Interpretation of 

Carboxyhemoglobin 

and Cyanide 

Concentrations in 

Relation to Aviation 

Accidents

Dennis V. Canfield

Arvind K. Chaturvedi

Kurt M. Dubowski

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

May 2005

Final Report

DOT/FAA/AM-05/9

Office of Aerospace Medicine

Washington, DC 20591
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

Technical Report Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

���������������

4. Title and Subtitle 5. Report Date 

������������������������������������������������������������������

�������������������������������

���������

6. Performing Organization Code 

7. Author(s) 8. Performing Organization Report No. 

���������������������������������������

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

��������������������������������������

��������������� 11. Contract or Grant No. 

������������������������

12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered 

�����������������������������

��������������������������������

����������������������������

��������������������� 14. Sponsoring Agency Code AAI-1, 

AAM-1

15. Supplemental Notes 

���������������������������������������������������������������������

16. Abstract 

�������������������������������������������������������������������������������������������������

��������������������������������������������������������������������������������������������������������������

���������������������������������������������������������������������������������������������������������

������������������������������������������������������������������������������������������������������������������������

����������������������������������������������������������������������������������������������������������

�����������������������������������������������������������������������������������������������������������������

�����������������������������������������������������������������������������������������������������

�������������������������������������������������������������������������������������������������������������

����������������������������������������������������������������������������������������������������������

�����������������������������������������������������������������������������������������������������������������

�����������������������������������������������������������������������������������������������������������������

����������������������������������������������������������������������������������������������������������������

�����������������������������������������������������������������������������������������������������������

���������������������������������������������������������������������������������������������������������

�������������������������������������������������������������������������������������������������������

�������������������������������������������������������������������������������������������������������������

�����������������������������������������������������������������������������������������������������������

������������

17. Key Words 18. Distribution Statement 

�������������������������������������������������������

����������������������������������

������������������������������������������������

������������������������������������������������������

����������������������������������������������

������������������������������

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

������������� ������������� ��

Form DOT F 1700.7 (8-72) Reproduction of completed page is authorized

1

INTERPRETATION OF CARBOXYHEMOGLOBIN AND CYANIDE

CONCENTRATIONS IN RELATION TO AVIATION ACCIDENTS

INTRODUCTION

Carbon monoxide poisoning has been the cause of 

death in humans since the first human discovered the use 

of fire for warmth, protection, and cooking. It continues 

today as a cause of death in humans. The introduction 

of household products containing synthetic nitrogenous 

materials in the early 19th century has increased the chance 

of hydrogen cyanide (a toxic combustion gas) production 

in a fire. The Civil Aerospace Medical Institute (CAMI) 

became directly involved in combustion toxicology in 

1970, when a Capitol International Airways DC-8 aircraft crashed on takeoff at Anchorage, Alaska, and blood 

specimens from the victims were found to contain cyanide 

(CN-) inhaled during the post-crash fire. The Federal 

Aviation Administration (FAA), as well as the public, 

wanted to know the source of the cyanide and what could 

be done to prevent a recurrence of cyanide exposures. At 

that time, little was known about the potential for aircraft 

interior materials to produce toxic combustion gases. 

Even more surprising was the lack of documented data 

on the toxicity of individual combustion gases. During 

the ensuing years, scientists at the FAA Technical Center 

and CAMI have systematically researched this issue to 

answer some important questions (2, 3, 9, 10). 

MATERIALS AND METHODS

The CAMI Bioaeronautical Sciences Research 

Laboratory receives biological specimens from almost 

all aviation accidents that occur in the United States. 

Blood specimens received from aviation accidents were 

analyzed for carboxyhemoglobin (COHb) and cyanide 

using spectrophotometric methods. The results of these 

tests are maintained in a computerized searchable database. 

All aviation cases received between 1990 and 2002 were 

searched for positive COHb and CN- findings using the 

“Bioaeronautical Sciences Research Laboratory, Forensic Case Management System” (copyright 1998-2004, 

DiscoverSoft Development, LLC, Oklahoma City, OK). 

