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Safety and Health Topics > Small Business Training > Confined Space Hazards

 CONFINED SPACE HAZARDS

Introduction

◾ Overview

◾ Types of Confined Spaces

◾ Reasons for Entering Confined Spaces

Hazardous Atmospheres

◾ Flammable Atmospheres

◾ Toxic Atmospheres

◾ Irritant (Corrosive) Atmospheres

◾ Asphyxiating Atmospheres

General Safety Hazards

◾ Mechanical

◾ Communication Problem

◾ Entry and Exit

◾ Physical

◾ Thermal Effects

◾ Noise

◾ Vibration

◾ General/Physical 

Discussion/Overheads - 1.91 MB

Student Handouts -253 K

Self-Inspection Checklist

CONFINED SPACE HAZARDS 

INTRODUCTION 

Overview

The hazards encountered and associated with entering and working in confined spaces are capable of causing bodily 

injury, illness, and death to the worker. Accidents occur among workers because of failure to recognize that a confined 

space is a potential hazard. It should therefore be considered that the most unfavorable situation exists in every case 

and that the danger of explosion, poisoning, and asphyxiation will be present at the onset of entry. 

Before forced ventilation is initiated, information such as restricted areas within the confined space, voids, the nature 

of the contaminants present, the size of the space, the type of work to be performed, and the number of people 

involved should be considered. The ventilation air should not create an additional hazard due to recirculation of 

contaminants, improper arrangement of the inlet duct, or by the substitution of anything other than fresh (normal) air 

(approximately 20.9% oxygen, 78.1% nitrogen, and 1% argon with small amounts of various other gases). The terms 

air and oxygen are sometimes considered synonymous. However, this is a dangerous assumption, since the use of 

oxygen in place of fresh (normal) air for ventilation will expand the limits of flammability and increase the hazards of 

fire and explosion. 

Hazardous conditions covered in this discussion include: Hazardous Atmospheres (flammable, toxic, irritant, and 

asphyxiating), and General Safety Hazards (mechanical, communications, entry and exit, and physical). 

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contain mechanical equipment with moving parts. Any combination of these parameters will change the nature of the 

hazards encountered. Degreasers, pits, and certain types of storage tanks may be classified as open topped confined 

spaces that usually contain no moving parts. However, gases that are heavier than air (butane, propane, and other 

hydrocarbons) remain in depressions and will flow to low points where they are difficult to remove. Open topped water 

tanks that appear harmless may develop toxic atmospheres such as hydrogen sulfide from the vaporization of 

contaminated water. Therefore, these gases (heavier than air) are a primary concern when entry into such a confined 

space is being planned. Other hazards may develop due to the work performed in the confined space or because of 

corrosive residues that accelerate the decomposition of scaffolding supports and electrical components. 

Confined spaces such as sewers, casings, tanks, silos, vaults, and compartments of ships usually have limited access. 

The problems arising in these areas are similar to those that occur in open topped confined spaces. However, the 

limited access increases the risk of injury. Gases which are heavier than air such as carbon dioxide and propane, may 

lie in a tank or vault for hours or even days after the containers have been opened. Because some gases are odorless, 

the hazard may be overlooked with fatal results. Gases that are lighter then air may also be trapped within an 

enclosed type confined space, especially those with access from the bottom or side. 

Hazards specific to a confined space are dictated by: (1) the material stored or used in the confined space; as an 

example, damp activated carbon in a filtration tank will absorb oxygen, thus creating an oxygen deficient atmosphere; 

(2) the activity carried out, such as the fermentation of molasses that creates ethyl alcohol vapors and decreases the 

oxygen content of the atmosphere; or (3) the external environment, as in the case of sewer systems that may be 

affected by high tides, heavier than air gases, or flash floods. 

The most hazardous kind of confined space is the type that combines limited access and mechanical devices. All the 

hazards of open top and limited access confined spaces may be present together with the additional hazard of moving 

parts. Digesters and boilers usually contain power-driven equipment which, unless properly isolated, may be 

inadvertently activated after entry. Such equipment may also contain physical hazards that further complicate the 

work environment and the entry and exit process. 

Reasons for Entering Confined Spaces

Entering a confined space as part of the industrial activity may be done for various reasons. It is done usually to 

perform a necessary function, such as inspection, repair, maintenance (cleaning or painting), or similar operations 

which would be an infrequent or irregular function of the total industrial activity. 

Entry may also be made during new construction. Potential hazards should be easier to recognize during construction 

since the confined space has not been used. The types of hazards involved will be limited by the specific work 

practices. When the area meets the criteria for a confined space, all ventilation and other requirements should be 

enforced. 

