Any level of carbon monoxide exposure can be dangerous to your health so you need to have a carbon monoxide detector installed to be safe.
Make sure all vents to gas appliances are not bent, obstructed etc so that gas appliances can properly vent to the outside.
Most people though don't experience symptoms of carbon monoxide poisoning until they're exposed to 1 to 70 PPM and as you're exposed to 70 PPM or above the symptoms of carbon monoxide poisoning begin to become noticeable which include headaches, fatigue and nausea.
Carbon Monoxide is a very dangerous odorless gas that can kill you pretty quickly and without a carbon monoxide detector it's hard to tell if you have carbon monoxide in your house and it can sometimes be too late.
If you don't have a carbon monoxide detector you really should install a carbon monoxide detector in your home so that you can be alerted when the carbon monoxide is detected.
Carbon Monoxide in your house can make you feel very sick and you need to get to fresh air right away if you've been exposed to carbon monoxide.
The Symptoms of Carbon Monoxide Poisoning are, Headaches, Blurred Vision, Confusion, Nausea, Vomiting, Shortness of Breath, Weakness, Dizziness and even loss of Consciousness.
If you suspect carbon monoxide poisoning you need to get out of the house if possible and call 911 immediately because carbon monoxide poisoning can kill you in a short time.
A doctor can take blood tests and breath tests to detect carbon monoxide in your body.