Carbon Monoxide--Related Deaths --- United States, 1999--2004

Carbon monoxide (CO) is a colorless, odorless, tasteless toxic gas produced by incomplete combustion in fuel-burning devices such as motor vehicles, gas-powered furnaces, and portable generators (1). Persons with CO poisoning often overlook the symptoms (e.g., headache, nausea, dizziness, or confusion), and undetected exposure can be fatal (1). Unintentional CO exposure accounts for an estimated 15,000 emergency department visits and 500 unintentional deaths in the United States each year (1). The most recent state-level estimates of CO-related deaths were described in 1991 for the years 1979--1988 (2). Using the most recent mortality data available, this report updates national and state-specific unintentional, non--fire-related CO mortality rates and describes the demographic, seasonal, and geographic patterns for 1999--2004. During this period, an average of 439 persons died annually from unintentional, non--fire-related CO poisoning, and the national average annual death rate was 1.5 per million persons. However, rates varied by demographic subgroup, month of the year, and state. Rates were highest among adults aged >65 years, men, non-Hispanic whites, and non-Hispanic blacks. The average number of deaths was highest during January. Among the states, Nebraska had the highest reliable CO mortality rate. These findings indicate that improved population-based prevention measures, including educating the public about the dangers of CO exposure, are needed at the state and national levels.

Mortality rates were calculated from death certificate data obtained from the National Vital Statistics System (NVSS), using the record axis fields from the multiple cause-of-death files compiled by the National Center for Health Statistics (3). Records were searched for all deaths occurring among residents of 50 states and the District of Columbia during 1999--2004 that contained International Classification of Diseases, Tenth Revision (ICD-10) code T58 (toxic effect of CO) as a contributing cause of death. A case of unintentional CO-related death was defined as one for which both poisoning by accidental exposure to gases or vapors (code X47) and toxic effect of CO (code T58) were listed as causes of death. All records of deaths caused by intentional exposure, exposure of undetermined intent, or fire-related exposure to CO (codes X00--X09, X76, X97, Y26, and Y17) were excluded. Deaths that occurred among foreign residents in the United States and deaths among U.S. residents who died abroad also were excluded.

Crude and age-adjusted rates of unintentional, non--fire-related deaths from CO poisoning were calculated by age group, sex, and race/ethnicity for the period 1999--2004. To assess the seasonality of CO-related mortality, the average daily number of deaths was calculated by month for the period 1999--2004. The national Non-Notifiable Disease Surveillance System was used to identify states in which physicians, laboratories, or hospitals are mandated by law to report acute CO poisoning (4). In addition, age-adjusted CO death rates were calculated for each state for the period 1999--2004 (5,6). Populations at risk were defined using the U.S. intercensal population estimate for 1999, the U.S. Census 2000 population count, and population bridged-race estimates (3) for 2001--2004. Using the direct method, state mortality rates were age adjusted to the U.S. standard 2000 population (3,5). Rates based on small numbers of deaths (five or fewer) or with relative standard errors (RSEs) >50% were considered unreliable and were not included (7). Rates based on >20 deaths and with RSEs <30% were considered reliable. Rates based on six to 29 deaths and with RSEs from 30% to 50% should be interpreted with caution. Statistically significant differences between rates were inferred by comparing 95% confidence intervals.

During 1999--2004, CO poisoning was listed as a contributing cause of death on 16,447 death certificates in the United States. Of these, 16,400 (99.7%) deaths occurred among U.S. residents inside the United States, and 2,631 (16%) were classified as both unintentional and non--fire-related deaths. For the period 1999--2004, an average of 439 persons died annually from unintentional, non--fire-related CO poisoning (range: 400 in 1999 to 473 in 2003). The annual average age-adjusted death rate in the U.S. was 1.5 deaths per million persons (Table 1). Death rates were highest for adults aged >65 years and for men (Table 1). Age-adjusted death rates were higher for non-Hispanic blacks and non-Hispanic whites than for other subgroups; however, the difference between the rates for blacks and whites was not statistically significant (Table 1). The average daily number of CO-related deaths was greatest during the months of January (2.07 deaths) and December (1.97 deaths) and lowest during the months of July (0.67 deaths) and August (0.67 deaths). For the period 1999--2004, a total of 35 states had sufficient numbers of CO-related deaths to calculate reliable mortality rates (Table 2). The state with the highest reliable CO mortality rate was Nebraska, and the state with the lowest reliable rate was California. As of December 2007, reporting of acute CO poisoning by health-care providers was mandatory for 13 states; no clear pattern of differences in CO-related mortality was detected between states with mandatory reporting and those without.