One of the most difficult entries to control is that of unauthorized entry, especially when there are large numbers of 

workers and trades involved, such as welders, painters, electricians, and safety monitors. 

A final and most important reason for entry would be emergency rescue. This, and all other reasons for entry, must be 

well planned before initial entry is made and the hazards must be thoroughly reviewed. The standby person and all 

rescue personnel should be aware of the structural design of the space, emergency exit procedures, and life support 

systems required.

HAZARDOUS ATMOSPHERES

Hazardous atmospheres encountered in confined spaces can be divided into four distinct categories: Flammable, Toxic, 

Irritant and/or Corrosive, and Asphyxiating. 

Flammable Atmospheres

A flammable atmosphere generally arises from enriched oxygen atmospheres, vaporization of flammable liquids, 

byproducts of work, chemical reactions, concentrations of combustible dusts, and desorption of chemical from inner 

surfaces of the confined space. 

An atmosphere becomes flammable when the ratio of oxygen to combustible material in the air is neither too rich nor 

too lean for combustion to occur. Combustible gases or vapors will accumulate when there is inadequate ventilation in 

areas such as a confined space. Flammable gases such as acetylene, butane, propane, hydrogen, methane, natural or 

manufactured gases or vapors from liquid hydrocarbons can be trapped in confined spaces, and since many gases are 

heavier than air, they will seek lower levels as in pits, sewers, and various types of storage tanks and vessels. In a 

closed top tank, it should also be noted that lighter than air gases may rise and develop a flammable concentration if 

trapped above the opening. 

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Chemical reactions forming flammable atmospheres occur when surfaces are initially exposed to the atmosphere, or 

when chemicals combine to form flammable gases. This condition arises when dilute sulfuric acid reacts with iron to 

form hydrogen or when calcium carbide makes contact with water to form acetylene. Other examples of spontaneous 

chemical reactions that may produce explosions from small amounts of unstable compounds are acetylene-metal 

compounds, peroxides, and nitrates. In a dry state, these compounds have the potential to explode upon percussion 

or exposure to increased temperature. Another class of chemical reactions that form flammable atmospheres arise 

from deposits of pyrophoric substances (carbon, ferrous oxide, ferrous sulfate, iron, etc.) that can be found in tanks 

used by the chemical and petroleum industry. These tanks containing flammable deposits will spontaneously ignite 

upon exposure to air. 

Combustible dust concentrations are usually found during the process of loading, unloading, and conveying grain 

products, nitrated fertilizers, finely ground chemical products, and any other combustible material. High charges of 

static electricity, which rapidly accumulate during periods of relatively low humidity (below 50%), can cause certain 

substances to accumulate electrostatic charges of sufficient energy to produce sparks and ignite a flammable 

atmosphere. These sparks may also cause explosions when the right air or oxygen to dust or gas mixture is present. 

Toxic Atmospheres

The substances to be regarded as toxic in a confined space can cover the entire spectrum of gases, vapors, and finelydivided airborne dust in industry. The sources of toxic atmospheres encountered may arise from the following:

1. The manufacturing process (for example, in producing polyvinyl chloride, hydrogen chloride is used as will as 

vinyl chloride monomer, which is carcinogenic).

2. The product stored [removing decomposed organic material from a tank can liberate toxic substances, such as 

hydrogen sulfide (H2S)].

3. The operation performed in the confined space (for example, welding or brazing with metals capable of 

producing toxic fumes).

During loading, unloading, formulation, and production, mechanical and/or human error may also produce toxic gases 

which are not part of the planned operation. 

Carbon monoxide (CO) is a hazardous gas that may build up in a confined space. This odorless, colorless gas that has 

approximately the same density as air is formed from incomplete combustion of organic materials such as wood, coal, 

gas, oil, and gasoline; it can be formed from microbial decomposition of organic matter in sewers, silos, and 

fermentation tanks. Carbon monoxide is an insidious toxic gas because of its poor warning properties. Early stages of 

CO intoxication are nausea and headache. Carbon monoxide may be fatal at 1000 ppm in air, and is considered 

dangerous at 200 ppm, because it forms carboxyhemoglobin in the blood which prevents the distribution of oxygen in 

the body. 

Carbon monoxide is a relatively abundant colorless, odorless gas, therefore, any untested atmosphere must be 

suspect. It must also be noted that a safe reading on a combustible gas indicator does not ensure that CO is not 

present. Carbon monoxide must be tested for specifically. The formation of CO may result from chemical reactions or 

work activities, therefore fatalities due to CO poisoning are not confined to any particular industry. There have been 

fatal accidents in sewage treatment plants due to decomposition products and lack of ventilation in confined spaces. 

