Confined Spaces & Atmospheric Hazards

Confined spaces are a major safety concern in water and wastewater operations because they can contain hazardous atmospheres, moving equipment, engulfment hazards, restricted exits, or other conditions that can quickly become life-threatening. Manholes, wet wells, tanks, vaults, pits, digesters, and similar structures may meet the definition of a confined space depending on their configuration and use.

The most important rule is simple: never assume a confined space is safe because it looks normal, smells normal, or was safe during a previous entry. Conditions can change rapidly, and some dangerous gases cannot be detected reliably by human senses.

What Is a Confined Space?

A confined space is generally a space that:

• is large enough for a worker to enter and perform assigned work;
• has limited or restricted means for entry or exit;
• is not designed for continuous employee occupancy.

Common examples in water and wastewater facilities may include:

• manholes;
• wet wells;
• tanks;
• vaults;
• digesters;
• pits;
• some process vessels;
• certain large pipes or enclosed structures.

Not every confined space is automatically a permit-required confined space, but every suspected confined space must be evaluated for hazards before entry.

Permit-Required Confined Spaces

A confined space may require a permit system when it contains, or has the potential to contain, a serious hazard.

Examples include a space that:

• contains or could develop a hazardous atmosphere;
• contains material that could engulf an entrant;
• has an internal configuration that could trap or asphyxiate a worker;
• contains another recognized serious safety or health hazard.

Water and wastewater operators should not decide informally that a space is safe enough to enter. The facility's confined-space program and entry procedures must be followed.

Atmospheric Hazards

A hazardous atmosphere may expose a worker to death, incapacitation, inability to escape without assistance, injury, or acute illness.

Atmospheric hazards can include:

• oxygen deficiency;
• oxygen enrichment;
• flammable gases or vapors;
• toxic gases or vapors;
• combustible dust;
• other immediately dangerous atmospheric conditions.

Wastewater environments deserve particular attention because decomposition and biological activity can produce or contribute to gases such as hydrogen sulfide, methane, and carbon dioxide.

Oxygen Deficiency and Oxygen Enrichment

Normal outdoor air contains about 20.9% oxygen.

For permit-required confined-space purposes, an oxygen concentration below 19.5% is considered oxygen deficient, while a concentration above 23.5% is considered oxygen enriched.

Low oxygen can impair judgment, coordination, consciousness, and eventually survival. Oxygen enrichment can greatly increase fire and combustion hazards.

An oxygen reading should never be considered separately from other atmospheric hazards. A space can have an acceptable oxygen concentration and still contain dangerous levels of toxic or flammable gas.

Flammable Atmospheres

Flammable gases and vapors are commonly evaluated relative to their lower flammable limit, or LFL.

A flammable gas, vapor, or mist above 10% of its LFL meets OSHA's definition of a hazardous atmosphere for permit-space purposes.

Methane can be a concern in wastewater systems, particularly where anaerobic decomposition occurs.

Ignition sources should be controlled where a flammable atmosphere could be present. Electrical equipment, tools, hot work, smoking, sparks, and static discharge can all become relevant depending on the hazard.

Hydrogen Sulfide

Hydrogen sulfide, or H2S, is a serious wastewater hazard. It may be generated when sulfur-containing material decomposes under anaerobic conditions.

Hydrogen sulfide can be toxic and may accumulate in low or poorly ventilated areas.

Although hydrogen sulfide is often associated with a rotten-egg odor at low concentrations, smell must never be used as the primary safety test. A worker's ability to detect the odor can become unreliable, and dangerous concentrations may be present without dependable warning from the senses.

Use appropriate atmospheric monitoring equipment and facility procedures rather than odor to determine whether entry conditions are acceptable.

Other Toxic Atmospheres

Confined spaces may contain toxic substances from treatment processes, cleaning chemicals, industrial discharges, decomposition, nearby operations, or previous use of the space.

Possible hazards can include:

• hydrogen sulfide;
• chlorine or other treatment chemicals;
• carbon monoxide;
• solvent vapors;
• ammonia;
• other site-specific contaminants.

The appropriate monitoring strategy depends on the hazards that could reasonably be present.

Atmospheric Testing Order

When testing a permit space for atmospheric hazards, OSHA specifies the following sequence:

1. oxygen;
2. combustible gases and vapors;
3. toxic gases and vapors.

This order is important because oxygen concentration can affect both the safety of the atmosphere and the performance or interpretation of some combustible-gas measurements.

Testing should be performed with suitable direct-reading instruments that are maintained and used according to applicable procedures and manufacturer instructions.

Test the Atmosphere Before Entry

Atmospheric testing is used to determine whether acceptable entry conditions exist before workers enter.

An operator should not lower only part of the body into a space before atmospheric evaluation simply because the worker has not yet reached the bottom. Breaking the plane of an opening with any part of the body can constitute entry under confined-space rules.

Testing should account for the possibility that gases may not be evenly distributed throughout the space.

