NIOSH HEALTH HAZARD EVALUATION REPORT 

HETA #2004-0123-2939 

City of Liberal Animal Shelter 

Liberal, Kansas 

May 2004

DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports
ii 

PREFACE

The Hazard Evaluation and Technical Assistance Branch (HETAB) of the National Institute for 

Occupational Safety and Health (NIOSH) conducts field investigations of possible health hazards in 

the workplace. These investigations are conducted under the authority of Section 20(a)(6) of the 

Occupational Safety and Health (OSHA) Act of 1970, 29 U.S.C. 669(a)(6) which authorizes the 

Secretary of Health and Human Services, following a written request from any employers or 

authorized representative of employees, to determine whether any substance normally found in the 

place of employment has potentially toxic effects in such concentrations as used or found. 

HETAB also provides, upon request, technical and consultative assistance to federal, state, and local 

agencies; labor; industry; and other groups or individuals to control occupational health hazards and 

to prevent related trauma and disease. Mention of company names or products does not constitute 

endorsement by NIOSH. 

ACKNOWLEDGMENTS AND AVAILABILITY OF REPORT

This report was prepared by Jane B. McCammon, MS, CIH, Director of the HETAB Denver Field 

Office, Division of Surveillance, Hazard Evaluation and Field Studies (DSHEFS). Desktop 

publishing was performed by Lisa Maestas. Review and preparation for printing were performed by 

Penny Arthur. 

Copies of this report have been sent to employee and management representatives at the City of 

Liberal Animal Shelter. This report is not copyrighted and may be freely reproduced. The report 

may be viewed and printed from the following internet address: 

www.cdc.gov/niosh/hhe/hhesearch.html. Single copies of this report will be available for a period of 

three years from the date of this report. To expedite your request, include a self-addressed mailing 

label along with your written request to: 

NIOSH Publications Office 

4676 Columbia Parkway 

Cincinnati, Ohio 45226 

800-356-4674 

After this time, copies may be purchased from the National Technical Information Service (NTIS) at 

5825 Port Royal Road, Springfield, Virginia 22161. Information regarding the NTIS stock number 

may be obtained from the NIOSH Publications Office at the Cincinnati address.

For the purpose of informing affected employees, copies of this report 

shall be posted by the employer in a prominent place accessible to the 

employees for a period of 30 calendar days.
iii 

Highlights of the NIOSH Health Hazard Evaluation

Evaluation of Carbon Monoxide (CO) Exposures during Euthanasia of Animals at the City of Liberal, 

Kansas, Animal Shelter

The City of Liberal, Kansas asked NIOSH for help in evaluating CO exposures of employees that operate their CO 

euthanasia chambers. NIOSH assisted the City by measuring CO near the chamber and in the breathing zone of workers 

that operated the empty chamber for this investigation. 

 

! We measured CO near the chamber and in the 

breathing zone of two employees. 

! We gathered information for the City about 

proper euthanasia methods; commercially 

available CO chambers; and the death of an 

operator of a homemade chamber in 

Chattanooga, Tennessee. 

! We provided educational materials about 

occupational safety and health to the workers. 

! We asked current and past employees about how 

they operated the chamber and symptoms they 

may have had when the chamber was operated. 

What NIOSH Found

! The CO gas chambers are a potentially serious 

hazard for the employees. 

! The death associated with the use of a similar 

chamber in Tennessee and the CO levels 

measured by NIOSH in this investigation 

indicate that the use of home-made uncontrolled 

gas chambers for animal euthanasia is not 

acceptable. 

 

! Use lethal injection instead of CO for euthanasia. 

! Follow existing euthanasia guidelines. 

! Purchase an acceptable, commercially-produced 

CO chamber for euthanasia if the decision is 

made to continue the use of CO. 

! Educate employees about risks and symptoms of 

CO exposure. 

! Assure safe use of such chambers by following 

all manufacturers’ guidelines related to 

maintenance and procedures.

! Comply with all guidelines and requirements for 

confined space relevant to operation of CO 

euthanasia chambers. 

What Liberal Animal Shelter Employees Can Do

! Immediately report symptoms of CO exposure to 

designated health and safety personnel (if CO 

use is continued). 

! Follow work practices defined in the written 

operational procedures for CO euthanasia 

chambers. 

What NIOSH Did What City Of Liberal Managers Can Do

What To Do For More Information:

We encourage you to read the full report. If you 

would like a copy, either ask your health and safety 

representative to make you a copy or call 

1-513-841-4252 and ask for 

 HETA Report #2004-0123-2939
iv 

Health Hazard Evaluation Report 2004-0123-2939 

City of Liberal Animal Shelter 

Liberal, Kansas 

May 2004 

Jane McCammon, MS, CIH

SUMMARY

In February 2004, the City of Liberal, Kansas, asked the National Institute for Occupational Safety and 

Health (NIOSH) for help in evaluating carbon monoxide (CO) exposures of employees that operate two 

euthanasia chambers at the city-run animal shelter. The Kansas Animal Health Department suspended 

use of the unvented chambers during a licensing inspection. The City requested NIOSH assistance in 

measuring employee CO exposure during operation of the chambers and modifying the chambers so that 

they could be again be used in a manner that would be acceptable to the Kansas Animal Health 

Department. 

In March 2004, NIOSH responded to the request by using direct-reading monitors to measure CO near the 

two chambers and in the breathing zone of workers that operated the empty chambers specifically for this 

investigation. (Operation of the chamber for this investigation was authorized by the licensing 

organization.) Past and present employees were interviewed about methods used to operate the chambers 

and symptoms experienced when the chambers were operated. NIOSH also gathered the following 

information: (1) details about the death of a Tennessee animal shelter worker during operation of a similar 

chamber; (2) the extent of use of CO euthanasia chambers across the United States; (3) policies and 

guidelines of the American Veterinary Medical Association (AVMA), the Humane Society of the United 

States (HSUS), and the National Animal Control Association (NACA) related to the appropriate use of 

CO for euthanasia in animal shelters; and (4) classification of such chambers as a “confined space” by 

NIOSH and the Occupational Safety and Health Administration (OSHA). 

When CO was introduced into the chamber, CO concentrations near the chamber rapidly exceed the 

NIOSH ceiling limit of 200 parts per million (ppm). The NIOSH Immediately Dangerous to Life and 

Health value of 1,200 ppm was exceeded in one instance. Peak CO concentrations in the general area 

during CO introduction were 800 ppm to 950 ppm, and greater than 1,200 ppm (the maximum range of 

the sampling instruments). Slowing the delivery rate of CO resulted in much lower concentrations near 

the chamber. When the chambers were opened, CO concentrations in the general area of the chambers 

ranged from 400 ppm to >1,200 ppm for several minutes. CO concentrations inside the chambers 

remained above 1,200 for an undetermined length of time. 

