Heavy metal exposure, such as with mercury in dental amalgam, has been suggested as a cause of ME/CFS in some people.
A new hypothesis paper suggests that cadmium (a widespread occupational and environmental pollutant) might be involved in the illness, based on the similarity between the neurological symptoms and the known effects of cadmium on the human body. For instance, brain grey matter has been shown to be decreased in ME/CFS patients compared with healthy people, and cadmium is said to induce neuronal death in cortical neurons in the brain. Again, ME/CFS has been associated with reduced cerebral blood flow, and cadmium has disruptive effects on the creation of new blood vessels.
This is all speculation, of course, but if high levels of cadmium were indeed measured in these patients, the hunt would be on for therapies capable of limiting or reversing cadmium exposure.
Reference: Could cadmium be responsible for some of the neurological signs and symptoms of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Pacini et al. Med Hypotheses 2012; 79(3): 403-7