Many times, in the middle of learning i will absentmindedly touch my scalp under my yarmulke, or my shoulder under my sleeve. Do i have to stop learning to wash my hands? may i continue without washing?
I personally asked Rav Herschel Schachter if one is scratching their scalp in the middle of learning if they need to interrupt their learning and go wash their hands.
He answered me that some poskim (he did not tell me which ones, though I link some opinions he may have been referring to below) rule that since nowadays we take showers on a much more regular basis one doesn't need to stop and wash their hands. As the halacha of washing hands after touching a "makom mechuse" was said in a time period where people took showers very infrequently.
The Shulchan Aruch 92:7 writes, “A dirty area is [defined as] the [normally] covered parts of a person’s body, because they contain beads of sweat. For instance, scratching one’s head [is considered touching a dirty place].” This refers to rubbing the roots of one’s hair, but if someone only touches the top of his head, he does not need to wash his hands (Shulchan Aruch HaRav; Mishnah Berurah 162:58, 164:10).
Kaf HaChaim 4, small paragraphs 75 and 98, is lenient even if one scratches the roots of his hairs which are not covered with a hat, because there is no sweat there.
Tzitz Eliezer, part 7, 2:14 concludes that if he washed his head and his hair is clean, even a person who scratches the roots of his hairs does not need to wash his hands.
For an interesting explanation why people in Mediaeval ages didnt bath see here:
This latter “disease” point was very common; it was believed in many parts of Europe that water could carry disease into the body through the pores in the skin. According to one medical treaty of the 16th century, “Water baths warm the body, but weaken the organism and widen pores. That’s why they can be dangerous and cause different diseases, even death.” It wasn’t just diseases from the water itself they were worried about. They also felt that with the pores widened after a bath, this resulted in infections of the air having easier access to the body. Hence, bathing became connected with spread of diseases, not just immorality.
For most lower class citizens, particularly men, this resulted in them completely forgoing bathing. During this time, people tended to restrict their hygienic arrangements to just washing hands, parts of the face, and rinsing their mouths. Washing one’s entire face was thought to be dangerous as it was believed to cause catarrh and weaken the eyesight, so even this was infrequent.