Employee exposures during this investigation were impacted by the fact that normal tasks were not 

conducted, and also that employees appropriately removed themselves from exposure when their CO 

monitors alerted them to the severity of the hazard. During two occasions of chamber operation, 

employees were exposed to maximum CO concentrations of 380 ppm and 945 ppm before they were able 

to move to safer locations. 
v 

NIOSH noted several other health and safety deficiencies. There were no confined space entry 

procedures. Respiratory protection was not available. Operating procedures were undefined. There was 

no hazard communication program. There was no program for employee training. There were no 

warning signs related to the CO hazard. There was no emergency action plan. The employer had not 

assessed the workplace to determine if hazards were present. 

The NIOSH investigator concluded that the use of homemade CO chambers, such as the 

one investigated here, presents an unacceptable health risk to animal shelter employees. 

Suggestions for reducing this risk are included in the Recommendations section of this 

report. Modification of the existing chambers was not an acceptable control method, and 

thus no recommendations were provided in that regard. 

Keywords: SIC 0752 (Animal specialty services / animal shelters), carbon monoxide, CO, 

carboxyhemoglobin, COHb, animal control officer, euthanasia, gas chamber 
Table of Contents 

Preface..........................................................................................................................................................ii

Acknowledgments and Availability of Report..........................................................................................ii

Highlights of the NIOSH Health Hazard Evaluation .............................................................................iii

Summary.....................................................................................................................................................iv

Introduction.................................................................................................................................................1

Background .................................................................................................................................................1

Process Description.....................................................................................................................................2

Methods........................................................................................................................................................3

Health Effects & Evaluation Criteria........................................................................................................3

Results ..........................................................................................................................................................6

Discussion ....................................................................................................................................................7

Conclusion ...................................................................................................................................................8

Recommendations.......................................................................................................................................8

References....................................................................................................................................................9

Tables & Figures.......................................................................................................................................11

Attachment 1 .............................................................................................................................................18

Attachment 2 .............................................................................................................................................19
Health Hazard Evaluation Report No. 2004-0123-2939 Page 1 

INTRODUCTION

On January 27, 2004, as part of a regular 

licensing inspection, the City of Liberal Animal 

Shelter was informed by the Kansas Animal 

Health Department that its carbon monoxide 

(CO) euthanasia chambers could no longer be 

used due to inadequate design. Deficiencies 

noted were the absence of purging and lack of 

ventilation systems. 

In response, a local veterinarian and the State 

Veterinarian suggested that the City request 

assistance from the National Institute for 

Occupational Safety and Health (NIOSH). 

Citing concern about employee exposure 

associated with use of the chambers as designed 

(no exhaust), the city requested assistance of 

NIOSH on February 2, 2004. NIOSH responded 

by conducting an investigation at the animal 

shelter on March 3 and 4, 2004. 

BACKGROUND

National euthanasia statistics are difficult to 

pinpoint because animal care and control 

agencies are not uniformly required to keep 

statistics on the number of animals taken in, 

adopted, euthanized, or reclaimed. There is no 

reporting structure enabling collection of such 

figures. However, it is estimated that shelters 

use euthanasia on 3 – 4 million dogs and cats 

annually to control the animal population in the 

United States.1

 This figure represents 

approximately 50% to 70% of all animals that 

enter a shelter.2

Regulation of euthanasia in animal shelters 

varies nationwide, but is often covered by a city, 

county, or state licensing agency. These 

agencies often incorporate policies of recognized 

animal welfare agencies for guidelines for 

acceptable euthanasia methods. Three such 

agencies are the American Veterinary Medical 

Association (AVMA), the Humane Society of 

the United States (HSUS), and the National 

Animal Control Association (NACA). These 

groups agree that injection of sodium 

pentobarbital is the preferred euthanasia method, 

but also allow for the use of CO euthanasia 

chambers. 

The Panel on Euthanasia convened by AVMA3

declared that the use of injectable agents is the 

most desirable euthanasia method for companion 

animals, but also stated that it is acceptable to 

use CO for individual animal or mass euthanasia 

for dogs, cats, and other small mammals. HSUS 

policies state that the use of CO, when delivered 

by a commercially manufactured and equipped 

chamber, is an acceptable method of euthanasia 

for some animals.4

 NACA acknowledges that 

there are agencies legally restricted in their 

ability to obtain sodium pentobarbital, and in 

such cases, if CO is the euthanasia method of 

choice, the shelter must use CO specifically 

designated for use in euthanasia. 

Occupational safety and health concerns 

associated with the use of CO euthanasia 

chambers heightened in 2000, when a 39-yearold Animal Humane Officer died while 

operating a homemade concrete block chamber 

in a Chattanooga, Tennessee shelter. He was an 

experienced operator of the chamber; he had 

used it for approximately 10 years. As part of 

the investigation of this fatality, the Tennessee 

Occupational Safety and Health Administration 

(TOSHA) determined that the animal control 

officer was exposed to CO in excess of 70,000 

parts of CO per million parts of air (ppm), which 

is rapidly fatal. Tennessee officials ruled that 

the chamber was a confined space, subject to 

permitting under OSHA regulations. The 

chamber operated by the Tennessee victim was 

slightly smaller (4’ X 4’ X 4’) than the large 

chamber used in Kansas (4’ X 4’ X 5’). 

The animal humane officer in Tennessee was 

found dead at the scene by two coworkers. 

Because his death was not witnessed, little is 

known about the circumstances under which he 

died. His carboxyhemoglobin (COHb) 

concentration reported in the medical examiner 

and laboratory report was >70%. The TOSHA 

inspection led to fines for extensive violations, 

including: no confined space permitting and 

procedures, absence of a hazard communication 

program; employee overexposure to CO; 

absence of an implemented written emergency 
Page 2 Health Hazard Evaluation Report No 2004-0123-2939. 

action plan; failure to properly assess workplace 

hazards; inadequate employee training; and 

inadequate signage. 

Liberal City representatives were under the 

impression that the use of CO euthanasia 

chambers was declining, and noted that theirs 

was the only one used in Kansas. A search of 

the internet indicated that CO chambers are 

extensively used in municipal shelters, 

particularly in counties removed from large city 

populations. Table 1 describes the extent of use 

or banning of CO chambers for euthanasia in 

U.S. animal shelters, listed by state. The listing 

was derived from various sources of internet 

information, including media coverage, letters, 

municipal government reports, etc. 

PROCESS DESCRIPTION

The City of Liberal Animal Shelter has been in 

operation for approximately 10 years, and was 

previously managed and/or owned by at least 

four private groups. The City of Liberal took 

over operation of the shelter in October 2003. 

The shelter employs 4 full-time employees, and 

has 27 animal bays, 12 portable dog cages, and 

18 cat cages. Approximately 60 animals are 

accepted by the shelter each month, with 

approximately 10 being claimed by their owner 

or adopted by new owners. Prior to the recent 

State inspection, most of the remaining animals 

were placed in one of two CO euthanasia 

chambers during twice weekly operation of the 

chambers. The same person has operated the 

chambers for the past nine months. 

The chambers are shown in Figures 1 and 2. 

Both chambers are housed in an outdoor 

enclosure (wooden fence sections approximately 

6 feet in height, no roof) adjacent to the 

building. Both homemade chambers are 

fabricated from sheet metal. The “dog chamber” 

measures 4’ X 4’ X 5’ and houses a maximum 

of seven caged dogs. The “cat chamber” 

measures 2’4” X 2’8” X 3’1” and houses a 

maximum of 5 caged cats. The chamber doors 

are fitted with rubber hosing fashioned as a seal. 

There is a plexiglass viewing window in the 

door. The operator loads animals into the 

chamber, and then locks the door with large 

butterfly clamps that are manually screwed tight 

to allow pressurization of the chamber. 

A tank of liquefied CO stands between the two 

cages. CO gas is introduced into either chamber 

by manual operation of valves controlling the 

flow of pure CO through small piping that led to 

two locations on each chamber (one at the 

bottom and top of each chamber). Each 

chamber is an otherwise solid metal box with a 

hinged door covering the full face of the 

chamber. Because there were no written 

standard operating procedures and there was 

considerable confusion about operating 

parameters, employees had no choice but to 

guess at how long to leave the valves open, 

whether to let the CO into the chamber very fast 

or gradually, how to open the bolted door safely, 

and how long to wait before removing the dead 

animals from the chamber. 

The delivery rate of CO into the chamber varied 

widely. The operator opens the valve, 

pressurizing the chamber at varying rates for 20 

to 30 seconds. At this point, the current operator 

leaves the area, while animals are left in the COfilled chamber for approximately 20 to 30 

minutes. The butterfly clamps are then loosened 

(with the operator leaning against it to keep the 

CO contained as long as possible) and the door 

is opened. The operator reports that this is 

usually done while holding your breath to avoid 

the CO. 

The doors remain open until the chamber is 

thought to have aired out (approximately 20 

minutes), at which point the dead animals are 

removed from the cages in the chambers. 

Interview of a past shelter employee indicated 

that very different methods were used to operate 

the chamber during his employment. He 

allowed the chamber to air out “long enough to 

be safe” which he considered to be 1 to 2 

minutes. He described experiencing symptoms 

that were consistent with CO overexposure, but 

also commented that it was difficult to know if 

the symptoms were related to CO or stress and 

smell associated with animal euthanasia . 
Health Hazard Evaluation Report No. 2004-0123-2939 Page 3 

Since the suspension of use of the CO chambers, 

the City has been conducting euthanasia by 

lethal injection through contracted veterinary 

services, which is cost-prohibitive for this 

financially troubled shelter. For example, the 

shelter used these services (comprised of once-aweek visits from the veterinarian to euthanize 41 

animals) for three weeks prior to the NIOSH 

investigation at a cost of $335.00 (which does 

not account for costs such as employee time, 

etc.). In comparison, the cost of use of the CO 

chambers from October 14, 2003, through 

January 7, 2004, was approximately $375.00 

(for 2 1⁄2 tanks of CO). 

METHODS

Employee CO exposures were measured using 

ToxiUltra Atmospheric Monitors (Biometrics, 

Inc.) with CO sensors. Similar monitors were 

also placed in the area of the CO chambers 

(generally one or two on top of the chamber 

operating at the time, one on the adjacent 

chamber, and one to the left of the small animal 

chamber). 

All ToxiUltra CO monitors were zeroed and 

calibrated before each use according to the 

manufacturer’s recommendations. These 

monitors are direct-reading instruments with 

data logging capabilities. The instruments were 

operated in the passive diffusion mode, with a 

one-minute sampling interval. The highest 

instantaneous reading of these instruments is 

approximately 1000 ppm. 

During operation, there were no animals in 

either chamber, and the operator did not perform 

the usual tasks associated with loading and 

unloading the chamber and disposing of the dead 

animals. During the morning of March 3, a light 

rain was falling, necessitating the use of a tarp to 

cover the area housing the chambers and related 

monitoring equipment. This tarp was purchased 

specifically for this testing, as the shelter does 

not normally conduct euthanasia in the rain. The 

tarp was removed prior to the afternoon 

operation of the chambers. 

On March 3, the small chamber was operated 

first (singly), with two full cycles of closing the 

chamber, introducing CO, allowing it to sit 

pressurized for a number of minutes, and then 

opening the chamber door. The large chamber 

was then operated (singly) for two full cycles. 

On March 4, a steady rain fell preventing 

sampling until late in the day. This sampling 

was also carried out in the absence of a tarp 

covering the area. One full cycle of operation of 

the large chamber was accomplished on this 

afternoon. 

HEALTH EFFECTS & 

EVALUATION CRITERIA

Health Effects of CO Exposure 

CO is a colorless, odorless, tasteless gas 

produced by incomplete burning of carboncontaining materials such as gasoline or propane 

fuel. The initial symptoms of CO poisoning 

may include headache, dizziness, drowsiness, or 

nausea. Symptoms may advance to vomiting, 

loss of consciousness, and collapse if prolonged 

or high exposures are encountered. If the 

exposure level is high, loss of consciousness 

may occur without other symptoms. Coma or 

death may occur if high exposures continue.5,6,7, 

8, 9, 10 The display of symptoms varies widely 

from individual to individual and may occur 

sooner in susceptible individuals such as young 

or aged people, people with preexisting lung or 

heart disease, or those living at high altitudes. 

Exposure to CO limits the ability of the blood to 

carry oxygen to the tissues by binding with the 

hemoglobin to form COHb. Once exposed, the 

body compensates for the reduced bloodborne 

oxygen by increasing cardiac output, thereby 

increasing blood flow to specific oxygendemanding organs such as the brain and heart. 

This ability may be limited by preexisting heart 

or lung diseases that inhibit increased cardiac 

output. 

Blood has an estimated 210-250 times greater 

affinity for CO than oxygen, thus the presence of 
Page 4 Health Hazard Evaluation Report No 2004-0123-2939. 