Another area where CO results as a product of decomposition is in the formation of silo gas in grain storage elevators. 

In another area, the paint industry, varnish is manufactured by introducing the various ingredients into a kettle, and 

heating them in an inert atmosphere, usually town gas, which is a mixture of carbon dioxide and nitrogen. 

In welding operations, oxides of nitrogen and ozone are gases of major toxicologic importance, and incomplete 

oxidation may occur and carbon monoxide can form as a byproduct. 

Another poor work practice, which has led to fatalities, is the recirculation of diesel exhaust emissions. Increased CO 

levels can be prevented by strict control of the ventilation and the use of catalytic converters. 

Irritant (Corrosive) Atmospheres

Irritant or corrosive atmospheres can be divided into primary and secondary groups. The primary irritants exert no 

systemic toxic effects (effects on the entire body). Examples of primary irritants are chlorine, ozone, hydrochloric acid, 

hydrofluoric acid, sulfuric acid, nitrogen dioxide, ammonia, and sulfur dioxide. A secondary irritant is one that may 

produce systemic toxic effects in addition to surface irritation. Examples of secondary irritants include benzene, carbon 

tetrachloride, ethyl chloride, trichloroethane, trichloroethylene, and chloropropene. 

Irritant gases vary widely among all areas of industrial activity. They can be found in plastics plants, chemical plants, 

the petroleum industry, tanneries, refrigeration industries, paint manufacturing, and mining operations. 

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Asphyxiating Atmospheres

The normal atmosphere is composed approximately of 20.9% oxygen and 78.1% nitrogen, and 1% argon with small 

amounts of various other gases. Reduction of oxygen in a confined space may be the result of either consumption or 

displacement. 

The consumption of oxygen takes place during combustion of flammable substances, as in welding, heating, cutting, 

and brazing. A more subtle consumption of oxygen occurs during bacterial action, as in the fermentation process. 

Oxygen may also be consumed during chemical reactions as in the formation of rust on the exposed surface of the 

confined space (iron oxide). The number of people working in a confined space and the amount of their physical 

activity will also influence the oxygen consumption rate. 

A second factor in oxygen deficiency is displacement by another gas. Examples of gases that are used to displace air, 

and therefore reduce the oxygen level are helium, argon, and nitrogen. Carbon dioxide may also be used to displace 

air and can occur naturally in sewers, storage bins, wells, tunnels, wine vats, and grain elevators. Aside from the 

natural development of these gases, or their use in the chemical process, certain gases are also used as inerting 

agents to displace flammable substances and retard pyrophoric reactions. Gases such as nitrogen, argon, helium, and 

carbon dioxide, are frequently referred to as non-toxic inert gases but have claimed many lives. The use of nitrogen to 

inert a confined space has claimed more lives than carbon dioxide. The total displacement of oxygen by nitrogen will 

cause immediate collapse and death. Carbon dioxide and argon, with specific gravities greater than air, may lie in a 

tank or manhole for hours or days after opening. Since these gases are colorless and odorless, they pose an 

immediate hazard to health unless appropriate oxygen measurements and ventilation are adequately carried out. 

Oxygen deprivation is one form of asphyxiation. While it is desirable to maintain the atmospheric oxygen level at 21% 

by volume, the body can tolerate deviation from this ideal. When the oxygen level falls to 17%, the first sign of 

hypoxia is a deterioration to night vision which is not noticeable until a normal oxygen concentration is restored. 

Physiologic effects are increased breathing volume and accelerated heartbeat. Between 14-16% physiologic effects are 

increased breathing volume, accelerated heartbeat, very poor muscular coordination, rapid fatigue, and intermittent 

respiration. Between 6-10% the effects are nausea, vomiting, inability to perform, and unconsciousness. Less than 

6%, spasmatic breathing, convulsive movements, and death in minutes. 

GENERAL SAFETY HAZARDS

Mechanical

If activation of electrical or mechanical equipment would cause injury, each piece of equipment should be manually 

isolated to prevent inadvertent activation before workers enter or while they work in a confined space. The interplay of 

hazards associated with a confined space, such as the potential of flammable vapors or gases being present, and the 

build-up of static charge due to mechanical cleaning, such as abrasive blasting, all influence the precautions which 

must be taken. 

