Industrial Hygiene: Identifying and Controlling Hazards

Industrial Hygiene Definition

• Industrial hygiene is a science dedicated to identifying, evaluating, and controlling workplace conditions that may cause illness or injury.
• Industrial hygiene involves predicting, recognizing, assessing, controlling, and preventing workplace environmental stressors that can lead to worker sickness or serious discomfort.
• An environmental stressor is any factor causing enough discomfort to result in lost time or illness.
• Environmental stressors include gases, fumes, vapors, dusts, mists, noise, and radiation.

Industrial Hygiene Phases

• Identification involves determining the presence or possibility of workplace exposures.
• Evaluation involves determining the magnitude of the exposure.
• Control involves applying appropriate technology to reduce workplace exposures to acceptable levels.

Industrial Hygienist

• Industrial hygienists can anticipate, recognize, evaluate, and control workplace environmental hazards.
• Anticipation involves the expectation of hazard existence.
• Recognition involves identifying the presence of workplace exposure.
• Evaluation involves determining the magnitude of exposure.
• Control involves implementing measures for hazard reduction to acceptable levels.
• Anticipation/recognition of potential hazards involves knowledge of materials, operations, processes, and conditions.
• Evaluation of environmental factors involves measuring exposure intensity, determining exposure frequency and duration, comparing to standards, and weighing all factors.
• Control involves reducing exposure to eliminate or reduce occupational diseases through engineering interventions, administrative/programmatic measures, and personal protective equipment.

OSHA: Process Safety Management (PSM)

• Process safety management (PSM) was developed after the Bhopal accident (1985) to prevent similar incidents.
• Emergency management activities include phases before, during, and immediately after an emergency.
• Emergency management is a part of PSM in chemical-related industries.

Purpose of PSM

• PSM aims to proactively and systematically prevent or minimize the consequences of a catastrophic release of toxic, flammable, reactive, or explosive chemicals.

PSM Standard Sections

• The PSM standard major sections include:
• Process Safety Information (PSI)
• Employee Participation
• Process Hazard Analysis (PHA)
• Operating Procedure
• Training
• Contractors Safety
• Pre-Startup Safety Review (PSSR)
• Mechanical Integrity
• Nonroutine Work Authorization (Hot Work Permits)
• Management of Change
• Emergency Planning and Response
• Incident Investigation
• Audit of PSM
• Trade Secrets

EPA: Risk Management Plan (RMP)

• The RMP regulation aims to decrease the number and magnitude of accidental releases of toxic and flammable substances.
• RMP protects off-site people and the environment, while PSM is designed to protect on-site people, though both regulations are similar.
• RMP elements: hazard assessment, prevention program, emergency response program, and documentation maintained on-site and shared with the local community.

Industrial Hygiene: Identification

• To safely handle hazardous chemicals in chemical plants, identify and control all potential hazards.
• Identification requires a thorough study of the chemical process, operating conditions, and operating procedures.
• Information sources: process design descriptions, operating instructions, safety reviews, equipment vendor descriptions, chemical supplier information, and operating personnel information.
• The quality of identification depends on the resources used and the questions asked.

Potential Hazards

• Potential hazards relate to chemical processes, operating conditions, and operating procedures.
• Chemical process hazards include process design, operating instructions, and safety reviews.
• Operating condition hazards include equipment description and chemical properties (MSDSs).
• Potential hazards can be liquids, vapors, dusts, or fumes
• The entry mode of toxicants can be inhalation, ingestion, or body absorption (skin or eyes) or injection.
• Potential damage can affect the lungs, ears, nervous system, kidneys, circulatory system, skin, liver, or reproductive organs.
• Data useful for health identification include threshold limit values (TLVs), odor threshold for vapors, physical state, vapor pressure of liquids, sensitivity of chemicals, rates/heats of reaction, hazardous by-products, reactivity, explosive concentrations, noise levels, and radiation types.
• Risk assessment is a potential hazard that can result in an accident.

Industrial Hygiene: Evaluation

• The evaluation phase determines the extent and degree of employee exposure to workplace toxicants and physical hazards.
• Evaluation studies existing control measures and their effectiveness.
• Ready access to a clean environment is important for sudden exposures to high concentrations.
• Preventing and controlling chronic effects from repeated exposures to low concentrations occurs through continuous, frequent, and periodic sampling and analysis.
• Obtain and compare exposure levels and compare them to acceptable occupational health standards to identify the potential hazards requiring better or more control measures.

