14.1 Confined Space Entry & Atmospheric Monitoring

14.1 Confined Space Entry & Atmospheric Monitoring

Core Function: Confined spaces in water and wastewater utilities—such as sewer manholes, lift station wet wells, valve vaults, storage tanks, and anaerobic digesters—present some of the most dangerous working environments in municipal operations. Operators must master OSHA 29 CFR 1910.146 regulations, understand the specific responsibilities of the entry team, execute rigorous chronological atmospheric testing, maintain multi-gas detection instrumentation, operate continuous positive-pressure ventilation, and implement non-entry rescue systems.

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1. OSHA Confined Space Standard (29 CFR 1910.146) & Utility Space Classifications

Under federal OSHA standard 29 CFR 1910.146 (Permit-Required Confined Spaces), municipal water and wastewater utilities must formally inventory, classify, and control all workspaces that present restricted entry or hazardous internal conditions.

                         CONFINED SPACE DEFINITION
        ┌─────────────────────────────────────────────────────────┐
        │ 1. Large enough to bodily enter and perform work        │
        │ 2. Limited or restricted means of entry or exit         │
        │ 3. Not designed for continuous employee occupancy       │
        └────────────────────────────┬────────────────────────────┘
                                     │
                     Does it contain ANY of the following?
                     • Hazardous or toxic atmosphere
                     • Engulfment hazard (liquid, sludge, lime)
                     • Entrapping inwardly converging walls/floor
                     • Other recognized serious safety hazard
                                     │
                    ┌────────────────┴────────────────┐
                    ▼                                 ▼
                 [ YES ]                            [ NO ]
                    │                                 │
                    ▼                                 ▼
       PERMIT-REQUIRED CONFINED SPACE         NON-PERMIT CONFINED
            (PRCS - Formal Permit,               SPACE (NPCS)
             Team, Rescue, Testing)          (Standard Safety Rules)

The Three Criteria for a Confined Space

To be legally classified as a Confined Space, a workspace must meet all three of the following conditions simultaneously:

1. Bodily Entry: It is large enough and so configured that an employee can bodily enter (head and torso) to perform assigned work.
2. Restricted Access: It has limited or restricted means for entry or exit (e.g., manhole chimneys, access hatches, vertical ladders, narrow doorways).
3. Non-Continuous Occupancy: It is not designed for continuous employee occupancy (lacks permanent engineered ventilation, lighting, and workspace habitability).

The Four Characteristics of a Permit-Required Confined Space (PRCS)

A confined space is legally classified as a Permit-Required Confined Space (PRCS) if it has one or more of the following four severe hazard profiles:

1. Hazardous Atmosphere: Contains or has the potential to contain a hazardous atmosphere (oxygen deficiency/enrichment, combustible gases, or toxic contaminants).
2. Engulfment Hazard: Contains a material that has the potential to engulf an entrant (raw sewage inflows, treated water, thickened sludges, dry bulk chemical lime, or activated carbon).
3. Entrapment Configuration: Has an internal configuration such that an entrant could be trapped or asphyxiated by inwardly converging walls, or by a floor that slopes downward and tapers to a smaller cross-section (e.g., hopper-bottom clarifiers, conical digester sumps, funnel-bottom wet wells).
4. Other Serious Hazards: Contains any other recognized serious safety or health hazard (such as unguarded rotating pump shafts, mechanical mixer impellers, exposed electrical conductors, steam lines, or hydraulic pressure lines).

+-----------------------+--------------------------+-----------------------+-----------------------------+
| Workspace Type        | OSHA Classification      | Dominant Hazards      | Standard Control Protocol   |
+-----------------------+--------------------------+-----------------------+-----------------------------+
| Sewer Manhole         | Permit-Required (PRCS)   | H2S, Methane, Low O2, | Continuous ventilation,     |
|                       |                          | Engulfment from surge | multi-gas meter, tripod/SRL |
+-----------------------+--------------------------+-----------------------+-----------------------------+
| Lift Station Wet Well | Permit-Required (PRCS)   | H2S, Engulfment,      | Mechanical isolation (LOTO),|
|                       |                          | Mixer blades, Falls   | forced blower, harness/winch|
+-----------------------+--------------------------+-----------------------+-----------------------------+
| Valve / Meter Vault   | Permit-Required (PRCS)   | High-pressure piping, | Atmospheric pre-test,       |
|                       | (Unless reclassified)    | CO2/CO, Flooding      | LOTO of motorized valves    |
+-----------------------+--------------------------+-----------------------+-----------------------------+
| Potable Storage Tank  | Permit-Required (PRCS)   | Low O2 (rusting),     | Full rigging, ventilation,  |
| / Standpipe           |                          | Falls, Engulfment     | line isolation / blinding   |
+-----------------------+--------------------------+-----------------------+-----------------------------+
| Anaerobic Digester    | Permit-Required (PRCS)   | Methane (explosive),  | Gas purging, LOTO,          |
|                       |                          | H2S, Asphyxiation     | continuous 4-gas monitoring |
+-----------------------+--------------------------+-----------------------+-----------------------------+

