iems are probably the worst as though they allow listening at lower volumes, if you wear them too long, your ear cannot breathe. too much moisture can make for infections which unchecked can breed hearing loss.
but otherwise give and take
"Occluding the ear canal with an earphone, earplug, or hearing aid will not cause infections in a healthy ear. It is possible they can aggravate the situation in an already unhealthy ear. If you do not experience ear infections currently, the use of earphones like the ER-4P will not cause an infection even when used for long periods of time. If you have a history of ear infections or other ear pathology you should consult your physician to determine if there any risks you need to be aware of."
The foam tips are not meant to be reused, and doing so could cause infection I guess.
I'd say in any sort of noisy environment, it's a lot easier to keep listening volumes low with IEMs.