4.4 Gravimetric, Microscopy, and Wet Chemical Methods

CIH Certified Industrial Hygienist Exam

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1 Chapter 1: The CIH Exam and How to Use This Guide

1.1 CIH Exam Format, Scoring, and Test-Day Logistics 1.2 Eligibility, Application Windows, Fees, and Certification Maintenance 1.3 The BGC Blueprint, Subject Areas, and a Calculation-First Study Plan

2 Chapter 2: Basic Science & Calculations

2.1 Gas Laws and Vapor Calculations 2.2 Unit Conversions: ppm, mg/m³, and Standard Temperature/Pressure 2.3 Fluid Mechanics and Airflow Fundamentals

3 Chapter 3: Air Sampling & Instrumentation

3.1 Active and Passive Air Sampling Strategies 3.2 Particulate and Aerosol Sampling 3.3 Direct-Reading Instruments and Calibration

4 Chapter 4: Analytical Chemistry & Lab Methods

4.1 Gas Chromatography and Mass Spectrometry 4.2 High-Performance Liquid Chromatography and Ion Chromatography 4.3 Spectroscopy and Elemental Analysis 4.4 Gravimetric, Microscopy, and Wet Chemical Methods 4.5 Laboratory Quality Assurance, Chain of Custody, and Desorption Efficiency

5 Chapter 5: Biostatistics & Epidemiology

5.1 Lognormal Distributions and Exposure Data Statistics 5.2 Upper Confidence Limits (UCL95) and Compliance Decision Rules 5.3 Epidemiological Study Designs and Measures of Association 5.4 Sampling Strategy: Sample Size, Power, and Worker Selection

6 Chapter 6: Toxicology & Biological Monitoring

6.1 Dose-Response Relationships and Points of Departure 6.2 Toxicokinetics: Absorption, Distribution, Metabolism, and Elimination 6.3 Aerosol Deposition, Clearance, and Respiratory Tract Dosimetry 6.4 Hepatic, Renal, Neurologic, and Hematologic Target Organ Toxicity 6.5 Pulmonary Toxicology, Carcinogenicity, and Reproductive Hazards 6.6 OEL Frameworks, Classifications, and Excursion Limits 6.7 Chemical Mixtures and Extended Work-Shift OEL Adjustments 6.8 Biological Exposure Indices (BEIs) and Biomonitoring

7 Chapter 7: Biohazards & Indoor Environmental Quality

7.1 Mold, Fungi, and Microbial Contaminants 7.2 Infectious Agents, Bloodborne Pathogens, and Biosafety Levels 7.3 Indoor Environmental Quality (IEQ) Investigations 7.4 Sanitation, Potable Water, and Personal Hygiene Programs

8 Chapter 8: Community Exposure & Environmental Health

8.1 Clean Air Act Framework, NAAQS, and New Source Review 8.2 Hazardous Air Pollutants, CEMS, and EPA Reference Methods 8.3 Atmospheric Stability, Lapse Rates, and Plume Morphology 8.4 Gaussian Plume Modeling, Plume Rise, and Environmental Fate 8.5 HAZWOPER Scope, Mandates, and Training Tiers 8.6 Protection Levels, Site Zoning, Incident Command, and Decontamination 8.7 Water Pollution, Hazardous Waste, and Community Right-to-Know

9 Chapter 9: Health Risk Analysis & Hazard Communication

9.1 GHS Hazard Classification: Health and Physical Hazard Criteria 9.2 GHS Labels, Safety Data Sheets, and Trade Secret Rules 9.3 The AIHA Exposure Assessment Strategy and Similar Exposure Groups 9.4 Qualitative Exposure Profiling and the AIHA Risk Matrix 9.5 Quantitative Health Risk Characterization: HQ, HI, and ELCR 9.6 NIOSH Occupational Exposure Banding 9.7 Risk Communication and Reporting to Affected Parties

10 Chapter 10: Industrial Ventilation Principles & LEV Design

10.1 Elimination, Substitution, Isolation, Containment, and Process Change 10.2 LEV Hood Taxonomy and Capture Velocity Selection 10.3 Hood Airflow Equations, Entry Losses, and Hood Static Pressure 10.4 Particulate Transport Aerodynamics and Duct Sizing 10.5 Duct Friction, Fitting Losses, and System Balancing 10.6 Fan Categories and Pressure Dynamics Across the Fan 10.7 System Curves, the Fan Laws, and Density Corrections

