750 Introduction to Industrial Hygiene

Permissible Exposure Limits (PELs)

OSHA 29 CFR 1910.1000 – Air Contaminants Tables Z-1, Z-2, and Z-3 sets the PEL legal limits, usually measured in parts per million (ppm) or milligrams per cubic meter (mg/m^³), for how much of a hazardous substance a worker can be exposed to during a specific time period—usually an 8-hour workday or a 40-hour workweek.

PELs are meant to prevent both short-term and long-term health problems caused by breathing in, touching, or sometimes swallowing dangerous chemicals. These limits are based on research into how certain substances affect the human body, especially the lungs, skin, eyes, and internal organs.

OSHA Regulation 29 CFR 1910.1000 – Air Contaminants

The regulation helps protect workers from breathing in harmful substances by requiring employers to keep air contaminant levels below the set limits listed in the tables below:

• Table Z-1: Lists common air contaminants and their exposure limits.
• Table Z-2: Lists substances with special exposure rules, like ammonia and hydrogen chloride.
• Table Z-3: Lists limits for mineral dusts, like coal dust and silica.

Examples of PELs include:

• Table Z-1 – Acetone: Acetone is a solvent commonly used in cleaning products, paint removers, and nail polish remover.
  Exposure Limit: 8-hour Time-Weighted Average (TWA): 1,000 ppm (2,400 mg/m³)
  Example: Workers in a factory that uses acetone for cleaning must ensure air levels stay below the PEL to avoid dizziness, headaches, or irritation.
• Table Z-2 – Benzene: Benzene is used in the production of plastics, resins, and synthetic fibers.
  Exposure Limits:
  - 8-hour TWA: 10 ppm
  - Ceiling limit: 25 ppm. Ceiling Limit is the highest amount a worker can be exposed to in the air at any moment during the workday.
  - Peak limit: 50 ppm (for up to 10 minutes, no more than twice per shift). The peak limit is usually higher than the ceiling limit because it accounts for situations where very short-term exposure is needed for certain tasks, and it is assumed that the body can handle a brief spike if it doesn't happen often. But it’s still very risky and closely monitored.
  Example: In a chemical plant, benzene levels must be carefully monitored due to its link to leukemia with long-term exposure.
• Table Z-3 – Respirable Crystalline Silica (Quartz): Found in sand, concrete, and stone, silica becomes dangerous when cut, drilled, or ground, creating dust.
  Exposure Limit: TWA: 10 mg/m³ ÷ (% silica + 2)
  Example: A construction worker cutting bricks must use dust controls and wear a respirator to stay below the PEL and prevent silicosis.

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