CO in the blood can interfere with oxygen 

uptake and delivery to the body. Once absorbed 

into the bloodstream, the half-time of CO 

disappearance from blood (referred to as the 

“half-life”) varies widely by individual and 

circumstance (i.e., removal from exposure, 

initial COHb concentration, partial pressure of 

oxygen after exposure, etc.). Under normal 

recovery conditions breathing ambient air, the 

half-life can be expected to range from 2 to 6.5 

hours. This means that if the initial COHb level 

were 10%, it could be expected to drop to 5% in 

2 or more hours, and then 2.5% in another 2 or 

more hours. If the exposed person is treated 

with oxygen, as happens in emergency 

treatment, the half-life time is decreased again 

by as much as 75% (or to as low as 

approximately 40 minutes). Delivery of oxygen 

under pressure (hyperbaric treatment) reduces 

the half-life to approximately 20 minutes. 

Carboxyhemoglogin measurements are typically 

made when a patient arrives at the hospital or a 

body arrives at the morgue. The time elapsing 

between CO exposure and COHb analysis is a 

likely explanation of the poor clinical correlation 

between symptoms of CO poisoning and COHb 

level. This complicates any discussion of 

“normal” COHb concentrations and COHb 

concentrations associated with symptoms. 

COHb concentrations among unexposed nonsmokers are typically between 1% - 2%, and for 

smokers typically between 3%-8%.11 Exposures 

resulting in COHb concentrations less than 10% 

usually cause no appreciable symptoms; 

exposures resulting in COHb greater than 50% 

are often fatal. However, COHb levels as low as 

1% - 10% have been associated with severe 

symptoms (including prolonged loss of 

consciousness), and COHb levels as high as 

47% resulted in no associated loss of 

consciousness.12 Thus, elevated COHb 

concentration can only be used to confirm 

exposure, not to confirm poisoning severity. 

CO Occupational Exposure 

Criteria 

As a guide to the evaluation of the hazards posed 

by workplace exposures, NIOSH field staff 

employ environmental evaluation criteria for the 

assessment of a number of chemical and 

physical agents. These criteria are intended to 

suggest levels of exposure to which most 

workers may be exposed up to 10 hours per day, 

40 hours per week for a working lifetime 

without experiencing adverse health effects. It 

is, however, important to note that not all 

workers will be protected from adverse health 

effects even though their exposures are 

maintained below these levels. A small 

percentage may experience adverse health 

effects because of individual susceptibility, a 

pre-existing medical condition, and/or 

hypersensitivity (allergy). In addition, some 

hazardous substances may act in combination 

with other workplace exposures, the general 

environment, or with medications or personal 

habits of the worker to produce health effects 

even if the occupational exposures are controlled 

at the level set by the criterion. These combined 

effects are often not considered in the evaluation 

criteria. Also, some substances are absorbed by 

direct contact with the skin and mucous 

membranes, which potentially increases the 

overall exposure. Finally, evaluation criteria 

may change over the years as new information 

on the toxic effects of an agent become 

available. 

The primary sources of environmental 

evaluation criteria for the workplace are: (1) 

NIOSH Recommended Exposure Limits 

(RELs)13, (2) ACGIH® Threshold Limit Values 

(TLVs®)14, and (3) the U.S. Department of 

Labor, Occupational Safety and Health 

Administration (OSHA) Permissible Exposure 

Limits (PELs).15 Employers are encouraged to 

follow the OSHA limits, the NIOSH RELs, the 

ACGIH TLVs, or whichever is the more 

protective criterion. 

OSHA requires an employer to furnish 

employees a place of employment that is free 

from recognized hazards that are causing or are 

likely to cause death or serious physical harm.16 

Thus, employers should understand that not all 

hazardous chemicals have specific OSHA 

exposure limits such as PELs and short-term 

exposure limits (STELs). An employer is still 

required by OSHA to protect their employees 
Health Hazard Evaluation Report No. 2004-0123-2939 Page 5 

from hazards, even in the absence of a specific 

OSHA PEL. 

A time-weighted average (TWA) exposure 

refers to the average airborne concentration of a 

substance during a normal 8- to 10-hour 

workday. Some substances have recommended 

STEL or ceiling values which are intended to 

supplement the TWA where there are 

recognized toxic effects from higher exposures 

over the short-term. 

Occupational criteria for CO exposure are 

applicable to employees who may be at risk of 

CO poisoning. The occupational exposure limits 

noted below should not be used for interpreting 

general population exposures because 

occupational standards are intended for healthy 

worker populations. The effects of CO are more 

pronounced in a shorter time if the person is 

physically active, very young, very old, or has 

preexisting health conditions such as lung or 

heart disease. Persons at extremes of age and 

persons with underlying health conditions may 

have marked symptoms and may suffer serious 

complications at lower levels of COHb.17 

The NIOSH REL for CO is 35 ppm for full shift 

TWA exposure, with a ceiling limit of 200 ppm 

which should never be exceeded.6

 The NIOSH 

REL of 35 ppm is designed to protect workers 

from health effects associated with COHb levels 

in excess of 5%.6

 NIOSH has established the 

immediately dangerous to life and health 

(IDLH) value for CO as 1,200 ppm.10 An IDLH 

value is defined as a concentration at which an 

immediate or delayed threat to life exists or that 

would interfere with an individual's ability to 

escape unaided from a space. 

The ACGIH recommends an 8-hour TWA TLV 

of 25 ppm based upon limiting shifts in COHb 

levels to less than 3.5%, thus minimizing 

adverse neurobehavioral changes such as 

headache, dizziness, etc., and to maintain 

cardiovascular exercise capacity.14 ACGIH also 

recommends that exposures never exceed five 

times the TLV (thus, never to exceed 125 

ppm).14 ACGIH recommends a Biological 

Exposure Index (BEI) for end of shift exhaled 

breath analysis in nonsmoking workers (exposed 

to CO) of 3.5% COHb (or 20 ppm).14 The BEI 

generally indicates a concentration below which 

nearly all workers should not experience adverse 

health effects. 

The OSHA PEL for CO is 50 ppm for an 8-hour 

TWA exposure.15

Other Relevant Criteria 

Certain HSUS and AMVA recommended 

standards for animal euthanasia impact on 

occupational exposure. HSUS lists the 

following minimum requirements and conditions 

for the use of CO chambers for euthanasia:4

“The Equipment: The chamber must 

be a commercially manufactured unit 

designed specifically for CO euthanasia 

and be properly maintained. 

It should be stationed and utilized in a 

well-ventilated area and exhausted to the 

outside. 

If it is designed to euthanize more than 

one animal at a time, it must be 

equipped with independent sections or 

cages to separate incompatible or 

frightened animals. 

The interior of the chamber must be 

well-lit, and equipped with view-ports, a 

regulator (which maintains the gas 

concentration), and flow-meter (which 

measures the gas concentration). 

Only bottled commercial grade gas must 

be used; engine or chemically-generated 

gas is not acceptable due to impurities 

and heat which are painful and 

inhumane. 

