How to block magnetic field under home office desk

The mains electric line runs directly under my home office, under my built in desk, and on the floor reads up to 12-15mG, and then connects to the main breaker panel about 10m away. I am sitting here for hours every day. I assume this is not good. What can I do? Move rooms... can be done, but I have a beautiful fitted office. Is there a material I can put on the floor under a rug? Can anything be done to stop the field electrically? Many thanks.