You can't see, taste, or smell it, but carbon monoxide (CO) can kill without warning in a matter of hours. Every home should have carbon monoxide detectors fitted to ensure you do not get caught by the silent killer. Remember to check your alarms weekly and to change the batteries every year. Better still use a model with a 10 year battery or mains wired.
You should ensure that your gas appliances and flues have been properly installed and are kept well-maintained and properly ventilated.