Comment by version_five

An MRI machine has a very uniform field inside the magnet, and a very strong gradient outside as the field curls back on itself. If you move your head back and forth outside the end of the bore, it causes dizziness, by inducing eddy currents in the brain (I've not done it myself, I'm very risk averse, but I've seen others doing it). I'd be much more worried about moving through a very strong gradient, spatially or temporally, than about the absolute field strength. Human made sustained field strengths are generally not dangerous, if the field is uniform. For example there is human and animal imaging at 7T , although lower field strengths are more common (the limiting factor, other than making the static field, is generally the RF frequency, which gets higher with field strength and introduces other issues as it does)