A common line is that 5 Gauss is the least a body responds to, and that below it the field does nothing. We use 5 Gauss only as a measurement reference for how far a field reaches, not as the point where the effect stops.
Why we leave it out
We picked 5 Gauss because it's a steady, repeatable edge, about ten times Earth's background field, so a reported reach reflects field that clearly stands above the noise. But published work has documented biological effects well below that level, down in the tens of microtesla. Calling 5 Gauss a floor for meaningful response would claim more than the number can carry. It's a reporting convention, and we say so.
The field is strongest right at the coil and weakens with distance in every direction. On the on-axis dipole model the report uses, it reaches half its peak strength at about three quarters of a coil radius from center and one tenth at about two radii. A larger coil holds the field up farther out.