Space Reclassification vs. Alternate Entry Procedures

• Reclassification to Non-Permit Space (1910.146(c)(7)): A permit-required space may be reclassified as a non-permit space only if it poses no actual or potential atmospheric hazards, and if all physical hazards are completely eliminated from outside the space (e.g., de-energizing and locking out pumps, closing and double-blocking incoming water valves). If an atmospheric hazard exists or could arise, the space can never be reclassified.
• Alternate Entry Procedures (1910.146(c)(5)): If the only hazard in the space is an actual or potential atmospheric hazard, and the employer demonstrates that continuous forced air ventilation alone is sufficient to maintain the space safe for entry, entry may proceed under streamlined alternate procedures without a full permit team, provided continuous atmospheric monitoring is maintained.

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2. The Confined Space Entry Team: Defined Roles & Legal Responsibilities

OSHA 29 CFR 1910.146 mandates strict role separation among entry personnel. Every permit-required entry must have at least three designated functional roles (though in small crews, the Entry Supervisor may also serve as an Authorized Entrant or Attendant if properly trained and equipped).

                       CONFINED SPACE ENTRY TEAM STRUCTURE

       ┌──────────────────────────────────────────────────────────────┐
       │                     ENTRY SUPERVISOR                         │
       │ • Authorizes entry & signs permit before work begins         │
       │ • Verifies atmospheric testing, ventilation, and LOTO        │
       │ • Ensures rescue services are confirmed available            │
       │ • Cancels permit and terminates entry upon condition change  │
       └──────────────────────────────┬───────────────────────────────┘
                                      │ Oversees
                                      ▼
       ┌──────────────────────────────────────────────────────────────┐
       │                   ATTENDANT ("HOLE WATCH")                   │
       │ • Stationed OUTSIDE at the portal at ALL times               │
       │ • Monitors entrant behavior and atmospheric readings         │
       │ • Maintains continuous communication (voice/radio)           │
       │ • Summons emergency rescue immediately if trouble arises     │
       │ • Operates non-entry mechanical retrieval equipment (winch)  │
       │ • CRITICAL RULE: NEVER enters the space to attempt rescue!   │
       └──────────────────────────────┬───────────────────────────────┘
                                      │ Monitors & Protects
                                      ▼
       ┌──────────────────────────────────────────────────────────────┐
       │                    AUTHORIZED ENTRANT                        │
       │ • Inspects PPE, harness, and personal gas monitor            │
       │ • Understands space hazards, symptoms, and exposure signs    │
       │ • Maintains continuous communication with the Attendant      │
       │ • Evacuates immediately upon alarm, order, or symptoms       │
       └──────────────────────────────────────────────────────────────┘

1. The Authorized Entrant

• Must be fully trained in recognizing early symptoms of toxic exposure (headache, dizziness, nausea, eye stinging) and oxygen deficiency (mental confusion, euphoria, shortness of breath).
• Must properly wear assigned personal protective equipment (PPE), including an OSHA-rated full-body harness connected to a mechanical retrieval line.
• Must maintain continuous two-way communication with the outside Attendant.
• Must exit the space immediately if an atmospheric monitor alarms, if ordered to evacuate, or if any unusual symptom or hazard is detected.

