1. City air is dirty and polluted, so it would be wise for you to shampoo your hair frequently. (OK)

Note, for expresses, It would be within your best interests; to your benefit.

2a. But the Phantom captured him and kept him in a prison, which was slowly filling with water. (OK)

2b. But the Phantom captured him and kept him in a prison, which was slowly was slowly being filled (by someone) with water. (OK)

3. It is the only man-made structure which/that can be seen from space. (OK)

4. However, after paying it into my account, my conscious began to trouble me. (Not OK) Try, conscience. :wink: