|Home | About | Journals | Submit | Contact Us | Français|
Carbon monoxide (CO) is a colorless, odorless, and tasteless toxic gas produced by incomplete combustion in fuel-burning devices. People with CO poisoning often overlook the symptoms (e.g., headache, nausea, dizziness, or confusion), yet undetected exposure can be fatal.1 In several countries, the maximum safe level of exposure to CO for a one-hour period is between 32 and 40 parts per million.2–5 In China, some health education programs claim that adequate ventilation alone can be an effective preventive measure against CO poisoning. However, this claim is not very comprehensive, as it takes other specific measures to prevent CO poisoning. Previous literature only noted that CO poisoning occurred in the absence of good ventilation and did not elaborate on the associated behavioral and conditional factors. This study attempted to determine the risk factors of behaviors and conditions that cause CO poisoning.
At the beginning of 2008, a massive ice storm hit a large region in Southern China. Soon after the disaster, the Chinese Centers for Disease Control and Prevention (CDC) conducted a series of rapid-filed investigations to determine the impact of the ice storm on public health in the affected areas. Preliminary results showed that non-occupational CO poisoning cases (hereafter, CO poisoning cases) increased dramatically (70 cases in 2008 vs. five cases in 2007 during the same time period) in three hospitals in Guiyang County, Hunan Province, which suffered serious damage from the ice storm (Figure). From April 14 to 25, 2008, we conducted an in-depth investigation to study the extent of CO poisoning cases in Guiyang County during the ice storm and to identify risk factors of CO poisoning.
Because the 2008 ice storm lasted from January 13, 2008—when rain, snow, and frost began—to March 6, 2008—when the electric power supply was restored—we defined a probable CO poisoning case as someone living in Guiyang County who had (1) non-occupational exposure to CO poisoning between January 13 and March 6, 2008, and (2) at least two of the following symptoms: fatigue, headache, dizziness, nausea, vomiting, cyanosis lips, disturbance of consciousness, and shock. A confirmed CO poisoning case was a probable case whose carboxyhemoglobin concentration in the blood was higher than 10% (normal concentration is <2% for nonsmokers and 5%–9% for smokers).6
A CO poisoning event may include single or multiple victims. If a number of CO poisoning cases occur during the same incident, these CO poisoning cases are considered one CO poisoning incident.
The limit of exposure to CO is 5% concentration of carboxyhemoglobin in the blood.7 We categorized cases into three groups according to the severity of poisoning. The first group experienced mild poisoning and had at least one of the following symptoms: severe headache, dizziness, floppy limbs, nausea, vomiting, mild or moderate disturbance of consciousness, or more than a 10% concentration of carboxyhemoglobin in the blood. The second group experienced moderate poisoning and included mild cases with disturbance of consciousness, those who recovered after rescue without significant complications, or those who had a 30%–50% concentration of carboxyhemoglobin in the blood. The third group experienced severe poisoning and included cases with any of the following symptoms: (1) a disturbance of consciousness up to the cerebral cortex or deep unconscious state; (2) a disturbance of consciousness with the presence of any of the following: cerebral edema, shock or severe myocardial damage, pulmonary edema, respiratory failure, upper gastrointestinal bleeding, focal brain damage (e.g., the cone system), or extrapyramidal signs of damage; and (3) more than 50% concentration of carboxyhemoglobin in the blood.8
We searched for cases in 45 hospitals in Guiyang County by collecting clinical symptoms records from outpatient logs and laboratory results. However, we found four cases in communities or villages where the case was located. Because we found four cases without a hospital record in the investigation, we only collected clinical symptoms from cases or cases' families.
We considered that residents in urban and rural areas may have had different behaviors when using coal-burning heating devices. As such, we conducted a 1:3 frequency-matched (by urban and rural area) case-control study to identify the risk factors for CO poisoning at the household level. Cases included households that had coal-burning stoves with CO poisoning (n=43, 93%). Controls (n=129) were households that had coal-burning stoves without CO poisoning. Controls were randomly selected by the village or community where the case households were located. We used household questionnaires to identify specific factors associated with coal burning at home (e.g., disposal of stove fire, gas appliance installations, fuel, ventilation, and other factors).
Meteorological data and information on power outages were collected from the local weather bureau and electric power company. We entered data using EpiData 3.09 and analyzed it using EpiInfo™ 188.8.131.52 We analyzed the differences between case and control households using Chi-square tests (>5 observations in all cells), Fisher's exact tests (≤5 observations in ≥1 cell), or t-tests. All tests were two-sided and an alpha level of p<0.05 was considered statistically significant.
We identified 89 cases from 86 CO poisoning incidents from the ice storm of 2008. Among them, 83 incidents involved one case and the other three involved two cases. We found a total of 85 cases from 45 hospitals, while the remaining four were from residential communities without hospital records. The cases ranged in age from 4 to 78 years. The mean age of cases was 39 (95% confidence interval 35, 43). About 73% of the cases were female. The case-fatality rate was slightly more than 3%. During the same time period in 2007, only 17 CO poisoning cases were reported. We interviewed 46 cases, among which 43 were coal-burning cases (Table 1). The remaining cases were not interviewed because a detailed address was not provided. We used the 43 coal-burning CO poisoning cases as the case group in our case-control study.
Table 2 shows a list of clinical symptoms of the 85 hospital-reported CO poisoning cases. Fatigue, headache, dizziness, and nausea were the main reported symptoms. Mild symptoms were of a higher proportion. Table 3 compares the incidence of CO poisoning in urban and rural areas, showing the incidence of CO poisoning to be higher in urban areas than in rural areas. Table 4 lists the severity of the 43 coal-burning cases, indicating that most cases (79%) were mild. Based on our finding that coal burning was the primary source of CO poisoning, we were able to identify the risk factors for indoor CO poisoning in the case-control study.
