How to block magnetic field under home office desk

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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.