Carbon monoxide (CO) is formed during the incomplete combustion of virtually any carbon-containing compound. Because it is an odorless, colorless, and tasteless gas, it is remarkably difficult to detect in the environment even when present at high ambient concentrations and is a leading cause of poisoning morbidity and mortality in the United States. Based on US national death certificate data, there were 439 annual deaths from unintentional non-fire exposure to CO from 1999 to 2004.27 The groups with the highest risk were male gender and elderly age, possibly because of occupational exposure and inability to discern CO symptoms, respectively. The CO-related mortality rate has remained essentially unchanged over the years despite increased CO detector use.22,26,28 More than half of these cases (64%) occurred in homes with faulty furnaces, usually in the fall or winter months. Many clusters were associated with power failures during catastrophic weather, such as ice storms, blizzards, and hurricanes.24,25 Analysis of the Centers for Disease Control and Prevention wide-ranging online data for epidemiologic research (WONDER) database showed total non–fire-related CO poisoning deaths decreased from 1,967 in 1999 to 1,319 in 2014.88 Of these non–fire-related deaths, unintentional cases continue at a rate of approximately 450 annually from 1999 to 2015.29,187
Just as important as mortality rates are the greater number of survivors from CO poisoning. Despite increased awareness for CO poisoning, there were still an average of 20,636 nonfatal, unintentional, non–fire-related CO exposures treated annually in the United States.28 More than 40% of cases occurred in the winter, with almost 75% occurring in residences. However, exclusion of intentional and fire-related cases severely underestimates the extent of the problem. Based on firsthand hospital data, a minimum of 50,000 patients with CO poisoning present to US emergency departments (EDs) each year, up to half resulting in hospitalization.96,111 More recent data using probable and suspected cases suggest that there were more than 230,000 ED visits in 2007 alone that were unintentional and related to non-fire CO poisoning.112
The bigger problem with CO poisoning is the associated morbidity that survivors risk even after acute treatment. The most serious complication is persistent or delayed neurologic sequelae (DNS) or delayed neurocognitive sequelae, which occurs in up to 50% of patients with symptomatic acute poisonings.81,174,226 At 1 year after exposure, of the 1,643 patients not treated with HBO 42% had cognitive sequellae at 6 weeks, 30% at 6 months and 18% at 12 months. Of the 75 treated with HBO 24% had cognitive sequellae at 6 weeks, 17% at 6 months and 14% at 12 months.228 There is still no highly reliable method of predicting who will have a poor outcome, requiring the threshold for HBO therapy for CO poisoning and follow-up be particularly low.