11 Chapter 11: General Ventilation, Air Cleaning & Diagnostics

11.1 Dilution Ventilation Principles and Steady-State Modeling 11.2 Transient Purging and Air Distribution Geometry 11.3 Particulate Air Cleaning: Cyclones, Baghouses, Scrubbers, and ESPs 11.4 Gaseous Emission Control and Air Cleaner Selection 11.5 Ventilation System Troubleshooting and Testing

12 Chapter 12: Noise Measurement & Hearing Conservation

12.1 Acoustic Physics and Decibel Mathematics 12.2 Sound Propagation, Octave Bands, and Frequency Weighting 12.3 Noise Dose Metrics and OSHA versus ACGIH Criteria 12.4 Exchange Rates, Extended Shifts, and Dosimetry Field Protocols 12.5 Engineering Noise Controls and Hearing Protector Attenuation 12.6 Hearing Conservation Programs and Standard Threshold Shifts 12.7 Hand-Arm and Whole-Body Vibration

13 Chapter 13: Thermal Stress Assessment & Control

13.1 Wet Bulb Globe Temperature (WBGT) and Heat Stress Indices 13.2 Heat-Related Disorders, Acclimatization, and Work/Rest Regimens 13.3 Cold Stress Evaluation and Wind Chill

14 Chapter 14: Ionizing Radiation Protection

14.1 Ionizing Radiation Physics and Radioactive Decay Kinetics 14.2 Radiation Units, Radiobiology, and Health Effects 14.3 ALARA, Stay-Time, and the Inverse Square Law 14.4 Shielding Physics, Material Selection, and Gamma Constants 14.5 Radiation Detection Instruments and Spectrometry 14.6 Personnel Dosimetry, Dose Limits, and Contamination Surveys

15 Chapter 15: Non-Ionizing Radiation & Laser Safety

15.1 The Non-Ionizing Spectrum and Static/ELF Field Exposures 15.2 Radiofrequency and Microwave Exposure: SAR, Standards, and Surveys 15.3 Ultraviolet, Visible, and Infrared Radiation Hazards 15.4 Laser Safety Classifications and Optical Density Calculations 15.5 Illumination, Lighting Design, and Visual Performance

16 Chapter 16: Non-Engineering Controls & Personal Protective Equipment

16.1 Respiratory Protection Programs and Assigned Protection Factors 16.2 Maximum Use Concentration, Oxygen Deficiency, and Breathing Air 16.3 Particulate Filters, Sorbent Cartridges, and Change Schedules 16.4 Respirator Fit Testing Protocols 16.5 Chemical Protective Clothing and Dermal Exposure Control

17 Chapter 17: Ergonomics & Workstation Design

17.1 Soft-Tissue Biomechanics and WMSD Pathophysiology 17.2 Ergonomic Risk Factors and Their Interaction Effects 17.3 The Revised NIOSH Lifting Equation 17.4 RULA, REBA, the Strain Index, and the ACGIH HAL TLV 17.5 Anthropometry and Workstation Geometry 17.6 VDT Workstations, Hand Tool Design, and Administrative Ergonomics

18 Chapter 18: Work Environments & Industrial Processes

18.1 Welding Fume Emissions, Metal Fume Fever, and Cr(VI)/Manganese Toxicology 18.2 Foundry Operations and Hot Work Fire Prevention 18.3 Vapor Degreasing, Pickling, Electroplating, and Cyanide Baths 18.4 Spray Coatings, Isocyanates, and Semiconductor Manufacturing 18.5 Confined Spaces and High-Hazard Industrial Operations 18.6 Asbestos, Lead, and Silica: Regulated Substance Programs 18.7 Abrasive Blasting and Surface Preparation

19 Chapter 19: IH Program Management & Professional Ethics

19.1 The BGC Code of Ethics: Canons and Enforcement 19.2 Professional Liability, Standard of Care, and Expert Testimony 19.3 Occupational Health & Safety Management Systems (ISO 45001, ANSI Z10) 19.4 Exposure Records Access and Medical Surveillance 19.5 OSHA Injury and Illness Recordkeeping and Enforcement

4.4 Gravimetric, Microscopy, and Wet Chemical Methods

Key Takeaways Explain This Topic AI

Gravimetric analysis requires 6-place microbalances (0.001 mg readability), 24-hour environmental conditioning (20°C, 50% RH), static neutralization via Po-210 alpha sources, and matched field blanks to measure particulate mass.