Atmospheric Stratification

Different gases can accumulate at different locations because of gas density, air movement, process conditions, temperature, and space configuration.

Atmospheric conditions near the opening may therefore differ from conditions deeper in the space.

Testing procedures should evaluate the areas where entrants will actually work and travel rather than relying on a single convenient measurement at the opening.

Continuous or Periodic Monitoring

Safe conditions at the beginning of an entry do not guarantee that conditions will remain safe.

Atmospheric hazards may develop because of:

• ongoing decomposition;
• process flow entering the space;
• chemical reactions;
• nearby operations;
• changes in ventilation;
• work performed inside the space;
• movement of accumulated material.

Permit-space conditions must be monitored as necessary to confirm that acceptable entry conditions are maintained throughout the entry.

Ventilation

Mechanical ventilation can be used to remove or control atmospheric hazards when appropriate.

Effective ventilation requires more than placing a blower near the opening. Air must reach the area occupied by the worker, and the air supplied must come from a clean source.

When entry is being conducted under conditions that depend on continuous forced-air ventilation, the ventilation must continue while employees remain in the space.

If ventilation stops or a hazardous atmosphere develops, workers must respond according to the entry procedure rather than assuming the atmosphere will remain safe.

Ventilation Does Not Replace Testing

A common mistake is assuming that running a blower automatically makes a confined space safe.

Ventilation may reduce atmospheric hazards, but atmospheric conditions still need to be evaluated and monitored as required.

If the ventilation system is poorly positioned, draws contaminated air, stops operating, or cannot adequately reach part of the space, dangerous conditions may remain or return.

Isolation of the Space

Atmospheric hazards are not the only confined-space hazards. Before entry, the space may need to be isolated from equipment, process flow, chemicals, electrical energy, hydraulic pressure, or other hazards.

Isolation measures may include:

• lockout/tagout;
• closing, locking, or otherwise securing valves;
• disconnecting or blocking lines where required;
• controlling mechanical equipment;
• preventing unexpected inflow;
• protecting the entry point from traffic or falling objects.

The exact isolation method depends on the space and the hazards present.

Engulfment and Flow Hazards

A worker can be endangered by water, wastewater, sludge, solids, or other material entering a space unexpectedly.

In sewer systems and wet wells, changing flows can create serious hazards even when the atmosphere was initially acceptable.

Operators should consider upstream pumps, automatic valves, rainfall, industrial discharges, process cycles, and other conditions capable of changing the environment during entry.

Entrants, Attendants, and Entry Supervisors

Permit-required confined-space entry involves defined responsibilities.

An authorized entrant is a worker authorized to enter the permit space.

An attendant remains outside the permit space and monitors the authorized entrants and conditions associated with the entry.

An entry supervisor determines whether acceptable entry conditions are present, authorizes the entry, verifies required precautions, and terminates the entry when necessary.

These roles require appropriate training and understanding of the hazards.

The Attendant's Role

For permit-space entry, at least one attendant is required outside the space for the duration of entry operations unless a specific regulatory provision permits a different approach.

The attendant must focus on protecting entrants. Important duties include:

• knowing the hazards;
• maintaining an accurate count of entrants;
• maintaining communication with entrants;
• monitoring conditions inside and outside the space;
• ordering evacuation when required;
• summoning rescue or emergency services;
• preventing unauthorized entry;
• performing assigned non-entry rescue functions.

The attendant should not be assigned unrelated duties that interfere with monitoring the entry.

When Entrants Must Leave

Authorized entrants must be prepared to leave immediately when conditions become unsafe.

Evacuation may be required when:

• a prohibited condition is detected;
• an atmospheric monitor alarms;
• an entrant recognizes a warning sign or symptom of dangerous exposure;
• the attendant orders evacuation;
• an evacuation alarm is activated;
• conditions outside the space create a new danger;
• the entry can no longer be safely monitored.

An operator should never silence an alarm and continue working simply because no one feels ill.

Communication

The attendant and entrants must be able to communicate effectively during entry.

The communication method depends on the space and may involve:

• direct voice communication;
• radio;
• hard-wired communication equipment;
• visual contact;
• another reliable method established by the entry procedure.

Communication is necessary both to monitor entrant status and to order evacuation if conditions change.

Entry Permits

A permit documents the conditions and precautions required for a permit-space entry.

Information may include:

• the space being entered;
• purpose of entry;
• date and authorized duration;
• authorized entrants;
• attendant and entry supervisor;
• identified hazards;
• isolation measures;
• acceptable entry conditions;
• atmospheric test results;
• communication methods;
• PPE and other required equipment;
• rescue and emergency arrangements.

A permit is not merely paperwork. It is a structured confirmation that hazards have been evaluated and required controls are in place.

Rescue Must Be Planned Before Entry

Confined-space rescue is dangerous because an unprotected rescuer can become a second victim.

Rescue arrangements must therefore be established before permit-space entry begins.

The rescue method and service must be appropriate for the hazards, configuration, and accessibility of the specific space.

