Moreover, the bigger ones tend to get stuck places. This wouldn't be too big an issue by itself (doctors have told me they are easy to remove if you know about them), but that they just sit there and burn into the flesh in the same spot.

To top it off, swallowing presents flu-like symptoms until it's too late.

The cell chemistry only plays an indirect role here (neither zinc nor lithium are terribly healthy in a child's body I guess).

The lithium batteries are thrown away with a fair bit of energy remaining: Internal resistance will go up too far to be useful to power something, but the cell can still electrolize when inside the body.

Zinc-air primaries on the other hand tend to be completely empty when removed from the hearing aid; they have a really nice flat discharge profile.