  A large and growing body of research shows that weak magnetic fields can
significantly influence various biological systems, including plants, animals,
and humans. However, the underlying mechanisms behind these phenomena remain
elusive. It is remarkable that the magnetic energies implicated in these
effects are much smaller than thermal energies. Here we review these
observations, of which there are now hundreds, and we suggest that a viable
explanation is provided by the radical pair mechanism, which involves the
quantum dynamics of the electron and nuclear spins of naturally occurring
transient radical molecules. While the radical pair mechanism has been studied
in detail in the context of avian magnetoreception, the studies reviewed here
show that magnetosensitivity is widespread throughout biology. We review
magnetic field effects on various physiological functions, organizing them
based on the type of the applied magnetic fields, namely static, hypomagnetic,
and oscillating magnetic fields, as well as isotope effects. We then review the
radical pair mechanism as a potential unifying model for the described magnetic
field effects, and we discuss plausible candidate molecules that might
constitute the radical pairs. We review recent studies proposing that the
quantum nature of the radical pairs provides promising explanations for xenon
anesthesia, lithium effects on hyperactivity, magnetic field and lithium
effects on the circadian clock, and hypomagnetic field effects on neurogenesis
and microtubule assembly. We conclude by discussing future lines of
investigation in this exciting new area of quantum biology related to weak
magnetic field effects.
