Chronic and occult carbon monoxide poisoning: we don't know what we're missing.

The purpose of this review is to examine the literature regarding occult and chronic exposure to low levels of carbon monoxide (CO)— that is, levels insufficient to cause emergency department attendance or admission to hospital with a clinical diagnosis of acute CO poisoning, although this in fact is the cause of the symptoms. More is known about acute CO poisoning, but knowledge and awareness of chronic poisoning is progressing slowly. There is a strong possibility that low level exposure to CO is responsible for widespread and significant morbidity, however the clinical syndrome produced is often overlooked because of a range of presentations, obscure symptoms, and a lack of awareness of the problem. In the USA, comparatively small changes in ambient levels of CO as a result of pollution have been shown to affect rates of presentation to emergency departments with various complaints. This raises the possibility that large numbers of patients may be seen in UK emergency departments with symptoms caused by, or disease states worsened by, exposure to CO without staff being aware of the fact. There also exists the issue of acute poisoning from domestic gas appliances and other sources, which result in symptoms, and illness not recognised by medical staff. This issue also falls into the remit of this review as cases of “occult poisoning” (see box 1 for definitions of CO poisoning).