How much tzoah is a problem for davening? Is there a minimum? For example if it were a trace that you can’t smell because it is such a small amount or not even be able to see, like somebody changed a child and liquid tzoah leaked through onto their hands and then they touched something else (walls etc), but so little that if they didn’t know that it happened they wouldn’t know there was a trace would that be a problem?
If the tzoah is too small to see, and it doesn’t have smell, it is permitted to say KS in front of it. However if it is on the person’s hands, then even if it doesn’t have a smell to it, he has to wash his hands before davening.
OC:H 76-4, M:B ibid 10. Although the M:B ibid 20 says that even if the tzoah is a mashehu, however the rule in the whole torah is that we are only commanded to be careful from things that the human eye can see. For example we are not commanded not to eat bugs (Y:D 84) that are too small for the human eye to see, (if we can see it but it isn’t clear what we are seeing then it is like we can see it). The same is with two letters of a sefer torah touching each other, and with the four minim (Chazon Ish O:CH145-11,Emek Bracha pg. 93b, Moadim U’zmanim 124).