Dry impurity does not pass to the one who touches it, so his garments do not become naajis and neither does his body. If a person sits on dried urine, his clothes do not become naajis as a result of that, and he does not have to wash them.
Najaasah (impurity) is only transmitted if it is wet. So if the impurity is wet or the hand of the one who touches it is wet, then it is transmitted as a result.
Shaykh Saalih al-Fawzaan (may Allaah preserve him) said:
If a person touches a wet impurity, then he must wash whatever of it touched his body, because the impurity has been transmitted to it. But if the impurity is dry, then he does not have to wash the part of the body that came in contact with it, because it is not transmitted to him. End quote.
Shaykh ‘Abd-Allaah ibn ‘Abd al-Rahmaan al-Jibreen (may Allaah preserve him) was asked:
Does dried urine not make clothes impure? i.e., if a child urinates on the ground and the urine remains there until it dries without being washed off, then someone comes and sits on the urine when it has dried, does his garment becomes impure?
Touching dry impurity does not affect the body or dry clothes, and it does not matter if one enters a dry washroom with dry feet, because the impurity is only transmitted when it is wet. End quote.
Fataawa al-Mar’ah al-Muslimah (1/194)
And Allaah knows best.