The chamber should achieve a minimum 

of 4%-6% gas (the concentration should 

never be above 10%, a level at which 

CO may become explosive) within 20 

seconds. The animals must be 

unconscious within 45-60 seconds, and 

death must occur within two to four 

minutes. 
Page 6 Health Hazard Evaluation Report No 2004-0123-2939. 

Monitoring equipment (such as an 

alarm) must be used during operation, 

and the chamber must be thoroughly 

cleaned after each use. 

The Euthanasia Process: A euthanasia 

technician should be present during the 

entire cycle in the event there is an 

equipment malfunction or other 

problem. 

Animals must be left in the chamber and 

exposed to the gas for a minimum of 30 

minutes to ensure death. 

The chamber must be fully exhausted 

before animals are removed. 

Positive verification of death must be 

determined for each animal prior to 

disposal. 

Staff Safety: Staff must be fully 

notified of the potential health risks 

involved with using CO. 

The use of a CO chamber should be 

closely monitored and in compliance 

with OSHA requirements.” 

AVMA recommendations are very similar to 

those of HSUS, with the following differences:3

Equipment: AVMA does not 

specifically require that the chamber be 

of commercial construction. Rather, 

they state that “the CO chamber must be 

of the highest quality construction.” 

AVMA does not specify the use of a 

regulator and flow meter for gas 

delivery to the chamber, but rather 

states: “The CO flow rate should be 

adequate to rapidly achieve a uniform 

CO concentration of at least 6% after 

animals are placed in the chamber.” 

The Euthanasia Process: AVMA does 

not address the process itself. 

Staff Safety: AVMA wording about 

staff training differs in that they 

recommend that personnel using CO be 

instructed thoroughly in its use and 

understand its hazards and limitations. 

AVMA adds the following safety 

information: “Safeguards must be taken 

to prevent exposure of personnel. Any 

electrical equipment exposed to CO (eg, 

lights and fans) must be explosion 

proof.” 

RESULTS

Results of CO sampling during operation of the 

chambers are presented in Figures 3 – 7. 

CO concentration during introduction of gas 

into the chamber: Each time the gas was 

introduced to either chamber, CO poured into 

the air around the positively pressured chamber 

through openings in the chamber door. Figures 

3 – 6 demonstrate that CO concentrations near 

the chamber rapidly exceed the NIOSH ceiling 

limit of 200 ppm during introduction of CO and 

the NIOSH IDLH value of 1,200 ppm was 

exceeded in one instance. This happens because 

there are many leaks around the chamber doors. 

Peak CO concentrations in the general area 

during introduction were 800 ppm to greater 

than 1200 ppm (the maximum range of the 

sampling instruments). 

On March 4, when the large chamber was 

operated for one more full cycle (shown in 

Figure 7), CO concentration in the area of the 

chambers was noticeably lower, with a 

maximum of approximately 100 ppm (the 

operator’s peak exposure was 170 ppm). During 

this cycle, the operator introduced CO into the 

chamber with more control than before with less 

of a burst of pressure and a slower delivery rate. 

CO concentration when the chambers were 

opened: Because these chambers are not 

vented, opening the chamber door represents the 

largest release of CO into the area occupied by 

the operator. Peak concentrations following 
Health Hazard Evaluation Report No. 2004-0123-2939 Page 7 

opening of the chamber ranged from 400 ppm to 

>1,200 ppm (the maximum capacity of the 

monitors) in the general area of the chambers. 

These concentrations remained above the 

NIOSH ceiling limit of 200 ppm for 2 to 10 

minutes. 

It is important to note that these concentrations 

were measured outside the chamber and that the 

operator did not conduct the regular activities of 

unloading dead animals. Figure 6 shows that the 

CO concentrations within the chamber remain 

above IDLH levels well past the time when 

external concentrations have dissipated. At 

16:19 on the graph in Figure 6, one of the CO 

monitors from the outside of the chamber was 

briefly placed within the chamber and indicated 

that the CO concentration was greater than 1,200 

ppm. To corroborate that information, one 

minute later a second monitor was briefly placed 

within the chamber, again measuring CO 

concentrations above the capacity of the 

instrument. Each monitor was quickly 

withdrawn to avoid damaging the sensor. This 

means that the operator is likely to be exposed 

above IDLH concentrations for an undetermined 

length of time if the chamber is not actively 

ventilated. 

Employee exposures: Employee exposures 

during this investigation were impacted by the 

fact that CO alarms alerted employees to the 

severity of the hazard during the tasks they were 

conducting. When alarms sounded, employees 

wearing monitors appropriately removed 

themselves from exposure. In addition, normal 

tasks were not conducted during the operation. 

Personal monitoring results should be viewed in 

light of this fact: typical exposures would most 

likely be higher when actual euthanasia is 

conducted in these chambers. 

The results shown in Figure 3 are perhaps the 

best indicator of employee exposure during 

normal operations with no CO alarm. When the 

valve of the first cylinder of CO was opened (at 

11:15), the operator thought the tank was empty. 

The valve was closed, and the chamber opened – 

at which point the operator was exposed to CO 

in excess of 380 ppm. After the tank was 

replaced with a full tank, and the valve was 

again opened to introduce CO into the smaller 

chamber (at 11:43), the operator’s exposure rose 

above the level of alarm (>200 ppm) twice (once 

during CO introduction and once when the door 

was opened). During each chamber operation 

after that, employees were then aware of the 

hazard and were either kept out of the area or 

quickly evacuated the area when CO was being 

delivered or chambers were being opened. 

Other findings: 

There were no confined space entry procedures. 

The employee operating the chamber did this 

task alone (no observers). 

Respiratory protection was not available. 

Operating procedures were undefined. 

There was no hazard communication program, 

although there was a Material Safety Data Sheet 

for the compressed liquid CO. 

There was no program for employee training. 

There were no warning signs related to the CO 

hazard. 

There was no emergency action plan. 

The employer had not assessed the workplace to 

determine if hazards were present. 

There was no way to tell if the CO concentration 

within the chamber was appropriate for humane 

euthanasia of the animals. 

DISCUSSION

Homemade CO euthanasia chambers and 

manually operated CO gas chambers with no 

exhaust ventilation, undetermined times of 

chamber evacuation, unknown level of chamber 

pressurization, unknown concentrations of CO 

in the chambers and breathing zones of the 

workers are highly hazardous. The body of 

information discussed in this report (i.e., the 

fatality associated with the use of a homemade 

chamber in Tennessee; the CO measurements 
Page 8 Health Hazard Evaluation Report No 2004-0123-2939. 

demonstrated in this NIOSH investigation; and 

accepted guidelines of organizations such as 

HSUS and AVMA indicate that the use of 

homemade uncontrolled gas chambers for 

euthanasia with no regard for employee safety is 

not acceptable. Consequently, there are no 

recommendations related to modification of the 

existing homemade chambers used at this animal 

shelter. 