The National Transportation Safety Board database was 

searched for information regarding the presence of fire 

during the study period. COHb levels at or above 10% 

and CN- levels at or above 0.25 ug/mL were considered 

positive. COHb concentrations were determined using 

a spectrophotometric method (1). CN- concentrations 

were determined using a spectrophotometric method 

(4, 7). In this study, we did not investigate postmortem 

changes of COHb and CN- prior to sample collection, 

or the stability of COHb and CN- in stored biological 

specimens. The Fractional Toxic Concentration (FTC) 

was calculated using the equation FTC = CN-(ug/mL)/

3.0 ug/mL + COHb(%)/70%, which is based on the 

Fractional Exposure Dose premise reflecting the additive 

effects of CN- and CO (2, 6). FTC values at or above 

1.0 are considered fatal. CN- concentrations at or above 

3.0 ug/mL are considered lethal concentrations (5). 

COHb concentration at or above 70% are considered 

to be lethal (8).

RESULTS

The laboratory found 223 cases (subjects) at or above 

10% COHb in blood out of 5945 cases (4%) received. 

The laboratory received specimens from 4482 aviation 

accidents. Not all of the 5945 cases received were suitable 

for CO analysis. CN- concentrations in blood ranged from 

0 to 6.3ug/mL, and positive CO concentrations were from 

10% to 60% carboxyhemoglobin. Fire was reported in 201 

of the cases with blood COHb at or above 10% and no 

fire in 21 of these cases. The fire status was not designated 

in 1 case. None of the 21 blood specimens from nonfire cases contained CN-

. The National Transportation 

Safety Board attributed 7 of the 21 non-fire fatalities to 

exhaust leaks. Nicotine or its metabolite was found in 9 

of the non-fire cases, and smoking may have contributed 

to the elevated blood COHb levels. All non-fire fatalities 

with COHb concentrations at or above 30% (4 cases) 

were determined by the NTSB to be from exhaust leaks. 

In some situations, damage to the aircraft prevented an 

examination of the exhaust system for leaks. FTCs values 

at or above 1.0 were only reached in 31 of the 223 cases 

and all of those 31 cases had elevated blood CN- levels. 

The correlation coefficient between FTC and CN- was 

0.92 (Figure 1); however, no such close correlation was 

observed between FTC and COHb with a correlation 

coefficient of 0.53 (Figure 2). Elevated blood CN- was 

detected in 103 of the 201 elevated CO cases with fire. 

Generally the high blood CN- concentrations were associated with the low COHb concentrations (Figure 3).
2 

The National Transportation Safety Board reported 

ground fires in 1716 aviation accidents, no fire in 3183 

aviation accidents, 71 in-flight fires, and fire unknown 

in 419 aviation accidents. Only 1 of the 71 in-flight 

fires had a person with a COHb at or above 10%. The 

1 in-flight fire case had 11% COHb and no CN- at or 

above 0.25 ug/mL. 

DISCUSSION AND CONCLUSION

Knowing the concentrations of COHb and CN- in 

blood is useful in evaluating the contribution of the 

combustion toxic gases, CO, and hydrogen cyanide 

(HCN) to the cause of aviation accidents. The presence 

of elevated COHb and CN- would indicate that the 

person was exposed to a fire environment while alive. 

Elevated COHb and CN- levels found in the blood of 

individuals who died on impact would lead an investigator to conclude that there was an in-flight fire, because 

CN- is not produced from an exhaust leak. An exhaust 

leak would be suspected in those cases with no fire, no 

CN-, and blood COHb levels above 20% because there 

is no plausible explanation for this concentration other 

than an exhaust leak. The fact that cyanide was found 

in all fire-related cases with an FTC level at or above 1.0 

would indicate that cyanide played an integral part in the 

fire-related fatal aviation accidents (Figure1). 

The fact that COHb at or above 10% was found in 

only 1 of the 71 in-flight fires investigated by the National Transportation Safety Board would indicate that 

in-flight fires are catastrophic events resulting in the death 

of the individual before COHb and CN- reach elevated 

concentrations.

However, the mere presence of COHb and CN- in 

blood, even in potentially toxic or lethal concentrations, 

does not per se indicate that exposure to CO and HCN 

caused the death. Fire-associated deaths are highly complex 

events. Flame, heat, and smoke are present in all fires, 

including CO and carbon dioxide. Other toxic gases are 

also generated in some fatal fires, including corrosive or 

irritant gases in others, in variable proportions. Fire-induced shock can contribute to mortality; reflex cardiac 

death from vagal inhibition has been associated with fires. 

