The two things that actually hurt people are chimney fires and carbon monoxide. Both are preventable, and both are found in a routine check.
Creosote is the tarry residue left when wood smoke cools inside the flue. It builds up in layers, and past a certain thickness it will catch fire — hot enough to crack a liner and set light to the timbers around the chimney. Sweeping removes it.
You would not. It has no smell, no colour and no taste, which is exactly what makes it dangerous. A blocked or cracked flue is the usual route. Fit an alarm on every floor, and have the flue checked once a year.
Once a year for a chimney in regular use, and always before the first fire of the season. More often if you burn unseasoned wood or use it daily through the winter.
If it has been more than a year, have it checked first. Most of what goes wrong with a chimney is invisible from the room — a cracked liner or a bird nest looks like nothing at all from a hearthside chair.
Stop using the fireplace and get a Level 2 inspection before lighting it again. Chimney fires are often brief and easy to miss, but they crack liners, and the next one goes into the structure.
If anything above sounds like your chimney, tell us and we will come and look.