The danger of carbon monoxide entering your home can arise due to multiple factors, and it is important to ensure these issues are prevented: having a fire without sufficient ventilation, whether it be due to a poor feed of fresh air into the chimney, or a fire that has a blockage and/or is restricted stopping the excess fumes leaving the house, therefore leaking out.
It is vital to have a carbon monoxide detector installed in your home, for safety and peace of mind. These test carbon monoxide levels in the air, very much like a smoke alarm, they are life savers. An audible carbon monoxide detector is recommended, as they will make you very much aware if there is any danger in the air. They can be purchased from many supermarkets, DIY stores and off the internet, I can help by advising on which specific detectors to choose, or supply them and install myself.