6.7 Chemical Mixtures and Extended Work-Shift OEL Adjustments

Chemical Mixtures and Extended Work-Shift OEL Adjustments

A single-agent, eight-hour comparison is the easy case. Real workplaces present several agents at once and shifts that are not eight hours long, and both situations have specific arithmetic that appears on every CIH exam.

1. Chemical Mixture Exposure Assessment

In industrial hygiene practice, workers are rarely exposed to single isolated chemicals. Painting, degreasing, printing, and composite fabrication involve multi-solvent vapors and chemical mixtures.

Toxicological Interaction Types

The Additive Mixture Formula (OSHA & ACGIH)

When two or more hazardous chemicals in a mixture share similar toxicological target organs or biological mechanisms, their combined effect is evaluated as additive:

$\mathbf{\sum_{i=1}^{n} \frac{C_i}{T_i} = \frac{C_1}{T_1} + \frac{C_2}{T_2} + \frac{C_3}{T_3} + \dots + \frac{C_n}{T_n} \le 1.0}$

Where:

• Ci = Measured airborne concentration of chemical i (in ppm or mg/m³)

• Ti = Corresponding Occupational Exposure Limit (PEL or TLV) for chemical i (in identical units)

• Interpretation:

  • If Σ Ci/Ti ≤ 1.0, the combined exposure is within acceptable limits.
  • If Σ Ci/Ti > 1.0, the exposure limit for the mixture is exceeded, even if every individual chemical concentration is below its respective standalone OEL (Ci < Ti).

Calculating Equivalent Mixture Exposure Limit (Tmix)

When evaluating a liquid solvent mixture with known liquid composition and vapor fractions, the equivalent mixture threshold limit (Tmix) in mg/m³ or ppm is:

$T_{\text{mix}} = \frac{C_{\text{total}}}{\sum_{i=1}^{n} \frac{C_i}{T_i}} = \frac{1.0}{\sum_{i=1}^{n} \frac{f_i}{T_i}}$

Where fi = Ci / Ctotal is the concentration fraction of component i in the airborne vapor mixture.

Independent Mixture Assessment

If the components act on entirely distinct physiological systems with completely unrelated health endpoints, additivity does not apply. Each substance is evaluated independently against its own standard:

$\frac{C_1}{T_1} \le 1.0, \quad \frac{C_2}{T_2} \le 1.0, \quad \dots, \quad \frac{C_n}{T_n} \le 1.0$

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2. Extended Work Shifts and OEL Adjustments

Traditional 8-hour workday / 40-hour workweek schedules are increasingly replaced in modern industrial facilities by non-traditional schedules (e.g., 10-hour shifts, 12-hour shifts, or 14-day rotational offshore shifts). Extended shifts cause a dual physiological stress:

1. Increased daily exposure duration (t_ extexp > 8 hours).
2. Reduced daily biological recovery and elimination time (24 - t_ extexp < 16 hours).

Consequently, body burden accumulates to higher steady-state levels than would occur under an 8-hour shift, requiring downward adjustment of exposure limits.

   +-------------------------------------------------------------+
   |                  EXTENDED SHIFT ADJUSTMENT MODELS           |
   +-------------------------------------------------------------+
   |                                                             |
   |  BRIEF & SCALA MODEL                                        |
   |  - Most widely used, highly conservative empirical model    |
   |  - Daily: RF = (8 / h) * ((24 - h) / 16)                    |
   |  - Weekly: RF = (40 / h_w) * ((168 - h_w) / 128)            |
   |  - Applies to systemic toxicants with t1/2 ~ 4 to 24 hours  |
   |                                                             |
   |  OSHA ADJUSTMENT MODELS                                     |
   |  - Category 1A (Irritants / Ceiling): NO reduction          |
   |  - Category 1B (Acute Toxicants): PEL_adj = PEL * (8 / h)   |
   |  - Category 2 (Cumulative Toxicants / Lead):                |
   |    PEL_adj = PEL * (40 / h_w) or PEL * (8 / h)              |
   |    (e.g., Lead PEL_adj = 400 / h ug/m3)                     |
   |                                                             |
   |  PHARMACOKINETIC MODELS (Hickey-Keith, Pharmacokinetic)     |
   |  - Uses specific biological elimination half-life (t1/2)    |
   |  - Avoids over-regulation for very long or very short t1/2  |
   +-------------------------------------------------------------+

