Carbon Monoxide can come from any carbon burning appliance that is not burning correctly and stops the blood carrying oxygen, which, depending on how much is inhaled, can be fatal or lead to serious injury. It is particularly dangerous because you cannot see it, smell it or taste it. It is known as the silent killer for that reason.
Symptoms of carbon monoxide poisoning can be easily confused with common illnesses such as flu and even similar to those experienced during a hangover. If the symptoms lessen when away from the home, that might be an indication of carbon monoxide poisoning.