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Nonfatal, Unintentional, Non--Fire-Related Carbon Monoxide Exposures --- United States, 2004--2006

Carbon monoxide (CO) is a colorless, odorless, nonirritating gas that is produced through the incomplete combustion of hydrocarbons. Sources of CO include combustion devices (e.g., boilers and furnaces), motor-vehicle exhaust, generators and other gasoline or diesel-powered engines, gas space heaters, woodstoves, gas stoves, fireplaces, tobacco smoke, and various occupational sources (1). CO poisoning is a leading cause of unintentional poisoning deaths in the United States; it was responsible for approximately 450 deaths each year during 1999--2004 and an estimated 15,200 emergency department (ED) visits each year during 2001--2003 (2,3). Health effects of CO exposure can range from viral-like symptoms (e.g., fatigue, dizziness, headache, confusion, and nausea) to more severe conditions (e.g., disorientation, unconsciousness, long-term neurologic disabilities, coma, cardiorespiratory failure, and death) (4,5). CO poisoning often is misdiagnosed and underdetected because of the nonspecific nature of symptoms (3). To update a previously published report (3) and provide national estimates of CO-related ED visits during 2004--2006, CDC analyzed data from the National Electronic Injury Surveillance System -- All Injury Program (NEISS-AIP) database. During 2004--2006, an estimated average of 20,636 ED visits for nonfatal, unintentional, non--fire-related CO exposures occurred each year. Approximately 73% of these exposures occurred in homes, and 41% occurred during winter months (December--February). Prevention efforts targeting residential and seasonal CO exposures can substantially reduce CO-related morbidity.

The NEISS-AIP database is maintained by the U.S. Consumer Product Safety Commission and includes data on all types and causes of injuries treated in U.S. hospital EDs. NEISS-AIP includes 63 of 100 NEISS hospitals selected as a stratified probability sample to represent hospitals in the United States and its territories (3,6). Based on the hospital ED records, NEISS personnel document information on sociodemographic characteristics, diagnosis, and patient disposition in a standardized data collection form. Consumer products that are mentioned in relation to the injury event (e.g., CO detectors) are recorded. Information on source of CO exposure, location of incident, and toxic effects is documented in a narrative section.

This analysis included data for ED visits to the 63 NEISS-AIP sample hospitals by all persons with potential exposure to CO. Cases were included in this analysis if 1) the injury was unintentional or the intent was unknown, 2) the principal diagnosis for the ED visit was anoxia or poisoning, and 3) when a related consumer product was mentioned in the ED record, the product was a CO detector or, when the mentioned product type was unknown, and exposure to CO was indicated in the narrative. Cases with fire-related CO exposures (e.g., those including burns or smoke inhalation) and cases including persons who were dead on arrival or who died in the ED were not included.

Cases were classified as CO poisoning, CO exposure, or possible CO exposure. A case was classified as CO poisoning if 1) CO poisoning was listed as a diagnosis or 2) CO exposure or possible CO poisoning was indicated in the narrative and toxic effects were noted. A case was classified as CO exposure if the narrative confirmed a CO exposure and indicated a CO source but noted no toxic effects. A case was classified as possible CO exposure if the narrative indicated a potential CO exposure but no source or toxic effects were mentioned.* Two CDC epidemiologists independently reviewed the data and narratives recorded during ED visits. Discrepancies between these records were reconciled by mutual agreement. The epidemiologists also classified CO source, location of incident, toxic symptoms, and CO detector presence and activation for each exposure.

This report provides estimates based on 1,072 records included in the analysis. Each case was assigned a sample weight based on the inverse of the selection probability; these weights were summed to provide national estimates of nonfatal, unintentional, non--fire-related CO exposures. Three years of data were used to provide stable rates. Confidence intervals were calculated by using a direct variance estimation procedure that accounted for the sample weights and complex sample design. Rates were calculated using the 2000 U.S. Census Bureau postcensal estimates as denominators for the respective years and categories. Stratum-specific estimates based on unweighted counts of less than 20, a coefficient of variation of >30%, or both, might be statistically unstable and were reported where applicable (3).