Phase Contrast Microscopy (PCM, NIOSH 7400) counts fibers with length > 5 µm and aspect ratio ≥ 3:1 using a Walton-Beckett graticule; PCM cannot distinguish asbestos from non-asbestos fibers.

Polarized Light Microscopy (PLM, EPA 600) identifies bulk asbestos minerals by optical crystallography (refractive index oils, dispersion staining, birefringence, sign of elongation).

Transmission Electron Microscopy (TEM, NIOSH 7402 / AHERA) provides definitive fiber identification via Selected Area Electron Diffraction (SAED) and Energy Dispersive X-ray Spectroscopy (EDS) down to 0.01 µm fiber diameter.

Limit of Quantitation (LOQ) is defined as 10 times the blank standard deviation (or 3.33 × LOD); sample results between LOD and LOQ are qualitatively present but carry elevated quantitative uncertainty.

Last updated: August 2026

3.3 Gravimetric, Microscopy, and Wet Chemical Methods

Gravimetric determinations, optical microscopy, electron microscopy, and fundamental detection-limit statistics are central to industrial hygiene practice. Industrial hygienists must know how to quantify respirable and total dust loadings, differentiate asbestos minerals from synthetic vitreous fibers, and apply analytical quality assurance principles to occupational exposure data.

1. Gravimetric Analysis for Total and Respirable Dusts

Gravimetric analysis measures the net particulate mass collected on a membrane filter by differential weighing before and after air sampling (C = Δ m / V). [Pre-Condition Filter (24h @ 20°C, 50% RH)] ──> [Neutralize Static (Po-210)] ──> [Tare Weigh (Microbalance ±1 µg)]

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• [Air Sampling]
• │

[Post-Condition Filter (24h @ 20°C, 50% RH)] ──> [Neutralize Static (Po-210)] ──> [Gross Weigh & Blank Correct]

Filter Media Selection & Properties

Filter Medium Pore Size Characteristics Target Analytes & IH Methods

Polyvinyl Chloride (PVC) 5.0 µm Hydrophobic; very low moisture absorption; low tare weight variability. Resistant to weak acids and bases. Total dust (NIOSH 0500), Respirable dust (NIOSH 0600), Crystalline Silica (NIOSH 7500/7602), Hexavalent Chromium.

Mixed Cellulose Ester (MCE) 0.8 µm or 0.45 µm Hydrophilic; hygroscopic (absorbs ambient moisture); easily dissolved in acids or cleared with acetone. Metals digestion (NIOSH 7300), Asbestos fiber counting by PCM (NIOSH 7400) and TEM (NIOSH 7402). Not suitable for gravimetric dust.

Polytetrafluoroethylene (PTFE / Teflon) 1.0 - 2.0 µm Chemically inert; highly hydrophobic; stable at elevated temperatures. Polynuclear aromatic hydrocarbons (PAHs), asphalt fumes, acid mists, specialized aerosols.

Glass Fiber / Quartz Fiber 0.3 - 1.0 µm High particulate loading capacity; binderless; withstands high temperatures (>500°C). Oil mists, pesticides, semi-volatile aerosols, high-volume ambient particulate sampling.

Microbalance Procedures & Environmental Controls

Microbalance Readability: Personal dust determinations require a 6-place microbalance (readability to 0.001 mg = 1 µg) or at minimum a 5-place balance (0.01 mg = 10 µg).

Environmental Conditioning: Filters must equilibrate in a temperature- and humidity-controlled weighing room or desiccator chamber at 20 ± 1°C and 50 ± 5% RH for at least 24 hours prior to both pre-sampling tare weighing and post-sampling gross weighing.

Static Charge Elimination: Static electrical charges on PVC membranes create phantom electrostatic forces against balance pans, causing weighing errors of 50 - 500 µg. Filters must be swept over a Polonium-210 (²¹⁰Po) alpha-ionizing source or an electronic bipolar ionizer immediately before placement on the balance pan.

