14.6 Arc-Generated Toxic Gases, Moving-Equipment Hazards & TWA Exposure Calculation

Toxic Gases Generated by the Arc

AWS B5.16 Clause 8.3.7 requires recognition of health hazards from fumes, toxic gases, noise and radiation. Fume particulate was covered above; the gaseous hazards are a separate exam topic because they are invisible, odourless at dangerous concentrations, and are produced by the arc itself rather than by the consumable.

The phosgene pathway deserves particular emphasis because it is the one that kills people who believed they were working safely. Degreasing tanks using chlorinated solvents must never share ventilation or floor space with arc welding: the solvent vapour travels, the arc's ultraviolet output cracks it, and the resulting phosgene has a permissible limit 500 times lower than carbon monoxide's. ANSI Z49.1 addresses this by requiring that parts cleaned with chlorinated solvents be thoroughly dried before welding and that welding be located away from such operations.

Exam Trap: "Ozone is a fume." Ozone, NOx, CO, phosgene and HF are gases — they pass straight through a particulate-filtering respirator and are not captured by a fume-rated cartridge. Controlling them requires ventilation, source separation or supplied-air respiratory protection, never a dust/fume filter. Answer choices that propose an N95 or P100 for a gaseous hazard are always wrong.

Mechanical & Moving-Equipment Hazards

Clause 8.3.7 also lists moving equipment among the safety hazards a welding engineer must recognize. Modern welding cells are mechanized, and the mechanization — not the arc — causes most of the severe-injury events:

• Positioners, turning rolls and manipulators. Rotating tables and turning rolls create crushing and in-running nip points between the workpiece and the rolls. Guarding, two-hand controls and travel limit switches are the accepted controls.
• Robotic welding cells. Safeguarding is governed by ANSI/RIA R15.06 (harmonized with ISO 10218): perimeter fencing, interlocked gates, light curtains, safe-speed teach modes and emergency stops. A welding engineer specifying a cell owns the risk assessment, not only the weld schedule.
• Energy isolation. Servicing any powered positioner, wire feeder, fixture actuator or exhaust fan requires lockout/tagout under OSHA 29 CFR 1910.147, including hydraulic and pneumatic stored energy, not just electrical.
• Cranes, rigging and material handling. Dropped or swinging weldments cause crush injuries during fit-up; lifting lugs that were tacked rather than welded to a qualified procedure are a recurring failure mode.
• Grinding and cutting equipment. Abrasive wheel guarding, correct wheel rating and eye protection under 29 CFR 1910.215 apply to the cleanup operations that surround every weld.

Comprehensive Worked Engineering Example: LEV Airflow & 8-Hour TWA Fume Exposure

Problem Statement

1. A welding workstation uses a flanged rectangular local exhaust hood (12 in × 18 in, area A = 1.50 ft²) positioned at a distance of X = 1.25 ft from a structural GMAW seam. Calculate the required volumetric flow rate Q (in CFM) to satisfy the OSHA mandatory capture velocity of V_c = 100 ft/min.

2. An industrial hygiene monitoring survey samples a welder fabricating 316L stainless steel vessels across an 8-hour shift. The personal breathing zone air sampler yields the following data:

   • Task 1 (SMAW with E316-16): Duration T_1 = 3.5 hours; Cr(VI) concentration C_1 = 8.40 μg/m3.
   • Task 2 (GTAW Root Pass): Duration T_2 = 2.5 hours; Cr(VI) concentration C_2 = 0.80 μg/m3.
   • Task 3 (Grinding / Setup): Duration T_3 = 2.0 hours; Cr(VI) concentration C_3 = 1.20 μg/m3.

   Calculate the 8-hour Time-Weighted Average (TWA) exposure to Hexavalent Chromium. State whether the welder has exceeded the OSHA Action Level (2.5 μg/m3) or the OSHA Permissible Exposure Limit (5.0 μg/m3), and outline mandatory engineering and administrative interventions.

