'In general, CO is produced when any material burns, but more is produced when there isnít enough oxygen for efficient burning. Common sources of CO in homes include fuel-burning devices such as furnaces, gas or kerosene space heaters, boilers, gas cooking stoves, water heaters, clothes dryers, fireplaces, charcoal grills, wood stoves, lawn mowers, power generators, camp stoves, motor vehicles and some power tools with internal combustion engines. Smoking is another common source of CO that impacts indoor air quality.'
'CO Poisoning Symptoms

Know the symptoms of CO poisoning. At moderate levels, you or your family can get severe headaches, become dizzy, mentally confused, nauseated, or faint. You can even die if these levels persist for a long time. Low levels can cause shortness of breath, mild nausea, and mild headaches, and may have longer term effects on your health. Since many of these symptoms are similar to those of the flu, food poisoning, or other illnesses, you may not think that CO poisoning could be the cause.'
'Carbon Monoxide:

Carbon monoxide is the same odorless, colorless gas that comes out the tailpipe of your car or a faulty gas heater. In high enough concentrations it is deadly; in lower doses it causes shortness of breath and increased heart rate.*'
'Air pollution exposes the human body to substances capable of endangering health and depletion of vitamin reserves. These substances include carbon monoxide, aldehydes, hydrocarbons, nitrogen oxides, sulphur dioxides and positive ions (also found in air conditioned rooms)'
'Persons living in most American cities are frequently exposed to 100 ppm (that is, 155 mg/cu mm) of carbon monoxide in the ambient air for varying periods of time and may attain carboxyhemoglobin blood levels up to 10 percent'

There's more info about various carbon monoxide sources at .
'According to Thomas L. Rodgers, the best medical literature states that, "Carbon monoxide inhibits the uptake of B12[...]".
'Drugs and Chemical offenses: Environmentally or pharmaceutically ingested, tactile, or respiratory organic and inorganic poisonings (including low level but chronic respiratory exposures e.g. formaldehyde, CL, CO (carbon monoxide), SOx, NOx, restricted oxygen, petrochemicals, tobacco, etc.) can set up low B12 intrinsic factor production, failed binding, transport or function anywhere in the B12 absorption / intrinsic factor chemistry, including its more evident hemoglobin catalyzation, myelinzation, and critical neuro-function paths.'

If you know anything about the possible link between carbon monoxide and B12, please post it in this thread!