In fire-associated fatal aviation crashes, there is also commonly blunt force and other forms of trauma such as 

fractures. Therefore, such deaths can involve asphyxia, 

blunt force trauma, chemical, thermal, and/or other injuries – sometimes in combination. Further, the combined 

effects of 2 or more irritant, corrosive, or toxic gases can 

be additive, or synergistic in some circumstances.

REFERENCES 

1. Canfield, D.V., Smith, M., Ritter, R.M., & Chaturvedi, 

A.K. (1999). Preparation of carboxyhemoglobin 

standards and calculation of spectrophotometric 

quantitation constants. J Forensic Sci, 44(2), 409-

12.

2. Chaturvedi, A.K., Sanders, D.C., Endecott, B.R., & 

Ritter, R.M. (1995). Exposures to carbon monoxide, hydrogen cyanide and their mixtures: Interrelationship between gas exposure concentration, time 

to incapacitation, carboxyhemoglobin and blood 

cyanide in rats. J Appl Toxicol, 15(5), 357-63.

3. Crane, C.R., Sanders, D.C., & Endecott, B.R. (1989). 

Inhalation Toxicology: IX. Times-to-incapacitation 

for rats exposed to carbon monoxide alone, to hydrogen cyanide alone, and to mixtures of carbon 

monoxide and hyrogen cyanide (Report No. DOT/

FAA/AM-89/4). Washington, DC: Federal Aviation 

Administration.1

4. Hartley, T.F., Philcox, J.C., & Willoughby, J. (1985). 

Two methods for monitoring the photodecomposition of sodium nitroprusside in aqueous and glucose 

solutions. J Pharm Sci, 74(6), 668-71.

5. Hartzell, G.E. (1989). Understanding of Hazards 

to Humans. In G.E. Hartzell (Ed.), Advances in 

Combustion Toxicology (Vol. 1, pp. 27). Lancaster: 

Technomic Publishing.

6. Hartzell, G.E., Switzer, W.G., & Priest, D.N. (1989). 

Modeling of toxicological effects of fire gases: V. 

Mathematical modeling of intoxication of rats by 

combined carbon monoxide and hydrogen cyanide atmospheres (Vol. 2). Lancaster: Technomic 

Publishing.

7. Lambert, J.L., Ramasamy, J., & Paukstelis, J.V. (1975). 

Stable reagents for the colorimetric determination 

of cyanide by modified konig reactions. Anal Chem, 

47(6), 916-8.

8. Nelson, G.L., Canfield, D.V., & Larson, J.B. (1993). 

Carbon monoxide and fatalities. In M.M. Hirschler 

(Ed.), Carbon monoxide and human lethality: Fire 

and non-fire studies (p. 186). London: Elsevier Applied Science.

1

This publication and all Office of Aerospace Medicine technical 

reports are available in full-text from the Civil Aerospace Medical 

Institute’s publications Web site: http://www.cami.jccbi.gov/aam400A/index.html
3

9. Sanders, D.C., Chaturvedi, A.K., Endecott, B.R., 

Ritter, R.M., & Vu, N. (1994). Toxicity of carboxyhemoglobin-hydrogen cyanide gas mixtures: 

exposure concentrations, time-to-incapacitation, 

carboxyhemoglobin, and blood cyanide parameters 

(Report No. DOT/FAA/AM-94/7). Washington, 

DC: Federal Aviation Administration. 1

10. Sanders, D.C., Endecott, B.R., Ritter, R.M., & 

Chaturvedi, A.K. (1993). Variations in time-to-incapacitation and carboxyhemoglobin values in rats 

exposed to two carbon monoxide concentrations 

(Report No. DOT/FAA/AM-93/7). Washington, 

DC: Federal Aviation Administration. 1

5

FIGURES

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

Blood Cyanide (ug/mL)

FTC

Figure 1. Fire cases showing relationship of FTC to cyanide concentrations. 

0.00

0.50

1.00

1.50

2.00

2.50

10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 60.00

COHb (%)

FTC

Figure 2. Fire cases showing relationship of FTC to COHb concentrations. 
6

Fire

0.00

1.00

2.00

3.00

4.00

5.00

6.00

10 15 20 25 30 35 40 45 50 55 60 65

Blood COHb (%)

Blood Cyanide ug/mL

Figure 3. Fire cases showing relationship of carboxyhemoglobin to cyanide concentrations.