Step-by-Step Engineering Solution

Part 1: LEV Volumetric Flow Rate Calculation

• DallaValle equation for flanged hood:

  Q = 0.75 * V_c * (10 * X² + A)
  

• Given: V_c = 100 ft/min, X = 1.25 ft, A = 1.50 ft²:

  X² = (1.25)² = 1.5625 ft²
  10 * X² + A = 10 * (1.5625) + 1.50 = 15.625 + 1.50 = 17.125 ft²
  Q = 0.75 * 100 * 17.125 = 75 * 17.125 = 1,284.38 CFM
  

• Engineering Result: The exhaust fan must deliver at least 1,285 CFM of airflow through the ductwork.

Part 2: 8-Hour Time-Weighted Average (TWA) Calculation

TWA = [Sum of (C_i * T_i)] / [Sum of T_i] = (C_1 * T_1 + C_2 * T_2 + C_3 * T_3) / 8.0 hours
TWA = [(8.40 μg/m3 * 3.5 hr) + (0.80 μg/m3 * 2.5 hr) + (1.20 μg/m3 * 2.0 hr)] / 8.0 hr
TWA = (29.40 + 2.00 + 2.40) / 8.0 = 33.80 / 8.0 = 4.225 μg/m3

Regulatory and Industrial Hygiene Evaluation:

• OSHA Action Level (2.5 μg/m3): EXCEEDED. (4.225 > 2.5).
• OSHA Permissible Exposure Limit (5.0 μg/m3): NOT EXCEEDED. (4.225 ≤ 5.0).
• Mandatory Corrective Action per 29 CFR 1910.1026: Because the Action Level was breached, the employer is legally obligated to: (1) enroll the welder in an annual medical surveillance program, (2) perform periodic exposure monitoring every 6 months, (3) institute mandatory Cr(VI) hazard training, and (4) upgrade engineering controls (e.g., repositioning LEV extraction source capture closer to the SMAW station) to drive the TWA below the 2.5 μg/m3 threshold.

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Industrial Scenarios & Certified Welding Engineer Exam Pitfalls

Real-World Field Disaster Scenario

A maintenance crew entered the ballast tank of an ocean barge to repair internal stiffener web cracks using SMAW. The crew ran high-pressure oxygen hoses into the tank to "freshen the air" and sweeten the humid atmosphere. Within 30 minutes, an arc spark bounced off a stiffener and landed on a worker's grease-stained leather glove. In the 28% oxygen-enriched environment, the leather glove and jacket exploded into intense, uncontrollable flames. Both welders suffered fatal third-degree burns before the attendant could react. Atmospheric testing had not been performed, and oxygen had been used as ventilation in violation of OSHA 1910.146 and ANSI Z49.1.

Common Exam Traps

Exam Trap 1: The 35-Foot Rule and Fire Watch Duration A standard exam question asks: "Under ANSI Z49.1 and OSHA 1910.252, what is the required combustible clearance and minimum post-welding fire watch duration?" The required clearance is 35 feet (10.7 meters), and the minimum post-work duration is 30 minutes (never 15 minutes!).

Exam Trap 2: Oxygen Enrichment Boundary Questions frequently ask candidates to identify the maximum allowable oxygen concentration for safe confined space entry. Options often include 20.9%, 21.5%, 23.5%, and 25.0%. The correct statutory limit per OSHA 1910.146 is 23.5%. Any reading above 23.5% constitutes an oxygen-enriched atmosphere.

Exam Trap 3: SMAW vs. GTAW Hexavalent Chromium Candidates are often tricked into assuming that GTAW produces the most dangerous fumes because it welds high-purity stainless steels. In reality, SMAW and FCAW generate far higher fractions of Hexavalent Chromium than GTAW due to alkali metal salts in the flux coatings that chemically oxidize chromium into its hexavalent state.