Carbon monoxide is produced whenever any fuel such as gas, kerosene, wood or charcoal is burned. If the appliances that burn fuel are maintained and used properly, the amount of CO produced is usually not hazardous. However, if appliances are not working properly or used incorrectly, dangerous levels of CO can result.

Symptoms:

• Low levels: shortness of breath, mild nausea and mild headaches. May have long term effects on your health
• Moderate levels: severe headaches, dizziness, mental confusion, nausea or fainting. Death can occur if this level persists for a long time

If you experience symptoms that you may be CO poisoning:

• GET FRESH AIR IMMEDIATELY
• GO TO AN EMERGENCY ROOM
  

Prevention is the Key:

• Have all fuel burning appliances inspected yearly by trained professional
• Make certain all flues and chimneys are in good condition
• Read and follow all instructions that accompany any fuel burning device
• Vent all fuel burning appliances to the outside, have them properly installed and maintain them according to manufacturers’ instruction
• If you cannot avoid using an unvented gas or kerosene heater, follow all instructions and crack a window to ensure enough air for ventilation and proper fuel-burning
• Don’t idle a car in an attached garage
• Don’t use an oven to heat your home
• Don’t use a charcoal or gas grill inside to heat the home
• Don’t use any gasoline powered engines in enclosed spaces
• Don’t sleep in any room with an unvented gas or kerosene space heater
  

http://www.epa.gov/iaq/pubs/coftsht.html