This question already has an answer here:
If Dementors are controlled by the ministery and looking for prisoners, why did they do that to Harry in the train in Harry Potter and The Prisoner of Azkaban? Is that their expedient effect? (When they accept Dementors to search the students train it means it shouldn’t be harmful or fatal! Wasn’t it?)
Was the thing they did to Harry the Dementor's kiss or is it just the thing they did to Sirius Black? When do they use the Dementor's kiss?
Is their effect permanent or is it just for the time they are around?
(I read Pottermore but I didn't get my answers. This link can be useful but because English is not my language it was hard for me to read it.)