The Brief & Scala Model

The Brief & Scala Model is an empirical mathematical adjustment developed to modify 8-hour TLVs for non-standard schedules:

1. Daily Reduction Factor (RFdaily) for shifts > 8 hours/day:

$\mathbf{RF_{\text{daily}} = \left(\frac{8}{h}\right) \times \left(\frac{24 - h}{16}\right)}$

$\mathbf{OEL_{\text{adj}} = OEL \times RF_{\text{daily}} = OEL \times \left(\frac{8}{h}\right) \times \left(\frac{24 - h}{16}\right)}$

Where h is the daily work shift length in hours (h > 8).

2. Weekly Reduction Factor (RFweekly) for schedules > 40 hours/week:

$\mathbf{RF_{\text{weekly}} = \left(\frac{40}{h_w}\right) \times \left(\frac{168 - h_w}{128}\right)}$

Where hw is the total hours worked per 7-day week (168 hours total in a week, 128 hours non-work in standard schedule). If both daily and weekly hours are non-standard, the more restrictive (smaller) RF is selected.

Reduction Factor Matrix for Common Work Schedules

Critical Restrictions on OEL Adjustments

1. Ceiling Limits (C): NEVER adjusted. Peak concentrations causing immediate acute damage or irritant threshold breach are independent of work shift duration.
2. Pure Sensory Irritants: Exposure limits based strictly on upper respiratory or ocular irritation (e.g., Ammonia, Formaldehyde) are NOT adjusted, because sensory irritation depends on instantaneous peak concentration rather than cumulative dose over time.
3. Substances with Very Long Half-Lives (t(1/2) > 1,000 hours): For cumulative toxins like Lead or Cadmium, daily shift fluctuations do not change steady-state bone/organ burdens; adjust based purely on weekly hours (40 / hw) rather than daily Brief & Scala.

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3. Worked Step-by-Step Calculation Examples

Worked Example 5.3.1: 8-Hour Time-Weighted Average (TWA) Calculation

Problem: An industrial hygiene personal air monitoring survey is performed for a degreasing operator exposed to trichloroethylene (TCE, OSHA PEL = 100 ppm, ACGIH TLV = 10 ppm). The monitoring results across the shift are as follows:

• 08:00 to 10:00 (120 min): C1 = 15.0 ppm
• 10:00 to 12:00 (120 min): C2 = 30.0 ppm
• 12:00 to 13:00 (60 min, lunch in clean breakroom): C3 = 0.0 ppm
• 13:00 to 16:00 (180 min): C4 = 8.0 ppm

1. Calculate the employee's 8-hour Time-Weighted Average exposure (TWA8).
2. Evaluate compliance against both the OSHA PEL (100 ppm) and ACGIH TLV (10 ppm).

Solution Steps:

1. Calculate total cumulative concentration-time product: $\sum (C_i \times t_i) = (15.0 \times 120) + (30.0 \times 120) + (0.0 \times 60) + (8.0 \times 180)$ $\sum (C_i \times t_i) = 1,800 + 3,600 + 0 + 1,440 = 6,840\,\text{ppm}\cdot\text{min}$

2. Divide by full 8-hour shift duration (480 minutes): $TWA_8 = \frac{6,840\,\text{ppm}\cdot\text{min}}{480\,\text{min}} = 14.25\,\text{ppm}$

3. Compliance Evaluation:

   • OSHA PEL (100 ppm): 14.25 ppm ≤ 100 ppm → In Compliance.
   • ACGIH TLV (10 ppm): 14.25 ppm > 10 ppm → Exceeds TLV.