Field Blank Adjustments: Field blanks (minimum of 2 per sampling set or 10% of samples) undergo identical storage, transport, and conditioning. The mean weight change of the field blanks (Δ mblank) is subtracted from sample weight gains: Δ m s a m p l e , c o r r e c t e d = ( m p o s t − m p r e ) s a m p l e − ( m p o s t − m p r e ) ‾ b l a n k s \Delta m_{sample, corrected} = (m_{post} - m_{pre})_{sample} - \overline{(m_{post} - m_{pre})}_{blanks} Δ m s am pl e , correc t e d ​ = ( m p os t ​ − m p re ​ ) s am pl e ​ − ( m p os t ​ − m p re ​ ) ​ b l ank s ​

2. Microscopy Techniques for Asbestos and Fibers

Workplace fiber analysis distinguishes between regulated asbestos minerals (serpentine chrysotile and amphiboles: amosite, crocidolite, tremolite, actinolite, anthophyllite) and non-asbestos fibers (fiberglass, rock wool, gypsum needles, cellulose). [Airborne or Bulk Fiber Sample]

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• ┌─────────────────────────────────────────┼─────────────────────────────────────────┐
• ▼ ▼ ▼
• [Phase Contrast Microscopy (PCM)] [Polarized Light Microscopy (PLM)] [Transmission Electron Microscopy (TEM)]
• NIOSH Method 7400 • EPA 600/R-93/116, NIOSH 9002 • NIOSH Method 7402 / AHERA
• Personal air monitoring (400x) • Bulk material identification (>1%) • Personal air & clearance monitoring
• Walton-Beckett Graticule • Refractive index oils & dispersion • SAED crystal structure + EDS elements
• Non-specific: Counts ALL fibers • Distinguishes asbestos types • Resolves thin fibrils down to 0.01 µm
• Phase Contrast Microscopy (PCM) — NIOSH Method 7400

Principle: Converts subtle phase shifts in light passing through transparent specimens into amplitude (brightness) differences using an annular diaphragm and phase-shifting ring.

Optics & Magnification: Positive phase-contrast microscope operating at 400x - 450x magnification with a green bandpass filter (540 nm). Resolution is verified using an HSE/NPL Phase Calibration Test Slide (block 5 must be visible; block 6 or 7 partially visible).

Filter Preparation: Air collected on 25-mm conductive cowl MCE filters (0.8 or 0.45 µm). A wedge is cleared with hot acetone vapor (rendering the filter transparent) and mounted in triacetin (glycerol triacetate, refractive index n ≈ 1.45) under a glass coverslip.

Walton-Beckett Graticule & Counting Rules (Rule A - Occupational):

Fiber Definition: Any particle with an aspect ratio (length-to-width) ≥ 3:1 , length > 5 µm , and diameter < 3 µm.

Field Area (Af): Standard Walton-Beckett graticule field area is calibrated with a stage micrometer (typically 0.00785 mm²).

In/Out Rules:

Fiber entirely inside graticule = 1.0 fiber .

Fiber with one end inside and one end outside = 0.5 fiber .

Fiber with both ends outside = Do not count (0) .

Fiber bundles count as 1.0 if individual fibers cannot be resolved.

Stopping Rules: Count 100 fibers or 100 fields , whichever occurs first, with a mandatory minimum of 20 fields counted . Target fiber loading: 100 to 1,300 fibers/mm² .

Critical Limitation: PCM is completely non-specific. It counts all qualifying particles (synthetic vitreous fibers, cellulose, gypsum, cotton) and cannot visualize thin asbestos fibers with diameters < 0.25 µm.

Polarized Light Microscopy (PLM) — EPA 600 / NIOSH 9002

Application: Qualitative mineral identification and semi-quantitative percent estimation in bulk building materials (Asbestos-Containing Material, ACM defined as > 1% asbestos).

Diagnostic Crystallographic Properties:

Morphology: Curly, wavy bundles (Chrysotile) vs straight, needle-like acicular fibers (Amphiboles: Amosite, Crocidolite).

Refractive Index (n(∥), n(⊥)): Evaluated using calibrated Cargille refractive index immersion liquids (e.g., n = 1.550, 1.605, 1.680).

Birefringence (Δ n = |n(∥) - n(⊥)|): Brightness and interference colors viewed between crossed polarizers (90°).

Sign of Elongation (using a 530 nm full-wave First Order Red / Tint plate):

Length-Slow (Positive, +): Slow vibration direction parallel to fiber length (addition color: blue in NE-SW orientation). Chrysotile and Amosite are length-slow.

