Question: I always used to wash negel vasser in the morning from the bathroom sink. Recently, I have been leaving a cup of water in a bowl next to my bed so that I do not need to walk around before washing. Is this preferable even though the water is uncovered?
Answer: The Gemara (Avoda Zara 30b) writes that mayim shenisgalu¸ water that was left uncovered, must not be poured out into the street, used for building, drunk from or even used to wash oneself. This was out of concern that a dangerous snake or scorpion had entered the water and poisoned it.
While Rambam (Rotzeach Ushemiras Hanefesh 11:15) records this as the halacha, the Tur (YD 116) and Shulchan Aruch (YD 116:1) write that this does not apply nowadays when such incidents are so unlikely.
R’ Yaakov Chaim Sofer (Kaf Hachaim OC 160:2; YD 116:8) adds that one only needs to observe this halacha in places where snakes are common (See Mishna Berura 160:23).
R’ Chaim Falaji (Kaf Hachaim 8:11) writes that one must not use mayim shenisgalu. The Shaarei Teshuva (4:7) notes, too, that there are those who are still particular about this (See Pischei Teshuva YD 116:1). Nonetheless, the Shaarei Teshuva concludes that one does not need to be concerned about mayim shenisgalu nowadays. Likewise, the Aruch Hashulchan (YD 160:1) writes that one may use such water nowadays. The Levush (OC 160:5) writes that not only may one use it for washing one’s hands, but one may even drink from mayim shenisgalu nowadays.
The Magen Avraham (4:1) and Mishna Berura (1:2) write that one should not walk more than 4 amos prior to washing one’s hands in the water.
In conclusion, one does not need to be particular to cover their water for netilas yadayim. It is preferable to leave an uncovered cup next to one’s room than to leave one’s room to wash their hands.