If the prohibition on carrying on Shabbos can apply even to extra saliva in one's mouth,* why doesn't it apply to the many extremely small things that are inevitably on one's person, such as dust?
Please note that I am asking about dust, etc., on your skin, not your clothes. I found an answer for when it's on your clothes, particularly if it's attached.
*Source: "The 39 Melachot," by Rabbi Dovid Ribiat, Vol. 4, section three, chapter V, 29c:
Walking with an accumulation of saliva: One who feels an accumulation of saliva in his mouth that has collected and is ready to be expelled should not continue walking in R'shus Harabim, but should stop and expel the excess fluid (on the street, not in a trash can - see a above). He should certainly not walk home or to any R'shus Hayochid with the excess saliva. The reason here is because some Poskim regard the excess saliva in this state as a separate entity and no longer part of the body. In this context, it is considered a Masui, and walking with it through R'shus Harabim, or to a R'shus Hayochid is Hotzoa.
There are several footnotes to the text. I couldn't really read them, but gathered that sources are the Mishnah Berurah, S' shin nun, s"k yud beis and yud daled, and the Mishnah Berurah שם, not sure.