Length-Fast (Negative, -): Fast vibration direction parallel to fiber length (subtraction color: yellow). Crocidolite is length-fast.

Dispersion Staining: Central stop or annular stop objectives produce distinctive color halos when the refractive index of the immersion liquid matches the fiber at a specific wavelength (λ0).

Point Counting (400-Point Method): Used for samples estimated visually at 1% - 10% asbestos to establish legally defensible quantification against the 1% regulatory threshold.

Transmission Electron Microscopy (TEM) — NIOSH 7402 / AHERA

Principle: High-energy electron beam (80 - 120 keV) transmits through carbon-coated specimen grids at 10,000x - 20,000x magnification .

Definitive Identification:

Selected Area Electron Diffraction (SAED): Generates crystal lattice electron diffraction patterns verifying mineral crystalline symmetry.

Energy Dispersive X-Ray Spectroscopy (EDS / EDX): Detects characteristic X-rays emitted by sample elements, verifying atomic composition ratios (Mg:Si:Fe:Ca:Na).

Resolving Limit: Visualizes ultra-thin fibrils down to 0.01 µm (10 nm) in diameter.

AHERA Clearance Standard (40 CFR 763): School asbestos abatement clearance requires airborne asbestos levels below 70 structures/mm² on the filter.

3. Limit of Detection (LOD), Limit of Quantitation (LOQ), and Quality Control

Understanding statistical detection thresholds is essential when interpreting laboratory data near regulatory compliance action levels. ──┼──────────────────────────────┼──────────────────────────────┼──────────────────────────────> Concentration

• 0 LOD LOQ
• (3σ) (10σ)
• [ Non-Detect ] [ Trace / Semi-Quant ] [ Fully Quantifiable ]
• [ < LOD: Censor or MLE ] [ Qualitative Identification ] [ RSD ≤ 10%, Defensible ]
• Statistical Definitions & Formulations

Limit of Detection (LOD): The lowest analyte concentration that can be reliably distinguished from analytical background noise at a 99% confidence level (3σ): LOD = 3 × s blank = 3 × s y m \text{LOD} = 3 \times s_{\text{blank}} = \frac{3 \times s_y}{m} LOD = 3 × s blank ​ = m 3 × s y ​ ​ Where sblank is the standard deviation of field/reagent blanks, sy is the standard error of regression, and m is calibration slope.

Limit of Quantitation (LOQ): The lowest concentration at which analyte mass can be measured with acceptable quantitative precision (relative standard deviation RSD ≤ 10%): LOQ = 10 × s blank = 3.33 × LOD \text{LOQ} = 10 \times s_{\text{blank}} = 3.33 \times \text{LOD} LOQ = 10 × s blank ​ = 3.33 × LOD

Data Interpretation Zones:

< LOD (Non-Detect): Analyte not detected. For exposure statistics across similar exposure groups (SEGs), use Maximum Likelihood Estimation (MLE) or Robust Regression on Order Statistics (ROS) rather than simple zero substitution.

LOD ≤ Result < LOQ ("Trace" or "J-Flag"): Analyte is confirmed present with qualitative certainty, but the numerical value carries high uncertainty and is considered semi-quantitative.

≥ LOQ: Fully quantifiable, legally defensible measurement meeting rigorous precision criteria.

Quality Assurance & Quality Control (QA/QC) Protocols

Field Blanks: Handled identically to active samples without pulling air; identifies contamination from media manufacturing, storage, or transit.

Media / Reagent Blanks: Unexposed laboratory blanks that establish baseline instrument noise and reagent purity.

Matrix Spikes & Spike Duplicates: Known quantities of analyte added to sample media; evaluates percent recovery (accuracy) and Relative Percent Difference (RPD) (analytical precision): RPD (%) = ∣ C 1 − C 2 ∣ ( C 1 + C 2 ) / 2 × 100 % \text{RPD (\%)} = \frac{|C_1 - C_2|}{(C_1 + C_2)/2} \times 100\% RPD (%) = ( C 1 ​ + C 2 ​ ) /2 ∣ C 1 ​ − C 2 ​ ∣ ​ × 100%

AIHA Proficiency Analytical Testing (PAT) Program: External blind proficiency testing program ensuring industrial hygiene laboratories maintain inter-laboratory accuracy and ISO/IEC 17025 accreditation standards.

