Question:
I know that brushing one’s hair on Shabbos is forbidden because hairs may be pulled out. What about using my fingers to straighten or separate my hair? If I know that a few hairs may come out in the process, may I nevertheless run my fingers through my hair on Shabbos?
Answer:
Yes, so long as it is not inevitable that any hair will be pulled out in the process.
While the Shulchan Aruch rules that one may not brush hair with a brush on Shabbos because it is impossible to avoid pulling out hairs, the Rama explicitly permits separating and arranging one’s hair with his hands.
Accordingly, one may run his fingers through his hair in order to separate strands, straighten it, or make it presentable on Shabbos.
Even though it is possible that an occasional loose hair may fall out during the process, the Rama nevertheless permits using one’s hands. The prohibition applies specifically to brushing with a brush, which normally and inevitably removes hair, and not to arranging hair with one’s fingers. However, in the event that even using ones hands will for certain pull out hair, then even this may not be done. Hence, one who has very light hair and knows from experience that even slight brushing of their hand will end up certainly removing hair, may not do so.
Conclusion:
One may not brush hair with a brush on Shabbos because hair removal is unavoidable. However, one may use his fingers to separate, arrange, and straighten his hair on Shabbos. This remains permitted even though an occasional loose hair may incidentally fall out during the process, so long as it is not a known inevitable occurrence.
Sources:
Michaber and Rama O.C. 303:27; M”A 303:23; Rivash 394; Biur Halacha 328 end; Kaf Hachaim 340:9-10; Piskeiy Teshuvos 303:17 footnote 108-109