Result: The employee's 8-hour TWA is 14.3 ppm, complying with the OSHA PEL but exceeding the health-based ACGIH TLV.

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Worked Example 5.3.2: Multicomponent Solvent Mixture Evaluation (Additive Formula)

Problem: In a spray painting booth, air monitoring reveals the simultaneous presence of three solvents with additive central nervous system depression effects:

• Acetone: Measured C1 = 150 ppm (ACGIH TLV T1 = 250 ppm)
• Methyl Ethyl Ketone (MEK): Measured C2 = 60 ppm (ACGIH TLV T2 = 200 ppm)
• Methyl Isobutyl Ketone (MIBK): Measured C3 = 12 ppm (ACGIH TLV T3 = 20 ppm)

1. Calculate the additive mixture exposure index (Imix).
2. Determine whether the mixture exposure complies with ACGIH additive standards.

Solution Steps:

1. Calculate individual exposure ratios (Ci / Ti): $\frac{C_{\text{acetone}}}{T_{\text{acetone}}} = \frac{150\,\text{ppm}}{250\,\text{ppm}} = 0.60$ $\frac{C_{\text{MEK}}}{T_{\text{MEK}}} = \frac{60\,\text{ppm}}{200\,\text{ppm}} = 0.30$ $\frac{C_{\text{MIBK}}}{T_{\text{MIBK}}} = \frac{12\,\text{ppm}}{20\,\text{ppm}} = 0.60$

2. Apply the Additive Mixture Formula: $I_{\text{mix}} = \sum_{i=1}^{n} \frac{C_i}{T_i} = 0.60 + 0.30 + 0.60 = 1.50$

3. Compliance Determination:

   • Because every individual chemical is below its standalone TLV (150 < 250, 60 < 200, 12 < 20), a naive single-chemical assessment would assume compliance.
   • However, because Imix = 1.50 > 1.00, the mixture limit is exceeded by 50%, representing an unacceptable combined narcotic exposure.

Result: The mixture index is 1.50, indicating an overexposure under the additive mixture standard.

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Worked Example 5.3.3: Extended Work Shift (12-Hour) Adjustment via Brief & Scala Model

Problem: Chemical processing operators work 12-hour shifts (h = 12) at a petrochemical refinery. An operator is exposed to n-hexane (8-hour ACGIH TLV-TWA = 50 ppm, skin).

1. Calculate the Brief & Scala daily reduction factor (RFdaily).
2. Calculate the adjusted TLV-TWA for this 12-hour work schedule.
3. If 12-hour personal sampling demonstrates an unadjusted TWA concentration of 32 ppm, determine compliance.

Solution Steps:

1. Calculate the Brief & Scala daily reduction factor: $RF_{\text{daily}} = \left(\frac{8}{h}\right) \times \left(\frac{24 - h}{16}\right) = \left(\frac{8}{12}\right) \times \left(\frac{24 - 12}{16}\right) = \left(\frac{2}{3}\right) \times \left(\frac{12}{16}\right) = \frac{2}{3} \times \frac{3}{4} = 0.500$

2. Calculate the adjusted TLV (TLVadj): $TLV_{\text{adj}} = TLV \times RF_{\text{daily}} = 50\,\text{ppm} \times 0.500 = 25.0\,\text{ppm}$

3. Compliance Evaluation:

   • Measured 12-hour TWA = 32 ppm.
   • Compared to standard 8-hour TLV (50 ppm), it appears compliant (32 < 50).
   • Compared to adjusted 12-hour TLV (25.0 ppm), 32 ppm > 25.0 ppm → Non-Compliant.

Result: The Brief & Scala reduction factor is 0.500, lowering the allowable 12-hour limit to 25.0 ppm; the worker's exposure of 32 ppm represents a verified overexposure.