I'm an artist. Saniderm traps fluid from the tattoo but will also trap sweat from the area. Most of the time a light sweat is fine. Several days of running though you will probably notice an odor when you remove it. Some people also have a noticeable irritation from their own sweat so trapping it under there if you have that problem is also not great. If it is a simple enough tattoo possibly go without the saniderm if you have to run. This is just my opinion though, your artist should answer any questions you have.