  Near-zero magnetic fields, called hypomagnetic fields, are known to impact
biological phenomena, including developmental processes, the circadian system,
neuronal and brain activities, DNA methylation, calcium balance in cells, and
many more. However, the exact mechanism underlying such effects is still
elusive, as the corresponding energies are far smaller than thermal energies.
It is known that chemical reactions involving radical pairs can be magnetic
field dependent at very low intensities comparable to or less than the
geomagnetic field. Here, we review in detail hypomagnetic field effects from
the perspective of the radical pair mechanism, pointing out that under certain
conditions, they can be comparable or even stronger than the effects of
increasing the magnetic field. We suggest that hypomagnetic field effects are
an interesting avenue for testing the radical pair mechanism in biology.

Quantum Physics;Biological Physics;Chemical Physics;Neurons and Cognition