Dilution Ventilation for Chemical Exposure Control

When Dilution Ventilation Is Appropriate

Dilution ventilation is appropriate when ALL of the following are met:

• Contaminant is of low to moderate toxicity (TLV generally above 100 ppm). ACGIH VS Manual Chapter 10 limits dilution to Class 1–3 substances.
• Generation rate is relatively uniform and predictable
• Workers are far enough from the source for mixing before breathing zone exposure
• Contaminant is a gas or vapor, not particulate

Dilution is NOT appropriate for: highly toxic substances (TLV < 100 ppm), carcinogens or sensitizers, intermittent releases with peak concentrations, or dusts/fumes/mists.

The ACGIH VS Manual recommends local exhaust as the first choice. Dilution is the fallback when LEV is impractical -- open-area solvent use, large-volume low-toxicity vapor generation, or supplemental control for fugitive emissions.

The Dilution Ventilation Formula

The ACGIH equation for solvent evaporation:

Q = (403 × SG × ER × 106 × K) / (MW × C)

Where:

• Q = required ventilation rate (CFM)
• 403 = constant combining molar volume at standard conditions with unit conversions
• SG = specific gravity of the liquid
• ER = evaporation rate (pints per minute)
• MW = molecular weight of the vapor
• C = acceptable concentration (ppm), typically the TLV-TWA
• K = mixing factor (3–10)

Evaporation rate is the most difficult variable. In practice, estimate from material usage: if a cleaning operation uses 2 pints of solvent per 8-hour shift, the average rate is 2/480 = 0.0042 pints per minute.

For the simpler case of a known generation rate in CFM of pure gas:

Q = G × K / Cfraction

Where G is generation rate in CFM and C_fraction is acceptable concentration as a decimal (ppm ÷ 10⁶).

Mixing Factors and Room Air Distribution

The ACGIH VS Manual K values:

• K = 1: Perfect mixing -- never achievable. Academic use only.
• K = 3: Good mixing. Well-placed supply/exhaust, minimal obstructions, low toxicity.
• K = 5: Average conditions. Some obstructions, moderate complexity.
• K = 8–10: Poor mixing. Complex geometry, significant obstructions, high toxicity needing extra margin.

Factors that improve mixing (lower K): cross-flow air pattern (supply low on one side, exhaust high on opposite), broad-distribution diffusers, ceiling fans preventing stratification, open floor plan.

Factors that worsen mixing (higher K): supply and exhaust on same wall (short-circuiting), large equipment creating dead zones, contaminant released near the exhaust, heavy contaminant at floor level with ceiling exhaust.

Practical Design Considerations

Supply and exhaust placement: Clean air should enter near workers, flow across the source, and exit at the exhaust. Workers must be upstream of the source. Never place workers between source and exhaust.

Makeup air: Every CFM exhausted must be replaced. Without makeup air, the building goes negative, exhaust fans lose effectiveness, and contaminated air enters through cracks and doors.

Temperature: In cold climates, dilution volumes can be enormous (> 10,000 CFM). All that air must be tempered in winter. This is a primary argument for LEV -- a 150 CFM capture hood replaces a 5,000 CFM dilution system at 3% of the energy cost.

Mixture effects: For multiple solvents, ACGIH recommends the additive mixture formula:

(C₁/T₁) + (C₂/T₂) + ... + (Cn/Tn) ≤ 1

Where C is measured concentration and T is the TLV for each component. This means dilution ventilation must be designed for the sum of individual requirements.