To prevent vapor leaks, flashbacks, and other hazards, workers should completely isolate the space. To completely 

isolate a confined space, the closing of valves is not sufficient. All pipes must be physically disconnected or isolation 

blanks bolted in place. Other special precautions must be taken in cases where flammable liquids or vapors may recontaminate the confined space. The pipes blanked or disconnected should be inspected and tested for leakage to 

check the effectiveness of the procedure. Other areas of concern are steam valves, pressure lines, and chemical 

transfer pipes. A less apparent hazard is the space referred to as a void, such as double walled vessels, which must be 

given special consideration in blanking off and inerting. 

Communication Problems

Communication between the worker inside and the standby person outside is of utmost importance. If the worker 

should suddenly feel distressed and not be able to summon help, an injury could become a fatality. Frequently, the 

body positions that are assumed in a confined space make it difficult for the standby person to detect an unconscious 

worker. When visual monitoring of the worker is not possible because of the design of the confined space or location of 

the entry hatch, a voice or alarm-activated explosion proof type of communication system will be necessary. 

Suitable illumination of an approved type is required to provide sufficient visibility for work in accordance with the 

recommendations made in the Illuminating Engineering Society Lighting Handbook. 

Entry and Exit

Entry and exit time is of major significance as a physical limitation and is directly related to the potential hazard of the 

confined space. The extent of precautions taken and the standby equipment needed to maintain a safe work area will 

be determined by the means of access and rescue. The following should be considered: type of confined space to be 

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The hazards described in this section include thermal effects (heat and cold), noise, vibration, radiation, and fatigue 

while working in a confined space. 

Thermal Effects

Four factors influence the interchange of heat between people and their environment. They are: (1) air temperature, 

(2) air velocity, (3) moisture contained in the air, and (4) radiant heat. Because of the nature and design of most 

confined spaces, moisture content and radiant heat are difficult to control. As the body temperature rises 

progressively, workers will continue to function until the body temperature reaches approximately 102o

F. When this 

body temperature is exceeded, the workers are less efficient, and are prone to heat exhaustion, heat cramps, or heat 

stroke. In a cold environment, certain physiologic mechanisms come into play, which tend to limit heat loss and 

increase heat production. The most severe strain in cold conditions is chilling of the extremities so that activity is 

restricted. Special precautions must be taken in cold environments to prevent frostbite, trench foot, and general 

hypothermia. 

Protective insulated clothing for both hot and cold environments will add additional bulk to the worker and must be 

considered in allowing for movement in the confined space and exit time. Therefore, air temperature of the 

environment becomes an important consideration when evaluating working conditions in confined spaces. 

Noise

Noise problems are usually intensified in confined spaces because the interior tends to cause sound to reverberate and 

thus expose the worker to higher sound levels than those found in an open environment. This intensified noise 

increases the risk of hearing damage to workers which could result in temporary or permanent loss of hearing. Noise 

in a confined space which may not be intense enough to cause hearing damage may still disrupt verbal communication 

with the emergency standby person on the exterior of the confined space. If the workers inside are not able to hear 

commands or danger signals due to excessive noise, the probability of severe accidents can increase. 

Vibration

Whole body vibration may affect multiple body parts and organs depending upon the vibration characteristics. 

Segmental vibration, unlike whole body vibration, appears to be more localized in creating injury to the fingers and 

hands of workers using tools, such as pneumatic hammers, rotary grinders or other hand tools which cause vibration. 

General/Physical

Some physical hazards cannot be eliminated because of the nature of the confined space or the work to be performed. 

These hazards include such items as scaffolding, surface residues, and structural hazards. The use of scaffolding in 

confined spaces has contributed to many accidents caused by workers or materials falling, improper use of guard rails, 

and lack of maintenance to insure worker safety. The choice of material used for scaffolding depends upon the type of 

work to be performed, the calculated weight to be supported, the surface on which the scaffolding is placed, and the 

substance previously stored in the confined space. 

Surface residues in confined spaces can increase the already hazardous conditions of electrical shock, reaction of 

incompatible materials, liberation of toxic substances, and bodily injury due to slips and falls. Without protective 

clothing, additional hazards to health may arise due to surface residues. 

Structural hazards within a confined space such as baffles in horizontal tanks, trays in vertical towers, bends in 

tunnels, overhead structural members, or scaffolding installed for maintenance constitute physical hazards, which are 

exacerbated by the physical surroundings. In dealing with structural hazards, workers must review and enforce safety 

precautions to assure safety. 

Rescue procedures may require withdrawal of an injured or unconscious person. Careful planning must be given to the 

relationship between the internal structure, the exit opening, and the worker. If the worker is above the opening, the 

system must include a rescue arrangement operated from outside the confined space, if possible, by which the 

employee can be lowered and removed without injury.

Discussion/Overheads - 1.91 MB

Student Handouts -253 K

Self-Inspection Checklist

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