Phisherman wrote:
Sometimes the water level in the toilet bowl drops below the trap
letting sewer gas into the bathroom. How can the water level be
raised in the bowl? Why is it that sometimes the level is low--this
has happened recently and the toilet has been working fine the past 15
years. TIA

You don't need to raise the water, rather you need to keep it from going
down. Most of the time when this happens there is a fully or partly blocked
vent. You may also have a partly blocked drain.

--
Joseph Meehan

Dia duit