Answered by Shaykh Yusuf Weltch
If there is dry najasa on my body (for example, one drop or more on my leg), does it transfer by sweating?
Kindly consider the explanation below:
If one has dry impurity on their body, they sweat; the impurity does not spread. This ruling prevents undue hardship for those not performing istinja’ with water. [Shurunbulali, Maraqi al-Falah]
However, it would be cautious (not obligatory) to wash if the traces of the impurity reappear noticeably.
Hope this helps
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani