This article is from the Static Electromagnetic Fields and Cancer FAQ, by John Moulder firstname.lastname@example.org and the Medical College of Wisconsin with numerous contributions by others.
Yes. A few biological effects have been reported in laboratory systems
for fields as low as 20 mT, and some organisms appear to be able to
detect changes in the strength and/or orientation of the Earth¹s static
magnetic field (0.03-0.05 mT) [1,54]. In addition, the rates of some
chemical reactions can be affected by magnetic fields as low as 10 mT