This Is MS Multiple Sclerosis Community: Knowledge & Support

mice were placed in a chamber where they breathed carbon monoxide (CO) at a concentration of about 500 parts per million for 20 days...a similar concentration of the gas can cause headaches and fainting in humans

For comparison, undiluted cigarette smoke contains about 30,000 ppm of CO, undiluted warm car exhaust about 7,000 ppm, and the chimney of a home wood fire about 5,000 ppm. Clean countryside air contains about 0.02 ppm of CO. The smoke from one pack of cigarettes, if distributed uniformly throughout an average sized house, could result in a CO concentration of up to 14 ppm.

...Pharmaceutical companies are currently working on developing drugs that can deliver carbon monoxide locally within the nervous system...

Soares suspects that CO works in this fashion because it promotes the binding of iron to heme molecules within the nervous system. Heme molecules that lack iron can increase the production of free radicals, which damage cells.