4. Worked Example: PCM Airborne Fiber Concentration Calculation

Worked Problem: An industrial hygienist performs personal air monitoring for asbestos during pipe insulation removal. Air is sampled using a 25-mm conductive cowl cassette containing a 0.8 µm MCE filter at a calibrated flow rate of 2.00 L/min for 480 minutes.

Microscopic Analysis (NIOSH Method 7400, Rule A):

• Total Graticule Fields Examined on Sample (n): 100 fields
• Total Fibers Counted on Sample (F): 162.0 fibers
• Total Graticule Fields Examined on Field Blank (nb): 100 fields
• Total Fibers Counted on Field Blank (Fb): 2.0 fibers
• Calibrated Walton-Beckett Graticule Field Area (Af): 0.00785 mm²
• Effective Filter Collection Area (Ac): 385 mm² (standard for 25-mm cowl)

Tasks:

Calculate the sample fiber density (E in fibers/mm²) and verify that it falls within the optimal loading range.

Calculate the sampled air volume in liters and cubic centimeters (cm³ = mL).

Calculate the airborne fiber concentration in fibers/cc.

Evaluate the result against the OSHA 8-hour TWA Permissible Exposure Limit (0.1 fibers/cc) and the OSHA 30-minute Excursion Limit (1.0 fibers/cc).

Step-by-Step Solution:

Step 1: Calculate Sample Fiber Density (E) Mean fibers per field on sample: F/n = 162.0/100 = 1.620 fibers/field

• Mean fibers per field on blank: Fb/nb = 2.0/100 = 0.020 fibers/field
• Net fibers per field: 1.620 - 0.020 = 1.600 fibers/field

Fiber density on the filter (E): E = ( F n − F b n b ) A f = 1.600 fibers/field 0.00785 mm 2 / field = 203.82 fibers/mm 2 E = \frac{\left(\frac{F}{n} - \frac{F_b}{n_b}\right)}{A_f} = \frac{1.600 \text{ fibers/field}}{0.00785 \text{ mm}^2/\text{field}} = 203.82 \text{ fibers/mm}^2 E = A f ​ ( n F ​ − n b ​ F b ​ ​ ) ​ = 0.00785 mm 2 / field 1.600 fibers/field ​ = 203.82 fibers/mm 2 Validation: 203.82 fibers/mm² falls cleanly within the NIOSH 7400 valid quantification range of 100 to 1,300 fibers/mm².

Step 2: Calculate Air Sampling Volume (V) V = 2.00 L/min × 480 min = 960 L = 960 , 000 cm 3 V = 2.00 \text{ L/min} \times 480 \text{ min} = 960 \text{ L} = 960,000 \text{ cm}^3 V = 2.00 L/min × 480 min = 960 L = 960 , 000 cm 3

Step 3: Calculate Airborne Fiber Concentration (C) Using the NIOSH 7400 formula: C = E × A c V × 1000 (if V in L) = ( F n − F b n b ) × A c A f V ( in cm 3 ) C = \frac{E \times A_c}{V \times 1000 \text{ (if } V \text{ in L)}} = \frac{\left(\frac{F}{n} - \frac{F_b}{n_b}\right) \times \frac{A_c}{A_f}}{V (\text{in cm}^3)} C = V × 1000 (if V in L) E × A c ​ ​ = V ( in cm 3 ) ( n F ​ − n b ​ F b ​ ​ ) × A f ​ A c ​ ​ ​ C = 203.82 fibers/mm 2 × 385 mm 2 960 , 000 cm 3 = 78 , 470.7 total fibers 960 , 000 cm 3 = 0.0817 fibers/cc C = \frac{203.82 \text{ fibers/mm}^2 \times 385 \text{ mm}^2}{960,000 \text{ cm}^3} = \frac{78,470.7 \text{ total fibers}}{960,000 \text{ cm}^3} = 0.0817 \text{ fibers/cc} C = 960 , 000 cm 3 203.82 fibers/mm 2 × 385 mm 2 ​ = 960 , 000 cm 3 78 , 470.7 total fibers ​ = 0.0817 fibers/cc

Regulatory Assessment: The calculated 8-hour TWA exposure is 0.0817 fibers/cc, which complies with the OSHA 8-hour TWA PEL of 0.10 fibers/cc. However, because it exceeds the OSHA Action Level of 0.05 fibers/cc (or 50% of the PEL), employer medical surveillance and employee training obligations remain in effect.