2. The Attendant ("Hole Watch")

• Stationed continuously at the entrance portal outside the permit space.
• Non-Abandonment Mandate: The attendant cannot leave their post for any reason while entrants are inside, unless relieved by another qualified, designated attendant.
• No Conflicting Duties: The attendant must not be assigned secondary tasks (such as retrieving tools from a truck or running errands) that distract from monitoring entrants.
• Continuous Headcount: Maintains an exact count and identity of all entrants inside the space.
• Communication & Surveillance: Maintains continuous verbal, visual, or radio contact with entrants and monitors the exterior environment for hazards (such as vehicle exhaust drifting toward the blower intake).
• THE CARDINAL SAFETY RULE: The Attendant MUST NEVER ENTER THE PERMIT SPACE TO ATTEMPT A RESCUE. Historic NIOSH data reveals that over 60% of all confined space fatalities are would-be rescuers who entered without proper equipment. The attendant's sole duty in an emergency is to summon professional emergency rescue services and execute non-entry retrieval from outside using the mechanical hoist.

3. The Entry Supervisor

• Verifies that all pre-entry atmospheric tests have been performed and recorded.
• Confirms that all isolation procedures (Lockout/Tagout of pumps, blocking of valves) and forced air ventilation are operational.
• Verifies that rescue services are available and that the communications system for summoning them is tested.
• Signs and dates the written Confined Space Entry Permit, officially authorizing entry.
• Terminates the permit, removes entrants, and cancels entry whenever site conditions change or an unpermitted atmospheric condition develops.

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3. Atmospheric Hazards & Mandatory Testing Protocols

Atmospheric hazards represent the leading cause of sudden death in municipal water and wastewater confined spaces. Testing must be performed prior to opening the space (through pick holes or cracked covers) and continuously while workers are inside.

                     MANDATORY ATMOSPHERIC TESTING SEQUENCE

     ┌──────────────────┐       ┌──────────────────┐       ┌──────────────────┐
     │  STEP 1: OXYGEN  │ ───►  │ STEP 2: % LEL    │ ───►  │  STEP 3: TOXICS  │
     │ (19.5% - 23.5%)  │       │ (Flammability)   │       │   (H2S and CO)   │
     └──────────────────┘       └──────────────────┘       └──────────────────┘
      LEL sensors REQUIRE        Must be < 10% LEL.         H2S Ceiling: 20 ppm
      O2 to burn gas sample!     Explosion hazard           CO TWA: 50 ppm

Why the Chronological Testing Order is Non-Negotiable

1. Oxygen Content Must Be Tested FIRST (19.5% to 23.5%):
   • Sensor Physics: Catalytic bead combustible gas (LEL) sensors work by catalytically burning a tiny sample of the gas on an internal heated filament. This chemical oxidation reaction requires an oxygen concentration of at least 10% to 14%. If an atmosphere is severely oxygen-depleted (e.g., 4% O2), the LEL sensor will read zero or artificially low even if the space is saturated with explosive methane!
   • Health Thresholds: Normal atmospheric air contains 20.9% oxygen. OSHA mandates a safe entry envelope between 19.5% (minimum) and 23.5% (maximum):
     • Below 19.5% (Oxygen Deficient): Hypoxia, impaired motor function, mental confusion, and loss of consciousness.
     • Above 23.5% (Oxygen Enriched): Extreme fire and flash-explosion hazard. Organic materials, hair, and cotton clothing ignite violently with explosive speed.
2. Flammable Gases & Vapors Must Be Tested SECOND (<10% LEL):
   • Combustible gas is measured as a percentage of the Lower Explosive Limit (LEL)—the lowest concentration of gas in air that will burn or explode if ignited.
   • OSHA mandates that atmospheric flammability must remain below 10% of the LEL for entry.
   • Methane (CH4): The primary constituent of sewer gas and anaerobic digester gas. Methane has an LEL of 5.0% by volume (50,000 ppm) and an Upper Explosive Limit (UEL) of 15.0% by volume. Therefore, 10% of the LEL for methane corresponds to 0.5% methane by volume (5,000 ppm).
3. Toxic Contaminants Must Be Tested THIRD:
   • Hydrogen Sulfide (H2S): Produced by the anaerobic bacterial reduction of sulfates in wastewater collection systems.
     • OSHA PEL Ceiling: 20 ppm (acceptable ceiling concentration; must never be exceeded without supplied air).
     • NIOSH REL: 10 ppm (10-minute ceiling).
     • IDLH Level: 100 ppm (Immediately Dangerous to Life or Health).
     • The Olfactory Fatigue Trap: At low levels (0.01 to 1.5 ppm), H2S exhibits a distinctive "rotten egg" odor. However, at concentrations of 100 ppm or higher—or after several minutes of low-level exposure—H2S paralyzes the olfactory nerve in the nasal passages. Entrants mistakenly assume the gas has dissipated because they can no longer smell it, moments before suffering respiratory paralysis, immediate "knockdown," and death.
   • Carbon Monoxide (CO): Generated by the incomplete combustion of fossil fuels (e.g., exhaust fumes from portable generators, service trucks, or gas-powered dewatering pumps located near the manhole opening).
     • OSHA PEL: 50 ppm (8-hour Time-Weighted Average).
     • NIOSH REL: 35 ppm; IDLH: 1,200 ppm.
     • Physiological Hazard: Completely colorless, odorless, and non-irritating. CO binds to blood hemoglobin with approximately 210 times greater affinity than oxygen, forming carboxyhemoglobin (HbCO) and suffocating body tissues at the cellular level.

