesn’t this verse teach that sprinkling the water contaminates the sprinkler?
It does indeed; but it is not discussing someone who is sprinkling to purify a tamei person or object! This is why the Torah here is careful not to say that the water is being sprinkled on a tamei person or thing! (Rambam ibid.)
Moreover, it can be demonstrated that the second part of the verse is also not discussing “touching” the water in order to perform purification. How so?
Touching water is always treated like carrying the water, since the contact makes a ripple, and moves the water (Hil. Avos HaTumos 6:13). How is it then that one who touches the water contracts a lower level oftumah than one who carries it? The answer is that water-of-purification generates a severe level of tumahonly when there is a critical quantity of it present, the minimum necessary to perform a purification. But less than this critical quantity, does not generate a severe level of tumah. Consequently, even though touching water is treated like carrying, touching less than the critical quantity does not generate a high level of tumah (Hil. Parah Adumah 15:1). The verse that teaches that touching generates only a low level oftumah must be referring to the touching of less than the minimum needed for performing a purification. Clearly, then, this touching cannot be in order to effect a purification.
The rabbis therefore took the first part of the verse as likewise referring only to someone who is handling the water with no intention to purify anybody or anything. When the verse mentions “someone who sprinkles” and does not specify that he is sprinkling on a tamei person or object, he is not sprinkling in order to purify (Rambam ibid.; see also Rashi Bemidbar 19:21).
It may now be suggested that the tumah discussed in this verse is tumah that derives from wasting and abusing the huge positive potential with which the water of purification is endowed. A person who has this water in sufficient quantities to purify someone tamei, is in possession of great power; but by handling it for no reason at all he abuses this power and potential and is therefore deservedly rendered tamei. If a person has in his possession the capacity to heal and to purify, and destroys or wastes this opportunity – does he not deserve to be rendered tamei?