A primary issue for this animal shelter, and 

apparently shelters across the United States, is 

the relative financial burden of lethal injection 

versus CO chamber operation. HSUS presents 

guidelines for comparing costs of appropriate 

use of each method, based on the use of a 

commercially produced euthanasia chamber 

designed specifically for use with CO.4

 HSUS 

and TOSHA identified only one such 

commercially produced euthanasia chamber. 

Technical information about this chamber is 

included in Attachment 1. 

CONCLUSION

Environmental sampling during staged 

euthanasia at this animal shelter indicated an 

unacceptable risk of exposure to excessive levels 

of CO. 

RECOMMENDATIONS

The following recommendations are based on 

the findings of this investigation and are 

intended to reduce the health hazard for 

employees. 

1. Continue the use of lethal injection instead of 

CO for animal euthanasia. 

2. If CO is used for euthanasia, purchase an 

acceptable commercially-produced chamber 

specifically designed for use with CO. An 

example of such a chamber is included in 

Attachment 2. 

3. If the decision to use CO for euthanasia is 

based on comparative costs of each method, use 

appropriate cost comparison analyses for this 

decision.4

4. Assure the safe use of the commercially 

produced chamber by periodically measuring 

CO emitted when the chamber is operated, and 

by following all manufacturers’ guidelines for 

maintenance and continued use of the product. 

5. A CO euthanasia chamber is a confined 

space, and all relevant guidelines or 

requirements for such confined spaces should be 

adhered to when the CO chamber is operated. 

(Attachment 2). The operator should never 

unload the chamber until reliable measurements 

indicate that the chamber has been cleared of 

high concentrations of CO. 

6. Develop a hazard communication program 

using the OSHA hazard communication standard 

as a program guideline. Excellent sample 

programs can be found on the OSHA website.18

7. If CO chambers are used, employees 

operating these chambers should immediately 

report symptoms of CO exposure to designated 

health and safety personnel. 

8. Develop written operational procedures for 

animal euthanasia, regardless of the method 

chosen. Example written procedures of lethal 

injection are available from HSUS. Operating 

procedures supplied with commercially 

available CO chambers serve as an excellent 

starting point for written procedures if that is the 

chosen euthanasia method. 

9. Post warning signs related to the CO hazard 

if CO chambers are used. 

10. Develop an emergency action plan. 

11. Conduct a workplace hazard assessment to 

define workplace hazards and appropriate 

methods of control. 
Health Hazard Evaluation Report No. 2004-0123-2939 Page 9 

REFERENCES

1..

 Humane Society of the United States [2004]. 

HSUS pet overpopulation estimates. Available 

from URL: http://www.hsus.org/ace/11830

2. National Council on Pet Population Study and 

Policy [2003]. Shelter statistics survey, 1994-

97. Available from URL: 

http://www.petpopulation.org/statsurvey.html

3. American Veterinary Medical Association 

[2001]. 2000 report of the AVMA panel on 

euthanasia. JAVMA 218:5. March 1, 2001. 

Available from URL: 

http://www.avma.org/resources/euthanasia.pdf

4. Rhoades RH [2002]. The HSUS Euthanasia 

Training Manual. Humane Society Press, 2100 

L Street, NW, Washington DC 20037. 

5. ACGIH [2001]. Documentation of threshold 

limit values and biological exposure indices. 7th 

edition. Cincinnati, OH: American Conference 

of Governmental Industrial Hygienists. 

6. NIOSH [1972]. Criteria for a recommended 

standard: occupational exposure to carbon 

monoxide. Cincinnati, OH: U.S. Department of 

Health, Education, and Welfare, Health Services 

and Mental Health Administration, National 

Institute for Occupational Safety and Health, 

DHEW (NIOSH) Publication No. 73-11000. 

7. NIOSH [1977]. Occupational diseases: a 

guide to their recognition. Revised ed. 

Cincinnati, OH: U.S. Department of Health, 

Education, and Welfare, Public Health Service, 

Centers for Disease Control, National Institute 

for Occupational Safety and Health, DHEW 

(NIOSH) Publication No. 77-181.

8. NIOSH [1979]. A guide to work-relatedness 

of disease. Revised ed. Cincinnati, OH: U.S. 

Department of Health, Education, and Welfare, 

Public Health Service, Centers for Disease 

Control, National Institute for Occupational 

Safety and Health, DHEW (NIOSH) Publication 

No. 79-116. 

9. Proctor NH, Hughes JP, Fischman ML 

[1988]. Chemical hazards of the workplace. 

Philadelphia, PA: J.B. Lippincott Company. 

10. NIOSH [2004]. Immediately dangerous to 

life and health concentrations. DHHS (NIOSH) 

Publication No. 97-140, Pocket Guide to 

Chemical Hazards and other Databases, Fourth 

Printing, February 2004, with changes and 

updates. 

11. World Health Organization [1999]. 

Environmental health criteria 213 – carbon 

monoxide (Second Edition). WHO, Geneva. 

ISBN 92 4 157213 2 (NLM classification: QV 

662) ISSN 0250-863X. 

12. Norkool DM, Kirkpatrick JN [1985]. 

Treatment of acute carbon monoxide poisoning 

with hyperbaric oxygen; a review of 115 cases. 

Ann Emer Med 14: 1168-1171 

13. NIOSH [1992]. Recommendations for 

occupational safety and health: compendium of 

policy documents and statements. Cincinnati, 

OH: U.S. Department of Health and Human 

Services, Public Health Service, Centers for 

Disease Control and Prevention, National 

Institute for Occupational Safety and Health, 

DHHS (NIOSH) Publication No. 92-100. 

14. ACGIH [2004]. 2004 TLVs® and BEIs®: 

threshold limit values for chemical substances 

and physical agents. Cincinnati, OH: American 

Conference of Governmental Industrial 

Hygienists. 

15. CFR [1997]29 CFR 1910.1000. Code of 

Federal Regulations. Washington, DC: U.S. 

Government Printing Office, Office of the 

Federal Register. 

16. Occupational Safety and Health Act of 1970, 

Public Law 91-596, sec. 5(a)(1) 

17. Kales SN [1993]. Carbon monoxide 

intoxication. American Family Physician 

48(6):1100-1104. 
Page 10 Health Hazard Evaluation Report No 2004-0123-2939. 

18. Occupational Safety and Health 

Administration [2004]. Hazard communication 

quick start. Available from URL: 

http://www.osha.gov/SLTC/hazardcommunicati

ons/standards.html
Health Hazard Evaluation Report No.2004-0123-2939 Page 11 

TABLES & FIGURES

Table 1. Examples of Use or Prohibition of Carbon Monoxide Chambers for Euthanasia in U. S. 