+-----------------------+-----------------------+--------------------+---------------------+-----------------------------+
| Atmospheric Hazard    | Chemical Symbol / SG  | OSHA Standard      | IDLH Threshold      | Physiological Impact        |
+-----------------------+-----------------------+--------------------+---------------------+-----------------------------+
| Oxygen (Deficient)    | O2 (SG = 1.11)        | Minimum: 19.5%     | < 12.0% O2          | Confusion, hypoxia, coma    |
+-----------------------+-----------------------+--------------------+---------------------+-----------------------------+
| Oxygen (Enriched)     | O2 (SG = 1.11)        | Maximum: 23.5%     | N/A (Explosion)     | Accelerated combustion      |
+-----------------------+-----------------------+--------------------+---------------------+-----------------------------+
| Methane (Combustible) | CH4 (SG = 0.55)       | < 10% LEL          | 100% LEL (5.0% vol) | Explosion, simple asphyxiant|
+-----------------------+-----------------------+--------------------+---------------------+-----------------------------+
| Hydrogen Sulfide      | H2S (SG = 1.19)       | 20 ppm Ceiling     | 100 ppm             | Olfactory paralysis, arrest |
+-----------------------+-----------------------+--------------------+---------------------+-----------------------------+
| Carbon Monoxide       | CO (SG = 0.97)        | 50 ppm TWA         | 1,200 ppm           | Chemical asphyxiant (blood) |
+-----------------------+-----------------------+--------------------+---------------------+-----------------------------+

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4. Multi-Gas Detector Operation, Stratification Sampling & Maintenance

Modern electronic multi-gas detectors utilize four distinct sensor cells: an electrochemical cell for oxygen (O2), a catalytic bead or infrared sensor for flammability (% LEL), an electrochemical cell for hydrogen sulfide (H2S), and an electrochemical cell for carbon monoxide (CO).

                     ATMOSPHERIC STRATIFICATION IN A MANHOLE

            [ Manhole Opening ]
                     │
                     ▼
       ┌───────────────────────────────┐
       │           TOP ZONE            │ ◄── METHANE (CH4 - SG = 0.55)
       │        (0 to 4 feet)          │     Lighter than air; rises to ceiling
       ├───────────────────────────────┤
       │          MIDDLE ZONE          │ ◄── CARBON MONOXIDE (CO - SG = 0.97)
       │        (4 to 8 feet)          │     AIR / NITROGEN (SG = 1.00 / 0.97)
       │                               │     Disperses evenly in breathing zone
       ├───────────────────────────────┤
       │          BOTTOM ZONE          │ ◄── HYDROGEN SULFIDE (H2S - SG = 1.19)
       │        (8 to 12+ feet)        │     CARBON DIOXIDE (CO2 - SG = 1.52)
       │                               │     Heavier than air; pools in invert
       └───────────────────────────────┘

Stratification Sampling Protocol

Gases do not mix uniformly in deep vertical utility spaces; they stratify according to their vapor density / specific gravity (SG) relative to ambient air (Air = 1.00):

• Top Stratum (Lighter than air): Methane (CH4, SG = 0.55) rises to the crown of sewer pipes, roof domes of storage tanks, and upper manhole chimneys.
• Middle Stratum (Equal to air): Carbon monoxide (CO, SG = 0.97), nitrogen (N2, SG = 0.97), and oxygen (O2, SG = 1.11) blend throughout the central breathing zone.
• Bottom Stratum (Heavier than air): Hydrogen sulfide (H2S, SG = 1.19), carbon dioxide (CO2, SG = 1.52), and hydrocarbon/gasoline vapors (SG = 3.0 to 4.0) settle into deep sumps, wet well inverts, and floor depressions.