An estimated 61,907 nonfatal, unintentional, non--fire-related cases of CO exposure occurred in the United States during 2004--2006, for an average of 20,636 exposures each year (Table 1). Of these, 68.5% were classified as CO poisoning, 30.6% as CO exposure, and 0.9% as possible CO exposure (Table 2). Overall, 7.0 CO-related ED visits per 100,000 population occurred each year during 2004--2006. Children aged <5 years had the highest estimated rate of CO-related ED visits (11.6 cases per 100,000 population) among all age groups. Among adults, persons aged 25--34 years had the highest estimated rate of CO-related ED visits (10.4 cases per 100,000 population). For older age groups, the estimated rate declined as age increased. Females had a higher estimated rate of CO-related ED visits (7.2 cases per 100,000 population), compared with males (6.7 cases per 100,000 population). The majority (90.4%) of the patients were released from the ED after examination and treatment, but 8.2% were either hospitalized or transferred to other hospitals for specialized care. The highest percentage of CO exposures (41.4%) occurred during the winter months (Table 2) of December (110 per day), January (96 per day), and February (76 per day) (Figure). The lowest percentage of exposures (16.8%) was observed during the summer. The majority (72.8%) of exposures occurred in homes; approximately 13.4% occurred at workplaces (Table 2).

Data regarding CO source, detector presence and activation, and toxic effects of CO exposures were missing for >30% of cases. Based on unweighted counts, the primary source of CO exposure was home heating systems (16.4%), which included furnaces, boilers, and unspecified heaters. Motor vehicles were reported as the second most common source of CO exposure (8.1%). CO detectors were reported present and activated in 17.8% of all exposures. More than half (54.1%) of all persons visited the ED with one or more symptoms indicating toxic effects of CO exposure, and 29.4% reported having two or more such symptoms. Headache (27.4%), nausea (14.6%), and dizziness (11.8%) were the most frequently reported symptoms.

Reported by: J Annest, PhD, T Haileyesus, MS, Office of Statistics and Programming, National Center for Injury Prevention and Control; J Clower, MPH, F Yip, PhD, A Stock, PhD, M Lucas, Div of Environmental Hazards and Health Effects, National Center for Environmental Health; S Iqbal, PhD, EIS Officer, CDC.

Editorial Note:

This report provides the most recent estimates of CO-related ED visits in the United States. During 2004--2006, an average of 20,636 ED visits for nonfatal, unintentional, non--fire-related CO exposures occurred each year. These estimates are higher than the estimated average of 15,200 CO-related ED visits per year reported for 2001--2003 (3). Better case ascertainment, increased reporting, or differential in sampling errors might account for this apparent increase; however, the data in this report do not allow drawing of conclusions regarding the cause of the increased visits. During 2004--2006, children aged <5 years had the highest estimated rates of CO-related ED visits and females had higher rates than males. These findings do not correspond to findings on fatal CO exposures, which indicate higher death rates among males and persons aged >65 years (2). Further research is needed regarding why certain population subgroups are at higher risk for CO exposure.

During 2004--2006, approximately 41% of reported cases of CO exposure occurred during the winter. This finding is consistent with previously published data on CO exposure (3,4,7). Increased use of home heating systems during winter, exposure to motor-vehicle exhaust by stranded motorists during blizzards, use of gasoline-powered generators during and after winter storms, and indoor use of charcoal grills, portable stoves, and space heaters all have contributed to the increase in CO exposures during winter (3,4,7). These findings highlight the importance of initiating and evaluating public health awareness campaigns for reducing CO exposures before and during winter months. The majority (72.8%) of patients were exposed in their homes; accordingly, prevention of residential CO exposures could substantially decrease CO-related morbidities (2,3).

The findings in this report are subject to at least three limitations. First, NEISS-AIP data did not include measurements of CO levels at the location of the incident or laboratory data for biologic indicators of CO exposure. ED documentation and narratives were used as a surrogate to assign exposures. Second, toxic effects, CO source, and detector presence and activation were not reported in NEISS-AIP for >30% of cases. Although the estimates for these variables might represent the population sampled, because of missing data, they might not represent national estimates. However, distributions for these factors were similar to those previously reported (3). Finally, the NEISS-AIP sample represents patients treated in hospital EDs; patients who sought treatment in other types of facilities (e.g., outpatient settings) or those who did not seek treatment were not included in this report.

Harmful exposures to CO, especially those occurring at home, are preventable. Basic preventive measures, including properly installing and maintaining home heating systems, installing CO detectors, and venting cooking and fuel-burning appliances, can minimize exposures (2,3). Additional public health messages geared toward at-risk populations might help reduce the number of CO exposures, especially residential and seasonal exposures. Continued surveillance of CO exposure will aid in developing prevention measures and targeted interventions.