Animal Shelters, by State. HETA 2004-0123-2939, Liberal Kansas 

State Description of Use 

CA Banned the use of CO chambers effective January 1, 2001. In 1999, Sacramento 

County used a CO euthanasia chamber for 17,000 unwanted pets. They 

discontinued use of the chamber in 2000. 

FL Lethal injection is specified as the only method allowed. 

MD Prohibits the use of CO chambers. 

ME Lethal injection is specified as the only method allowed. 

MN Hennepin County uses CO euthanasia chambers for 10,000 to 12,000 dogs and 

cats annually. 

MO The city of St. Joseph uses CO chambers. 

NC Most county animal shelters use gas euthanasia chambers on 37 animals per 

1,000 people statewide, or 70% to 91% of those animals making it to the shelter. 

In the Charlotte region, only Mecklenburg and Burke Counties use lethal 

injection. Wake County used CO euthanasia for 2,800 animals in 2002. 

Johnston used gas on 3,614 animals euthanized in 2001. Chatham used gas 

euthanasia for 98 % of 1,628 animals in 2002. 

OH Of Ohio’s 88 counties, 20 use CO chambers to euthanize companion animals, 

including Richland, Fairfield (30 dogs a week), and Johnston (3,614 animals 

euthanized with CO in 2001) Counties. 

OK Allows the use of CO chambers. 

OR Lethal injection is specified as the only method allowed. 

PA Lethal injection is specified as the only method allowed. 

SC Allows the use of CO chambers. 

TN Banned the use of CO chambers in 2002. 

TX The manufacturer of one of the only commercially available CO chambers 

reported that market research estimates indicate that 65% of Texas counties use 

homemade CO chambers. 

UT In Utah County, CO chambers are the main method used to euthanize animals in 

animal shelters. In 2002, CO was used on 60% (480) of the 800 animals 

euthanized monthly. Although Enoch used truck-generated CO to kill 450 

animals in 1998, with the annual number declining as publicity about the Enoch 

chamber rose. According to the chamber manufacturer mentioned above, every 

UT County has at least one CO chamber. 

VA Of Virginia’s 95 counties, 25 to 27 use a CO gas chamber for euthanasia. 

WA The City of Emporia uses CO euthanasia chambers. 

WY Casper recently purchased a CO chamber ($7,500) for euthanasia of animal 

shelter animals. 

General: The only manufacturers of euthanasia chambers specifically designed for use with CO (same 

company mentioned above) reports having sold 8 units in the past 6 months, with overall sales of 

approximately 300 such chambers in the previous 26-year history of the company. 
Page 12 Health Hazard Evaluation Report No 2004-0123-2939. 

Figure 1. Large CO Euthanasia Chamber 

Figure 2. Large and small CO euthanasia chambers and associated CO supply system 
Health Hazard Evaluation Report No.2004-0123-2939 Page 13 

Figure 3. CO Concentration during Small Euthanasia Chamber Operation 

City of Liberal Animal Shelter - Liberal, Kansas March 3, 2004

CO introduced @ 11:13 but appeared to be no CO in tank

CO introduced for 20 seconds @11:43 - Chamber Opened @ 11:48

-100

0

100

200

300

400

500

600

700

800

900

Time

11:11:1411:14:2911:17:4411:20:5911:24:1411:27:2911:30:4411:33:5911:37:1411:40:2911:43:4411:46:5911:50:1411:53:2911:56:4411:59:5912:03:1412:06:2912:09:4412:12:5912:16:1412:19:29

Time

CO Concentration (ppm)

Employee 1 On large chamber / area covered On small chamber / area covered
Page 14 Health Hazard Evaluation Report No 2004-0123-2939. 

Figure 4. CO Concentration during Small Euthanasia Chamber Operation 

City of Liberal Animal Shelter - Liberal, Kansas March 3, 2004

CO introduced for 30 seconds @ 13:58 - Chamber door opened @ 14:03

-200

0

200

400

600

800

1000

1200

1400

13:58:0913:58:5413:59:3914:00:2414:01:0914:01:5414:02:3914:03:2414:04:0914:04:5414:05:3914:06:2414:07:0914:07:5414:08:3914:09:2414:10:0914:10:5414:11:3914:12:24

Time

CO Concentration (ppm)

Employee 1 Employee 2 On large chamber
Health Hazard Evaluation Report No.2004-0123-2939 Page 15 

Figure 5. CO Area Concentrations during Large Euthanasia Chamber 

Operation

Liberal Animal Shelter - Liberal, Kansas

March 3, 2004

CO introduced for 25 seconds @ 15:29 - Door opened @ 15:44

-200

0

200

400

600

800

1000

1200

1400

1600

1800

15:27:3615:28:3615:29:3615:30:3615:31:3615:32:3615:33:3615:34:3615:35:3615:36:3615:37:3615:38:3615:39:3615:40:3615:41:3615:42:3615:43:3615:44:3615:45:3615:46:3615:47:3615:48:3615:49:3615:50:36

Time

CO Concentration (ppm)

Employee 1 Employee 2 On large chamber 

On small chamber Near small chamber
Page 16 Health Hazard Evaluation Report No 2004-0123-2939. 

Figure 6. CO Concentration during Large Euthanasia Chamber Operation 

City of Liberal Animal Shelter - Liberal, Kansas March 3, 2004

CO introduced for 30 seconds @ 16:07 - Chamber door opened @ 16:15

-200

0

200

400

600

800

1000

1200

1400

1600

16:06:0616:06:5116:07:3616:08:2116:09:0616:09:5116:10:3616:11:2116:12:0616:12:5116:13:3616:14:2116:15:0616:15:5116:16:3616:17:2116:18:0616:18:5116:19:3616:20:2116:21:0616:21:5116:22:3616:23:2116:24:06

Time

CO Concentration (ppm)

Employee 1

On large chamber (operating)

On small chamber (not operating) / Checking inside chamber at 16:20

Near small chamber / Checking inside chamber at 16:19
Health Hazard Evaluation Report No.2004-0123-2939 Page 17 

Figure 7. CO Concentration during Large Euthanasia Chamber Operation

City of Liberal Animal Shelter - Liberal, Kansas March 4, 2004

CO introduced for 30 seconds @ 14:40 - Chamber opened @ 14:58

-200

0

200

400

600

800

1000

1200

1400

14:49:5014:50:5014:51:5014:52:5014:53:5014:54:5014:55:5014:56:5014:57:5014:58:5014:59:5015:00:5015:01:5015:02:5015:03:5015:04:5015:05:5015:06:5015:07:5015:08:5015:09:5015:10:50

Time

CO Concentration (ppm)

Employee 1 Employee 2 On large chamber On large chamber
Page 18 Health Hazard Evaluation Report No 2004-0123-2939. 