Sampling Rule: Atmospheric testing must be performed at the top, middle, and bottom at vertical intervals of no more than 4 feet (1.2 m) throughout the entire depth of the space.

Calculating Sample Tubing Response Lag

When using a multi-gas monitor equipped with an motorized internal sampling pump and external tubing, operators must account for sample transport time:

Total Test Time = (Tubing Length in feet × 1 to 2 seconds/foot) + Sensor Response Time (T90)

• Standard Tygon or Teflon sampling tubing requires 1 to 2 seconds of travel time per linear foot.
• The electrochemical and catalytic sensors require an additional 15 to 30 seconds (T90) to stabilize after the gas reaches the manifold.
• Example Calculation: If sampling an 18-foot wet well with a 20-foot sample hose, the operator must allow: (20 ft × 2 sec/ft) + 20 sec sensor lag = 60 seconds at each 4-foot sampling elevation before reading the display.

Bump Testing vs. Full Calibration

+-----------------------+---------------------------------------+---------------------------------------+
| Parameter             | Daily Bump Test (Functional Check)    | Full Electronic Calibration           |
+-----------------------+---------------------------------------+---------------------------------------+
| Frequency             | Before EACH day's use                 | Monthly, quarterly, or when bump fails|
+-----------------------+---------------------------------------+---------------------------------------+
| Purpose               | Verifies sensor response and audible/ | Adjusts internal sensor span curves to|
|                       | visual/vibratory alarm activation     | NIST-traceable certified gas values   |
+-----------------------+---------------------------------------+---------------------------------------+
| Procedure             | Expose to challenge gas; check that   | Apply zero gas (clean air), then apply|
|                       | sensors respond within ±10% to ±15%   | span gas; instrument recalibrates     |
+-----------------------+---------------------------------------+---------------------------------------+
| Failure Consequence   | Monitor CANNOT be used; must undergo  | If calibration fails, sensor cell is  |
|                       | immediate full calibration            | expired and must be replaced          |
+-----------------------+---------------------------------------+---------------------------------------+

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5. Mechanical Forced Ventilation & Non-Entry Rescue Equipment

Atmospheric testing determines whether a space is safe to enter; mechanical forced ventilation ensures it remains safe throughout the entry.

                    FORCED POSITIVE-PRESSURE VENTILATION

               [ Fresh Air Blower (Upwind) ]
                            │
                            ▼ Flexible Ductwork
       ┌────────────────────┼────────────────────┐
       │                    │                    │
       │                    │                    │
       │                    ▼ Fresh Air Stream   │
       │                 ┌─────┐                 │
       │                 │     │ 1 to 2 feet     │
       │                 └─────┘ from floor      │
       │                    │                    │
       │   ◄── Contaminants swept UP & OUT ──►   │
       └─────────────────────────────────────────┘

Positive-Pressure Ventilation Rules

1. Positive Pressure Only: Blowers must force clean, fresh outside air into the space. Negative-pressure exhaust fans can pull explosive vapors through the motor housing and fail to supply fresh oxygen to the breathing zone.
2. Clean Air Sourcing: The blower intake must be positioned in uncontaminated outdoor air, located strictly upwind from idling service trucks, portable generator exhausts, and sewer vents.
3. Duct Placement: The flexible delivery duct must extend down into the space to within 1 to 2 feet of the bottom. Because hazardous gases like H2S are heavier than air, introducing fresh air at the floor sweeps heavy contaminants upward and out through the manhole opening.
4. Continuous Operation: Ventilation must run continuously before entry (for at least 10 to 15 air changes) and for the entire duration of the work. Blowers must never be shut down while workers remain inside.
5. MANDATORY LIFE-SAFETY PROHIBITION: NEVER ventilate a confined space with pure oxygen! Using compressed oxygen cylinders to ventilate a space enriches the atmosphere above 23.5% O2, creating a catastrophic flash-fire and explosion hazard where work boots, clothing, and grease ignite instantly from a static spark.

