2.5 Human Factors & Ergonomic Hazard Analysis

Human Factors & Ergonomic Hazard Analysis

Introduction to Ergonomics

Ergonomics (from the Greek ergon meaning work, and nomos meaning laws) is the applied science of equipment design, intended to maximize productivity by reducing operator fatigue and discomfort. In occupational safety, the fundamental philosophy of ergonomics is simple yet profound: Fit the job to the worker, do not force the worker to fit the job.

When a mismatch occurs between the physical requirements of a task and the physical capabilities of the human body, the result is often a class of injuries known as Musculoskeletal Disorders (MSDs).

Musculoskeletal Disorders (MSDs)

MSDs are injuries and disorders that affect the human body's movement or musculoskeletal system (muscles, tendons, ligaments, nerves, discs, blood vessels). Common examples include:

• Carpal tunnel syndrome (compression of the median nerve in the wrist).
• Tendinitis (inflammation of a tendon).
• Rotator cuff injuries (shoulder joint).
• Epicondylitis (tennis elbow).
• Herniated spinal discs.

Unlike an acute injury (like a broken leg from a fall), MSDs are typically cumulative trauma disorders. They develop gradually over time due to repeated micro-traumas. Because the onset is slow, workers often ignore early warning signs (aching, tingling, numbness) until the damage is severe and potentially irreversible.

Primary Ergonomic Risk Factors

OHSTs must be adept at recognizing the three primary ergonomic risk factors in the workplace:

1. High Force: Tasks that require high muscle effort increase fatigue and the risk of muscle strain. Examples include heavy lifting, pushing heavy carts, or using forceful pinch grips.
2. Repetitive Motion: Performing the same motions repeatedly, especially when combined with force or awkward postures, does not allow sufficient time for tissues to recover. The frequency and duration of the task are critical variables.
3. Awkward Postures: Postures that place stress on the body. A "neutral posture" is where joints are naturally aligned (e.g., standing straight with arms resting at sides). Awkward postures include working with hands above the head, twisting the torso, bending at the waist, or extreme wrist deviation.

Secondary risk factors include contact stress (e.g., resting the wrist on a sharp desk edge), vibration (from power tools), and cold temperatures (which decrease tissue flexibility and blood flow).

Ergonomic Assessment Tools

To move beyond subjective observations, safety professionals utilize standardized tools to quantify ergonomic risk.

The NIOSH Lifting Equation

Developed by the National Institute for Occupational Safety and Health, this is the gold standard for evaluating two-handed manual lifting tasks. The equation calculates a Recommended Weight Limit (RWL) for a specific lifting scenario.

The RWL represents the maximum weight that nearly all healthy workers could lift over an 8-hour shift without an increased risk of lower back pain.

The formula requires measuring several variables:

• Horizontal Multiplier (HM): Distance of the load away from the body.
• Vertical Multiplier (VM): Height of the lift from the floor.
• Distance Multiplier (DM): Total vertical distance the load is lifted.
• Asymmetric Multiplier (AM): The angle of torso twisting required.
• Frequency Multiplier (FM): How often the lift is performed.
• Coupling Multiplier (CM): The quality of the worker's grip on the load.

By comparing the Actual Weight being lifted to the calculated RWL, a Lifting Index (LI) is generated. An LI greater than 1.0 indicates an increased risk of injury, requiring ergonomic redesign of the task.

RULA and REBA

For evaluating postures, particularly in the upper body, two common tools are used:

• RULA (Rapid Upper Limb Assessment): Specifically evaluates the posture, force, and repetition of the neck, trunk, and upper limbs (often used for seated assembly or computer tasks).
• REBA (Rapid Entire Body Assessment): Evaluates posture for the whole body, including the legs, making it useful for dynamic, unpredictable tasks like healthcare patient handling or construction.

Both tools generate a score; higher scores dictate a more urgent need for ergonomic intervention.

Anthropometry and Workstation Design

Anthropometry is the scientific measurement of the human body (e.g., reach distances, eye height, sitting elbow height). Because human dimensions vary wildly, designing a "one-size-fits-all" workstation guarantees that it will fit almost no one properly.

Effective ergonomic design relies on adjustability. Workstations should be designed to accommodate the 5th percentile female (for reach requirements—ensure smaller workers can reach items) to the 95th percentile male (for clearance requirements—ensure larger workers have enough legroom). Adjustable chairs, height-adjustable tables, and articulated monitor arms are engineering controls that allow the workstation to adapt to the specific dimensions of the user.

Ergonomic Controls

Applying the hierarchy of controls to ergonomics:

• Engineering Controls (Preferred): Utilizing mechanical hoists or vacuum lifts to eliminate manual lifting, redesigning tool handles to maintain neutral wrist posture, providing adjustable workstations.
• Administrative Controls: Job rotation (rotating workers through different tasks to use different muscle groups), mandating micro-breaks, enlarging the task to add variety.
• PPE: (Least effective). Anti-vibration gloves or knee pads. Note: Back belts are generally not considered effective PPE by NIOSH for preventing back injuries and may induce a false sense of security.