Evaluating Exposure to Volatile Toxicants

• Air concentrations of toxicants are continuously monitored online in a work environment.

Time-Weighted Average (TWA)

• TWA = (1/8) ∫C(t) dt
• C(t) is the concentration (in ppm or mg/m³) of the chemical in the air.
• t is the worker shift time in hours.
• For one chemical, assuming concentration C₁ is fixed over time T₁, the TWA concentration is TWA = (C1T1 + C2T2 + ... + CnTn) / 8 hrs.

Multiple toxicants exposure

• The combined exposures from multiple toxicants with different TLV-TWAs is determined from the equation: Σ [Ci / (TLV-TWA)i]; where n is the total number of toxicants, Ci is the concentration of chemical i with respect to the other toxicants, and (TLV-TWA)i is the TLV-TWA for chemical species i.
• Workers are overexposed if the sum in the above Equation exceeds 1.
• When the sum of concentration of toxicants are mixed the TLV-TWA can be computed from
• (TLV-TWA)mix = ∑ Ci / {∑ [Ci/ (TLV-TWA)]}
• Workers are overexposed if (TLV-TWA)mix < ∑ Ci

Evaluation of Worker Exposure to Dust

• Dust: Evaluation calculations are performed identically to volatile vapors.
• Instead of ppm, use mg/m³ or mppcf (millions of particles per cubic foot) as a concentration unit.

Evaluation of Worker Exposure to Noise

• Noise evaluation: calculations are performed identically to calculations for vapors.
• dBA is used instead of ppm, and hours of exposure is used instead of concentration.

Estimating Worker Exposure to Toxic Vapors

• Cppm = (Qm R T / k Qv P M) * 10^6
  • C is the concentration of volatile vapor in the enclosure,
  • V is the volume of the enclosure,
  • Qv is the ventilation rate,
  • k is the nonideal mixing factor, and
  • Qm is the evolution rate of volatile material.
  • R is the ideal gas constant. T is the absolute ambient temperature,
  • P is the absolute pressure, and
  • M is the molecular weight of the volatile species.
• A steady-state condition is assumed, K varies from 0.1 to 0.5 for most practical situation.
• For perfect mixing k = 1.

Estimating the Vaporization Rate of a Liquid

• Vaporization rate is proportional to the difference between the saturation vapor pressure and the partial pressure of the vapor in the stagnant air
• Qm ∝ (Psat - p).
• Qm = MKA(Psat - p) / (R g T L)
  • Qm is the evaporation rate (mass/time), M is the molecular weight of the volatile substance,
  • K is a mass transfer coefficient (length/time) for an area A, Rg is the ideal gas constant, and TL is the absolute temperature of the liquid.

Industrial Hygiene: Control

• Control requires the application of appropriate technology for reducing workplace exposures.
• During the design process, ensure that the newly designed control technique provides the desired control.
• The two major control techniques are environmental controls and personal protection.
• Respirators and Ventilation

Typical Control Techniques

• Enclosures: Enclose hazardous operations .
• Local ventilation: Use properly designed hoods.
• Dilution ventilation: Design locker rooms with good ventilation and special areas or enclosures for contaminated clothing.
• Wet methods: Clean vessels chemically vs. sandblasting Use water sprays for cleaning frequently.
• Good housekeeping: Use dikes around tanks and pumps.
• Personal protection: Use safety glasses and face shields. Wear appropriate respirators; airline respirators are required when oxygen concentration is less than 19.5%.

Ventilation

• Ventilation can remove dangerous concentrations of flammable and toxic materials.
• Ventilation can be highly localized, reducing the quantity of air moved and the equipment size.
• Ventilation equipment is readily available and can be easily installed.
• Ventilation equipment can be added to an existing facility.
• Ventilation is based on two principles: dilute the contaminant below the target concentration and remove the contaminant before workers are exposed.
• The major disadvantage of ventilation is the operating cost.
• Ventilation systems are composed of fans and ducts.
• Fans produce a small pressure drop (less than 0.1 psi) that moves the air.
• The best system is a negative pressure system, with the fans located at the exhaust end of the system, pulling air out.

Local Ventilation Methods

• Enclosing i.e contain and separate
• Receiving i.e receive, contain and empty.
• Capturing i.e capture.

Reasons for Systems to Fail

• Incorrect type of hood is chosen and can never provide sufficient protection.
• The airborne contaminant isn't contained or captured.
• LEV hood design is incorrect
• Insufficient airflow

Air Cleaners Filter

• Air Cleaners filter the air in enclosed area.