Non-Entry Rescue Rigging & Equipment

OSHA mandates that non-entry retrieval systems must be used whenever an authorized entrant enters a permit space with a vertical depth greater than 5 feet (1.5 m), unless the equipment would increase the overall hazard.

                    NON-ENTRY RETRIEVAL SYSTEM SETUP

                           [ Tripod / Davit Arm ]
                                     │
                                     ├──► Mechanical Winch (4:1 Advantage)
                                     │
                              [ Lifeline (SRL) ]
                                     │
                                     ▼
                        [ Full-Body Safety Harness ]
                         (Attached to Dorsal D-Ring)
                                     │
                              [ Entrant Body ]

• Tripod / Davit Arm: Engineered aluminum anchor rated for a minimum 5,000-pound breaking strength, erected symmetrically over the manhole opening.
• Mechanical Winch / Hoist: Equipped with a steel cable or synthetic rope lifeline, self-retracting lifeline (SRL) fall arrester, and a minimum 4:1 mechanical advantage, allowing a single attendant outside to hoist an incapacitated 250-lb worker without entering the space.
• Full-Body Harness: ANSI-approved harness with the retrieval line connected directly to the center rear dorsal D-ring (positioned between the shoulder blades) to ensure the entrant is pulled vertically through the opening. In narrow openings (under 18 inches in diameter), wristlets with spreader bars may be used to lift the entrant with arms extended overhead.

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6. Practical Operational Scenarios & Exam Traps

Practical Operational Scenario

A collection system crew arrives at a 16-foot-deep sanitary sewer lift station wet well to replace a failed submersible pump level float. The crew consists of an Entry Supervisor, an Attendant, and an Authorized Entrant.

• Pre-Entry Verification:
  1. The supervisor verifies that the submersible pumps and mechanical grinders are locked out and tagged out (LOTO) at the motor control center.
  2. The multi-gas detector is bump-tested using certified quad-gas (58L cylinder: 20.9% O2, 50% LEL methane, 25 ppm H2S, 100 ppm CO). All four sensors respond within ±10% and alarms sound.
  3. Testing through the access hatch pick-hole reveals: Top: 20.9% O2, 0% LEL, 0 ppm H2S; Middle: 20.8% O2, 0% LEL, 2 ppm H2S; Bottom (15 ft): 19.8% O2, 4% LEL, 16 ppm H2S. Because H2S is below the 20 ppm ceiling and flammability is <10% LEL, entry is permissible once ventilated.
  4. The positive-pressure blower is positioned 15 feet upwind of the utility truck exhaust, and the duct is lowered to 18 inches above the wet well floor. After 15 minutes of forced ventilation, re-testing shows 20.9% O2, 0% LEL, and 0 ppm H2S throughout.
  5. The supervisor signs the permit, the entrant dons a full-body harness attached to the tripod hoist, and enters with a continuous personal 4-gas monitor.
• Emergency Event: Ten minutes into the repair, an upstream industrial slug enters the wet well. The entrant's personal monitor alarms with H2S flashing 35 ppm. The entrant signals distress and stumbles. The Attendant immediately shouts the evacuation order, summons the local fire department rescue squad via two-way radio, engages the tripod winch, and cranks the entrant out of the well within 45 seconds—never entering the hatch. The entrant recovers safely in fresh air.

Critical Exam Traps

• Trap 1: The Rescuing Attendant. Exam questions frequently describe an unconscious entrant and ask what the attendant should do first. The incorrect trap choice is "Quickly descend the ladder to pull the worker's head above sewage." The correct answer is always summon rescue and operate the external mechanical retrieval hoist from outside. Attendants must NEVER enter the space.
• Trap 2: Olfactory Fatigue. Questions often ask why an operator stopped smelling hydrogen sulfide after several minutes in a manhole. The correct explanation is olfactory fatigue / sensory paralysis of the olfactory nerve, NOT that the gas cleared or drifted away.
• Trap 3: Testing Order. Questions test the sequence of atmospheric checks. Remember: Oxygen FIRST, Flammability SECOND, Toxics THIRD. Catalytic bead flammability sensors require oxygen to function.
• Trap 4: Pure Oxygen Ventilation. Watch out for choices suggesting ventilating with "compressed pure oxygen" to rapidly raise oxygen levels. This is strictly illegal and lethal due to explosive flash fire hazard; only ambient fresh air may be used.