  1. US Environmental Protection Agency. Basic information: carbon monoxide (CO). Available at
  2. CDC. Carbon monoxide--related deaths---United States, 1999--2004. MMWR 2007;56:1309--12.
  3. CDC. Unintentional non--fire-related carbon monoxide exposures---United States, 2001--2003. MMWR 2005;54:36--9.
  4. CDC. Carbon monoxide: a model environmental public health indicator. The National Workgroup on Carbon Monoxide Surveillance. Augusta, ME: Maine Department of Health and Human Services; 2006. Available at
  5. Raub JA, Mathieu-Nolf M, Hampson NB. Carbon monoxide poisoning---a public health perspective. Toxicology 2000;145:1--14.
  6. US Consumer Product Safety Commission. The NEISS sample (design and implementation) 1997 to present 2001. Bethesda, MD: US Consumer Product Safety Commission; 2001. Available at
  7. Daley WR, Smith A, Paz-Argandona E, Malilay J, McGeehin M. An outbreak of carbon monoxide poisoning after a major ice storm in Maine. J Emerg Med 2000;18:87--93.

* CO exposure and possible CO exposure cases likely included persons who had no toxic effects but who 1) visited EDs because they were involved in events in which they believed they might have been exposed to CO, 2) accompanied CO-exposed household members to EDs, or 3) were complying with recommendations of emergency response personnel (e.g., fire department personnel or emergency medical technicians) when high levels of CO were measured in their homes.

Bridged-race postcensal population estimates available at,, and

Table 1

TABLE 1. Average annual estimated number,* percentage, and
rate† of nonfatal, unintentional, non–fire-related carbon monoxide
(CO) exposure cases, by selected characteristics —
United States, 2004–2006
Characteristic No. (%) Rate (95% CI§)
Age group (yrs)¶
0–4 2,344 (11.4) 11.6 (7.7–15.4)
5–9 1,407 (6.8) 7.2 (3.8–10.5)
10–14 1,577 (7.6) 7.6 (3.3–11.8)
15–24 3,341 (16.2) 7.9 (5.7–10.1)
25–34 4,183 (20.3) 10.4 (6.5–14.3)
35–44 2,775 (13.5) 6.3 (4.2–8.4)
45–54 2,229 (10.8) 5.2 (3.4–7.1)
55–64 1,444 (7.0) 4.8 (2.9–6.6)
>65 1,328 (6.4) 3.6 (2.3–4.9)
Male 9,770 (47.3) 6.7 (5.0–8.4)
Female 10,866 (52.7) 7.2 (5.0–9.4)
Treated and released 18,646 (90.4) 6.3 (4.6–8.0)
Hospitalized/Transferred 1,695 (8.2) 0.6 (0.3–0.9)
Other/Unknown†† 294 (1.4) — —
Total 20,636 (100.0) 7.0 (5.1–8.8)
* National estimated number of persons with nonfatal, unintentional, non–
fire-related carbon monoxide exposures treated in hospital emergency
departments, based on 1,072 cases reported by the National Electronic
Injury Surveillance System – All Injury Program (NEISS-AIP).
† Rate per 100,000 population calculated using U.S. Census Bureau
postcensal population estimates with bridged-race categories.
§ Confidence interval.
¶ Age data were missing for eight cases.
** Numbers do not sum to total because of rounding of weighted data.
†† Estimates might be unstable because of unweighted counts of <20,
coefficient of variation >30%, or both.
Return to top.
Table 2

TABLE 2. Average annual estimated number* and percentage of
nonfatal, unintentional, non–fire-related carbon monoxide (CO)
exposure cases, by exposure status, season, and location of
incident — United States, 2004–2006
Characteristic No. (%) (95% CI†)
Total 20,636 (100.0) —
Exposure status
CO poisoning 14,127 (68.5) (48.0–88.9)
CO exposure 6,320 (30.6) (19.1–42.1)
Possible CO exposure§ 189 (0.9) —
Season¶ **
Winter 8,538 (41.4) (30.2–52.5)
Spring 4,175 (20.2) (12.3–28.2)
Summer 3,474 (16.8) (9.5–24.2)
Fall 4,448 (21.6) (14.0–29.2)
Location of incident**
Residence 15,030 (72.8) (53.7–91.9)
Workplace 2,769 (13.4) (6.0–20.8)
Other 1,162 (5.6) (3.1–8.2)
Unknown/Not stated/Missing 1,674 (8.1) (4.3–11.9)
* National estimated number of persons with nonfatal, unintentional, non–
fire-related carbon monoxide exposures treated in hospital emergency
departments, based on 1,072 cases reported by the National Electronic
Injury Surveillance System – All Injury Program (NEISS-AIP).
† Confidence interval.
§ Estimates might be unstable because of unweighted counts of <20,
coefficient of variation >30%, or both.
¶ Winter: December–February; Spring: March–May; Summer: June–
August; Fall: September–November.
** Numbers do not sum to total because of rounding of weighted data.
Return to top.

FIGURE. Average estimated number* of nonfatal, unintentional,
non–fire-related carbon monoxide exposure cases per day, by
month† — United States, 2004–2006
Return to top.

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Date last reviewed: 8/20/2008


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