3.3 Industrial Hygiene & Chemical Hazards

Industrial hygiene is the science and art devoted to the anticipation, recognition, evaluation, and control of those environmental factors or stresses arising in or from the workplace, which may cause sickness, impaired health and well-being, or significant discomfort among workers or among the citizens of the community.

1. Anticipation: Involves identifying potential hazards in the design or planning phase before a process or chemical is introduced.
2. Recognition: Involves identifying existing hazards in the workplace through inspections, reviews of safety data sheets (SDSs), and analysis of work processes.
3. Evaluation: Involves measuring the magnitude or severity of the hazard, typically through quantitative exposure monitoring (such as air sampling or noise dosimetry) and comparing the results to occupational exposure limits.
4. Control: Involves implementing measures to eliminate or reduce the hazard to acceptable levels.

Understanding Occupational Exposure Limits (OELs)

To evaluate chemical hazards, industrial hygienists compare measured concentrations of airborne chemicals to established exposure limits. There are three primary organizations that publish these limits, and understanding their differences is critical for regulatory compliance and safety:

1. OSHA Permissible Exposure Limits (PELs)

OSHA PELs are legally enforceable federal standards codified in 29 CFR 1910.1000 (Tables Z-1, Z-2, and Z-3). Many OSHA PELs were adopted in 1971 from the existing 1968 ACGIH guidelines and have not been updated since. As a result, they may not represent the most current toxicological research, but they remain the legal maximums that employers must not exceed.

• Time-Weighted Average (TWA): The average airborne concentration of a substance over a standard 8-hour workday, for a 40-hour workweek, to which nearly all workers may be repeatedly exposed without adverse effects.
• Short-Term Exposure Limit (STEL): The maximum concentration to which workers can be exposed continuously for a short period of time (typically 15 minutes) without suffering irritation, chronic or irreversible tissue damage, or narcosis. Exposure to the STEL must not occur more than 4 times per day, and there must be at least 60 minutes between successive exposures.
• Ceiling Limit (C): The concentration that must not be exceeded during any part of the working exposure. If instantaneous monitoring is not feasible, the ceiling is assessed as a 15-minute TWA.

2. ACGIH Threshold Limit Values (TLVs)

Published by the American Conference of Governmental Industrial Hygienists, TLVs are peer-reviewed, scientific guidelines rather than legal standards. They are updated annually based on new toxicological studies and are generally much lower (more protective) than OSHA PELs. OSHA encourages employers to adopt TLVs to protect their workers, even if the higher PEL is legally met.

3. NIOSH Recommended Exposure Limits (RELs)

Published by the National Institute for Occupational Safety and Health (the federal research agency under the CDC), RELs are scientific recommendations designed to protect worker health over a 10-hour workday in a 40-hour workweek. NIOSH also establishes Immediately Dangerous to Life or Health (IDLH) values, which represent concentrations of airborne contaminants from which a worker could escape within 30 minutes without suffering debilitating or irreversible health effects.

Calculating the 8-Hour Time-Weighted Average (TWA)

To determine compliance with an 8-hour TWA PEL, industrial hygienists use the following formula:

TWA = [ (C1 x T1) + (C2 x T2) + ... + (Cn x Tn) ] / 8

Where:

• C is the concentration of the chemical measured during interval T.
• T is the duration of that exposure interval in hours.
• The sum of all intervals T1 + T2 + ... + Tn must equal 8 hours (or the total shift duration, though it is normalized to an 8-hour scale for comparison with 8-hour standards).

Example Calculation: An employee is exposed to toluene (OSHA PEL = 200 ppm TWA; NIOSH REL = 100 ppm TWA). An industrial hygienist conducts personal air sampling over an 8-hour shift and records the following exposure levels:

• 2 hours of exposure at 300 ppm (during a mixing operation)
• 4 hours of exposure at 80 ppm (during routine monitoring)
• 2 hours of exposure at 20 ppm (during administrative work)

Let's calculate the TWA:

TWA = [ (300 x 2) + (80 x 4) + (20 x 2) ] / 8 TWA = [ 600 + 320 + 40 ] / 8 = 960 / 8 = 120 ppm

Conclusion: The employee's 8-hour TWA exposure is 120 ppm. This is in compliance with the legally enforceable OSHA PEL of 200 ppm, but it exceeds the more protective NIOSH REL of 100 ppm. The employer should implement controls to reduce exposure below 100 ppm to ensure optimal health protection.

The Hierarchy of Controls

When chemical hazards are recognized and evaluated as exceeding exposure limits, employers must implement controls. Under OSHA standards (such as 29 CFR 1910.1000(e)), employers must follow the Hierarchy of Controls. They cannot simply hand out respirators to employees as a first step; engineering controls must be implemented first.

1. Elimination/Substitution: The most effective control. Involves removing the chemical entirely or substituting it with a less hazardous substance (e.g., replacing lead-based paints with water-based acrylic paints, or replacing a chlorinated solvent with a citrus-based solvent).
2. Engineering Controls: Design and physical modifications to the workspace or equipment to isolate the worker from the hazard.
   • Local Exhaust Ventilation (LEV): Captures contaminants at the source before they reach the worker's breathing zone (e.g., fume hoods, spray booths).
   • Isolation/Enclosure: Placing a physical barrier between the worker and the process (e.g., glove boxes, acoustic enclosures).
3. Administrative Controls: Changes to the way people work to reduce exposure duration or frequency.
   • Worker Rotation: Rotating workers out of high-exposure areas to limit their individual TWA.
   • Housekeeping and Wet Methods: Implementing regular wet sweeping to prevent combustible dust or silica from becoming airborne.
4. Personal Protective Equipment (PPE): The least effective control, used only as a last resort or when other controls are infeasible or being installed. PPE does not eliminate the hazard; if the PPE fails (e.g., a respirator seal breaks), the worker is immediately exposed.

Routes of Entry and Physiological Impacts

Chemicals can only cause harm if they enter the body. There are four primary routes of entry:

1. Inhalation: The most common and dangerous route in industrial settings. Workers breathe in airborne gases, vapors, mists, fumes, or dusts.
   • Physiological Impact: Contaminants travel down the trachea into the lungs, where they can damage the respiratory system (e.g., chlorine gas causing pulmonary edema) or pass directly into the bloodstream through the alveoli (e.g., carbon monoxide binding with hemoglobin to form carboxyhemoglobin, preventing oxygen transport).
2. Absorption (Skin/Eye Contact): Chemicals contact the skin or eyes and pass through the outer layers into the blood or cause localized damage.
   • Physiological Impact: Direct contact can cause localized irritation or chemical burns (e.g., sulfuric acid). Organic solvents (like benzene or dimethyl sulfoxide) can penetrate intact skin, entering the bloodstream and causing systemic toxicity, such as bone marrow damage or leukemia.
3. Ingestion: Chemicals enter the body through the mouth, typically via contaminated hands, food, beverages, or cosmetics in the work area.
   • Physiological Impact: Hand-to-mouth contact transfers chemicals to the digestive system. Heavy metals like lead are commonly ingested when workers eat lunch or smoke cigarettes without washing their hands first. This leads to chronic poisoning, affecting the nervous and reproductive systems.
4. Injection: Chemicals enter the body through a break in the skin, such as punctures from needles, cuts from contaminated glass/metal, or high-pressure spray equipment.
   • Physiological Impact: Direct entry into the deep tissue or bloodstream bypasses the skin's protective barrier, leading to rapid systemic distribution and risk of infection or acute poisoning.