Table 5 lists risk factors for the occurrence of CO poisoning when burning coal in the house. Using a mud stove, putting the stove in the bedroom, having sealed window gaps, and having unsealed tunnel joints were risk factors for indoor CO poisoning. Tunnel clearing, awareness of CO poisoning, knowledge of CO poisoning prevention, and proper ventilation measures were all shown to be protective factors. When the mud stove was in use, CO leaked out at the top and the bottom, as shown in the Photo. Table 6 shows the dose-response effect of the frequency of ventilation. We found a negative correlation between the frequency of opening windows or doors and the possibility of CO poisoning.
During the 2008 ice storm, there was a sharp increase in CO poisoning occurrence than during the same time period in 2007 in Guiyang County. Inadequate ventilation measures, such as closing windows and doors when using coal-burning heaters, sealing window gaps, and using stoves inappropriately, were all risk factors for CO poisoning. Protective factors included a clear smoke tunnel, adequate ventilation, awareness of CO poisoning, and knowledge of prevention.
As a comparison, the CO source in Guiyang County (i.e., coal burning) during the 2008 ice storm differed from the CO source in Maine during its 1998 ice storm. In Maine, the CO source was gasoline-powered electric generators in 30 incidents (71%), kerosene heaters in eight incidents (19%), and propane heaters in four incidents (10%).11 We are not aware of the number of CO incidents caused by using gasoline-powered electric generators to keep warm in Maine, but we know that these electric generators were a risk factor. Studies in Guiyang County and Maine have demonstrated the importance of knowledge regarding how to keep warm safely in cold weather.
We believe that the local government has a responsibility to educate residents about appropriate usage of CO-generating heating appliances during cold weather. The local government should remind people who use these heating appliances at night of the importance of seeking fresh air immediately when CO exposure is suspected.12
In this study, we identified inadequate ventilation as a risk factor for CO poisoning when using CO-generating heating appliances. This result was similar to the investigation of two cases of non-occupational CO poisoning in Shanghai, China.13 We also identified a negative correlation between the frequency of opening windows or doors when using CO-generating heating appliances and the possibility of CO poisoning. Based on interviews concerning the average frequency of ventilation in using heating appliances, we defined a frequency of one to three times per use of heating appliances as inadequate ventilation, four to seven times as frequent ventilation, and more than seven times as adequate ventilation. It showed that with increased frequency of ventilation, the probability of CO poisoning decreases.
A large population in Guiyang County uses mud stoves for heating. When this type of stove is used, CO gas leaks from the top and bottom of the stove (Photo). Using CO-generating heating appliances in the bedroom increases the possibility of CO poisoning. Sealing window edges when using CO-generating heating appliances leads to poor ventilation. Using heating appliances with unsealed pipeline joints causes poisonous CO gas to leak. These dangerous behaviors or conditions ultimately cause CO poisoning.
This study had several limitations. We did not determine whether or not the lack of a smoke-exhaust system when using heating devices was a risk factor for CO poisoning (no statistical significance). However, common sense indicates that the lack of a flue system when using heating devices is likely to enhance CO exposure. It is possible that the control group ventilated through doors or windows when using heating equipment without smoke-exhaust attachments, while the case group did not ventilate.
We also found that the mud stove, which has no flue, was used more often in the case group than in the control group. The ventilation gate at the bottom of the stove is usually covered by an iron sheet and cannot be closed tightly. As such, CO gas would easily leak from the top and bottom of the stove when burning. We considered this stove to be a CO poisoning hazard even with good ventilation.
Also, we did not thoroughly investigate why the CO poisoning rate was higher in urban than in rural areas. It could be that the ventilation condition in rural areas is better, as we found that many rural households did not have glass on their windows. It is also possible that we underestimated the CO poisoning rate in rural areas, as we did not include cases reported by some rural hospitals without clinical symptom records and laboratory results. In the case-control study, the number of cases (incidents) was not sufficient, and some cases could not be interviewed because a detailed address was not provided.
The cause of death in many cases of CO poisoning can be attributed to faulty equipment or a lack of awareness of risk. Proper installation and regular maintenance of home-heating appliances; cleaning of obstructed chimneys; and careful attention to ventilation during the use of butane and kerosene space heaters, wood stoves, and charcoal grills will reduce risk.14 These effective measures can be applied to prevent the occurrence of CO poisoning cases.
Based on the findings of this study, we suggest that the local government should promote health education, informing local residents about the dangers of CO poisoning, with a focus on prevention. Some points should be emphasized. First, smoke-exhaust pipes must be cleaned frequently. This suggestion is similar to that of the investigation of 327 CO poisoning cases in He Ji Hospital.15 Second, the smoke tunnel joints should be sealed. Third, mud stoves should not be used, as they can easily leak CO gas. Fourth, adequate ventilation should be ensured when using stoves. Fifth, stoves should not be used in the bedroom, especially when sleeping. The government should also put more effort into banning the sale of shoddy stoves. Future research should also examine the reason for the high CO poisoning rate in urban areas.
The authors are indebted to staff at the Chenzhou Centers for Disease Control and Prevention (CDC) and the Guiyang CDC for their assistance during this investigation. The authors also thank Dr. Zhu Baoping and Dr. Robert E. Fontaine, U.S. CDC, who provided many valuable suggestions and helped revise this article.