ATTACHMENT 1 

Commercially Produced CO Euthanasia Chamber 
Health Hazard Evaluation Report No.2004-0123-2939 Page 19 

ATTACHMENT 2 

Confined Space Entry Requirements 

Euthanasia chambers such as those described in this report, meet NIOSH and OSHA criteria for 

a permit-required confined space; therefore, permit-required confined space requirements should 

be followed before any workers enter this area. OSHA regulation 29 CFR 1910.146 defines a 

confined space as a space that meets three criteria: (1) a space that is large enough and 

configured so that an employee can bodily enter and perform any assigned work; (2) a space that 

has limited or restricted means for entry or exit (for example, tanks, vessels, storage bins, vaults, 

and pits that have limited means of entry); and (3) a space that is not designed for continuous 

employee occupancy. The standard then defines a permit–required confined space as a space 

that meets one or more of the following criteria: (1) a space that contains or has a potential to 

contain a hazardous atmosphere; (2) a space that contains a material that has the potential for 

engulfing (surrounding and capturing of a person by a liquid or finely divided solid substance 

that can be aspirated and cause death or that can exert enough pressure to cause death by 

strangulation, constriction, or crushing) the person entering the space; (3) a space with an 

internal configuration designed in a way that the person entering the space could be trapped or 

asphyxiated by inwardly converging walls or by a floor which slopes downward and tapers to a 

smaller cross section; or (4) a space that contains any other recognized serious safety or health 

hazard.1 

NIOSH defines a confined space as “an area which by design has limited openings for entry and 

exit, unfavorable natural ventilation which could contain (or produce) dangerous air 

contaminates, and which is not intended for continuous employee occupancy.2 The NIOSH 

criteria for working in confined spaces further classifies confined spaces based upon the 

atmospheric characteristics such as oxygen level, flammability, and toxicity. 

As shown in Table 1, if any of the hazards present a situation which is immediately dangerous to 

life or health (IDLH), the confined space is designated Class A. A Class B confined space has 

the potential for causing injury and/or illness, but is not an IDLH atmosphere. A Class C 

confined space is one in which the hazard potential would not require any special modification of 

the work procedure. Table 2 lists the confined space program elements which are recommended 

(or must be considered by a qualified person, as defined by the criteria) before entering and 

during work within confined spaces based on the established hazard classification. 

 

 

 

1 

Code of Federal Regulations [1997]. 29 CFR 1910.146. Washington, DC: U.S. Government Printing Office, Federal Register. 

2 

NIOSH [1979]. Criteria for a recommended standard: Working in Confined Spaces. Cincinnati, OH: U.S. Department of Health, Education, 

and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) 

Publication No. 80–106.
Page 20 Health Hazard Evaluation Report No 2004-0123-2939. 

Table 1 

CONFINED SPACE CLASSIFICATION TABLE 

Parameters Class A Class B Class C

Characteristics Immediately dangerous to life – rescue 

procedures require the entry of more 

than one individual fully equipped with 

life support equipment – maintenance of 

communication requires an additional 

standby person stationed within the 

confined space 

Dangerous, but not immediately life 

threatening – rescue procedures require 

the entry of no more than one 

individual fully equipped with life 

support equipment – indirect visual or 

auditory communication with workers 

Potential hazard – requires no 

modification of work procedures – 

standard rescue procedures – direct 

communication with workers, 

from outside the confined space 

Oxygen 16% or less 

*(122 mm Hg) or 

greater than 25% 

*(190 mm HG) 

16.1% to 19.4% 

*(122 – 147 mm Hg) 

or 21.5% to 25% 

(163 – 190 mm Hg) 

19.5 % – 21.4% 

*(148 – 163 mm Hg) 

Flammability 

 Characteristics 

20% or greater of LFL 10% – 19% LFL 10% LFL or less 

Toxicity **IDLH greater than contamination level, 

referenced in 29 CFR Part 1910 Sub 

Part Z – less than **IDLH 

less than contamination level 

referenced in 29 CFR Part 1910 

Sub Part Z

* Based upon a total atmospheric pressure of 760 mm Hg (sea level) 

** Immediately Dangerous to Life or Health – as referenced in NIOSH Registry of Toxic and Chemical Substances, 

Manufacturing Chemists data sheets, industrial hygiene guides or other recognized authorities. 

NIOSH [1979]. Criteria for a recommended standard: working in confined spaces. Cincinnati, OH: U.S. Department of Health, Education, 

and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) 

Publication No. 80–106.
Health Hazard Evaluation Report No.2004-0123-2939 Page 21 

Table 2 

CHECK LIST OF CONSIDERATIONS FOR ENTRY, 

WORKING IN AND EXITING CONFINED SPACES 

ITEM CLASS A CLASS B CLASS C

1. Permit X X X 

2. Atmospheric Testing X X X 

3. Monitoring X 0 0 

4. Medical Surveillance X X 0 

5. Training of Personnel X X X 

6. Labeling and Posting X X X 

7. Preparation 

 Isolate/lockout/tag 

 Purge and ventilate 

 Cleaning Processes 

 Requirements for special equipment/tools 

X 

X 

0 

X 

X 

X 

0 

X 

0 

0 

0 

0 

8. Procedures 

 Initial plan 

 Standby 

 Communications/observation 

 Rescue 

 Work 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

0 

X 

X 

X 

9. Safety Equipment 

and Clothing 

 Head protection 

 Hearing protection 

 Hand protection 

 Foot protection 

 Body protection 

 Respiratory protection 

 Safety belts 

 Life lines, harness 

0 

0 

0 

0 

0 

0 

X 

X 

0 

0 

0 

0 

0 

0 

X 

0 

0 

0 

0 

0 

0 

X 

 10. Rescue Equipment X X X 

 11. Recordkeeping/Exposure X X 

X = indicates requirement 

0 = indicates determination by the qualified person 

NIOSH [1979]. Criteria for a recommended standard: working in confined spaces. Cincinnati, OH: U.S. Department of Health, Education, 

and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) 

Publication No. 80–106. 
DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 

4676 Columbia Parkway 

Cincinnati, OH 45226-1998 

OFFICIAL BUSINESS 

Penalty for private use $300 

Delivering on the Nation's promise: 

Safety and Health at work for all people 

through research and prevention

To receive NIOSH documents or information 

about occupational Safety and Health topics 

contact NIOSH at: 

1-800-35-NIOSH (356-4674) 

Fax: 1-513-533-8573 

E-mail: pubstaft@cdc.gov 

or visit the NIOSH web site at: 

http://www.cdc.gov/niosh 

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