Reported by: M King, PhD, C Bailey, MS, National Center for Environmental Health, CDC.

Editorial Note:

Consistent with previous studies (1,2), the results of this analysis indicate that men and adults aged >65 years were more likely to die from CO poisoning than other persons. The higher rate in men has been attributed to high-risk behaviors among men, such as working with fuel-burning tools or appliances. The higher rate among older persons has been attributed to the likelihood of older adults mistaking symptoms of CO poisoning for other conditions common among persons in this age group (e.g., influenza-like illnesses or fatigue. CO deaths were highest during colder months, likely because of increased use of gas-powered furnaces and use of alternative heating and power sources used during power outages, such as portable generators, charcoal briquettes, and propane stoves or grills (1). Similar to previous findings (2), the highest CO death rates tended to be among western (e.g., Alaska, Montana, and Wyoming) and midwestern (e.g., Nebraska and North Dakota) states, likely because of variations in weather and geography and state-by-state variations in prevalence of certain risk behaviors.

The findings in this report are subject to at least three limitations. First, carboxyhemoglobin measurements are not a routine part of autopsies, and postmortem measurements often are unreliable because carboxyhemoglobin concentrations produced by different analytic methods vary (8), which might have resulted in misclassification of CO-related deaths. In addition, receipt of mortality data often is delayed, and the data might lack the circumstantial and clinical detail that could provide information about the specific mechanisms of CO poisoning, which might have resulted in misclassification. Second, because the symptoms of CO poisoning are nonspecific and clinical recognition is challenging, certain cases might not be recognized, resulting in underestimates. Finally, because ICD-10 coding has only one code specific to CO (T58), distinguishing between deaths caused by motor-vehicle exhaust and other CO-related deaths is not possible using the methods in this analysis.

Because persons are relying on CO alarms to prevent CO poisoning (9), additional research regarding their effectiveness is needed, including an evaluation of the cost effectiveness of CO alarms used in residences. As additional years of data become available, tracking of longitudinal trends in CO-related mortality should continue to guide public health measures aimed at preventing deaths from CO poisoning (10).

Exposure to CO can be prevented with basic precautions, including proper installation and maintenance of fuel-burning appliances (Box). CO detectors can alert occupants to accumulating gas and should be placed on every level of a home. Additional measures to educate the public regarding the dangers of CO are needed, particularly during the winter season. Additional surveillance that combines timely estimates of morbidity and mortality with situational information related to mechanisms of CO exposure (e.g., length of exposure, type of fuel-burning device involved, and behaviors or chain of events preceding exposure) could help target prevention measures and reduce CO poisonings.

References

1. CDC. Unintentional non--fire-related carbon monoxide exposures in the United States, 2001--2003. MMWR 2005;54:36--9.
2. Cobb N, Etzel RA. Unintentional carbon monoxide-related deaths in the United States,1979--1988. JAMA 1991;266:659--63.
3. CDC. US census populations with bridged-race categories. Hyattsville, MD: US Department of Health and Human Services, National Center for Health Statistics; 2004. Available at http://www.cdc.gov/nchs/about/major/dvs/popbridge/popbridge.htm.
4. Council of State and Territorial Epidemiologists. Non-notifiable disease surveillance system assessment 2005. Available at http://www.cste.org/nndsssurvey/2004nndss/nndssstatechrreporcondnona2005.asp.
5. Anderson RN, Rosenberg HM. Age standardization of death rates: implementation of the year 2000 standard. Natl Vital Stat Rep 1998;37(3).
6. Fay MP, Feuer EJ. Confidence intervals for directly standardized rates: a method based on the gamma distribution. Stat Med 1997;16:791--801.
7. CDC. Compressed mortality file 1979--1998 and 1999--2002. Available at http://wonder.cdc.gov/mortsql.html.
8. Levine B, D'Nicuola J, Kunsman G, Smith M, Stahl C. Methodologic considerations in the interpretation of postmortem carboxyhemoglobin concentrations. Toxicology 1996;115:129--34.
9. CDC. Use of carbon monoxide alarms to prevent poisonings during a power outage---North Carolina, December 2002. MMWR 2004;53:189--92
10. Mott JA, Wolfe MI, Alverson CJ, et al. National vehicle emission policies and declining US carbon monoxide-related mortality. JAMA 2002;288:988--95.

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Table 2


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Box


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Date last reviewed: 12/20/2007