I had a question regarding surma. What are some sahih benefits of it from hadiths and is the lead in it truly causes brain damage? Both in babies and adults.

The hadiths mention these benefits:

It sweetens the mouth, helps grow hair, improves vision, and helps prolong one’s sujud.

One should avoid using kohl that contains lead. There are types of kohl that are lead free or have very small and insignificant amounts of lead.