When breathing eucalyptus oil it's only bad to breathe it if you do it too much and too often.
But in moderation breathing in or inhaling eucalyptus oil is pretty safe and considered the best and safest way to get the eucalyptus oil into your blood stream.
However never ingest the eucalyptus oil directly because it can be fatal and if you or a child happens to ingest some eucalyptus oil accidentally you should call 911 or get to the emergency room as soon as possible.
Induce vomiting also to get the eucalyptus oil out of your system so that it doesn't cause any other health issues.
Breathing in or inhaling eucalyptus oil is a great way to help you fall asleep, help with colds and help you relax but just don't do it too much because then it could be harmful.