Hi Richard,
do you mean when inhaling or exhaling? Generally speaking it’s best to breathe gently from the diaphragm when playing harmonica, with your jaw and throat open, with the mouthpiece of the harmonica fully enclosed by your lips so that the front is really inside your mouth. The idea is to move a large volume of air under low pressure, rather than to force air through the instrument. It’s good to relax your shoulders and adopt an upright posture. The breath exercises I recommend to increase lung capacity work best on a diatonic harp, for example train rhythms, as the chords sound nicer than on the chrom. You may want to check out my video on the subject here:
Hope that’s a help
Steve