Calling emergency services after an incident occurs is not a substitute for evaluating beforehand whether the designated rescue service can respond appropriately to the identified hazard.

Non-Entry Rescue

Where appropriate, non-entry rescue allows an entrant to be removed without another worker entering the hazardous space.

Retrieval systems can include:

• full-body or chest harnesses;
• retrieval lines;
• lifting or retrieval devices;
• appropriate anchor systems.

OSHA requires retrieval systems or methods for permit-space entry unless the retrieval equipment would increase the overall risk or would not contribute to rescue.

Non-entry rescue is generally preferred when it can be performed safely because it avoids exposing another worker to the space.

Do Not Attempt an Improvised Rescue

One of the most dangerous confined-space responses is an untrained coworker entering impulsively to rescue a collapsed worker.

If the atmosphere caused the first worker to collapse, an unprotected rescuer can be overcome in the same way.

Follow the established rescue plan. Entry rescue should be performed only by personnel who are properly trained, equipped, and assigned to perform that function.

Personal Protective Equipment

PPE for confined-space entry depends on the hazards identified.

Equipment may include:

• protective clothing;
• gloves;
• eye and face protection;
• head protection;
• fall or retrieval equipment;
• hearing protection;
• respiratory protection when required under an appropriate respiratory-protection program.

PPE does not replace atmospheric testing, ventilation, isolation, communication, or rescue planning.

Respiratory Protection

Respirators should never be treated as a casual substitute for controlling a dangerous atmosphere.

Respiratory protection requires proper selection, training, medical evaluation, fit testing where applicable, equipment maintenance, and compliance with the relevant respiratory-protection requirements.

Some atmospheres may be immediately dangerous to life or health and require specialized respiratory protection and rescue provisions.

Operators should follow the facility's respiratory-protection program rather than selecting respiratory equipment independently.

External Hazards Around the Opening

The entrance itself may create hazards even when atmospheric conditions are acceptable.

Examples include:

• vehicle traffic around a manhole;
• pedestrians approaching the opening;
• fall hazards;
• tools or objects falling into the space;
• rain or runoff entering the opening;
• equipment positioned near the entrance.

Barriers, traffic control, guard systems, or other controls may be necessary to protect both entrants and workers outside the space.

Weather and Wastewater Collection Systems

Collection-system confined spaces deserve additional attention during changing weather.

Rainfall can rapidly increase sewer flow, alter atmospheric conditions, and create unexpected hydraulic hazards.

Entry decisions should consider both current conditions and conditions that could reasonably develop while employees are in the space.

Common Confined-Space Safety Mistakes

• Assuming a familiar manhole or tank is automatically safe.
• Using smell to determine whether toxic gas is present.
• Testing only at the opening and ignoring conditions deeper in the space.
• Entering before required atmospheric testing is complete.
• Testing for only one gas when several hazards may exist.
• Assuming ventilation eliminates the need for monitoring.
• Ignoring process flow or equipment that can create hazards during entry.
• Allowing the attendant to perform duties that interfere with monitoring entrants.
• Continuing work after an atmospheric monitor alarms.
• Entering impulsively to rescue a collapsed worker.
• Beginning entry without a workable rescue plan.
• Treating a permit as paperwork rather than as confirmation of safe entry conditions.

A Practical Entry Mindset

1. Determine whether the space meets the confined-space definition.
2. Determine whether it is permit-required.
3. Identify atmospheric and physical hazards.
4. Isolate hazardous energy, flow, and equipment as required.
5. Test the atmosphere in the correct sequence.
6. Ventilate or otherwise control hazards as required.
7. Verify acceptable entry conditions.
8. Establish communication, attendant coverage, and rescue arrangements.
9. Use required PPE and entry equipment.
10. Monitor conditions throughout the entry.
11. Evacuate immediately if prohibited or unsafe conditions develop.

What to Remember for the Exam

• A confined space has limited or restricted entry or exit, is large enough to enter for work, and is not designed for continuous occupancy.
• Some confined spaces require a permit because of atmospheric, engulfment, configuration, or other serious hazards.
• Never assume a confined space is safe based on appearance, odor, or previous experience.
• Oxygen below 19.5% is oxygen deficient.
• Oxygen above 23.5% is oxygen enriched.
• Flammable gas, vapor, or mist above 10% of the LFL meets OSHA's hazardous-atmosphere criterion for permit spaces.
• Atmospheric testing order is oxygen first, combustible gases and vapors second, and toxic gases and vapors third.
• Hydrogen sulfide odor is not a reliable safety indicator.
• Ventilation does not eliminate the need for required atmospheric evaluation and monitoring.
• Atmospheric conditions can change after entry begins.
• Permit spaces must be isolated from hazardous energy and process hazards as required.
• An attendant remains outside and monitors entrants during permit entry.
• Entrants must evacuate when prohibited conditions, alarms, symptoms, or evacuation orders occur.
• Rescue must be planned before entry begins.
• Untrained workers should not make improvised entry rescues.
• Use non-entry rescue when required and when it can be performed safely.