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Dilution ventilation: the mass balance approach to indoor air quality
Dilution ventilation is the strategy of controlling indoor contaminant concentration by continuously introducing fresh air to mix with and dilute contaminated room air, then exhausting the mixture. It's the most common approach for widely distributed, low-level contaminant sources where local exhaust capture at the source isn't practical, such as off-gassing from building materials, furniture, or general occupant-related emissions.
The governing relationship is a steady-state mass balance: at equilibrium, the rate of contaminant removal (airflow times concentration difference) equals the rate of contaminant generation. Rearranging gives the required dilution airflow: Q = G / (C_target − C_outdoor), where G is generation rate, C_target is your maximum acceptable concentration, and C_outdoor is the background level already present in incoming air.
Why the mixing safety factor matters so much
The basic mass balance formula assumes perfect, instantaneous mixing throughout the room — a theoretical ideal that real ventilation systems never achieve. In practice, contaminant concentration varies throughout a space, with pockets of higher concentration near the source or in poorly mixed corners. The mixing safety factor (K), typically ranging from 3 to 10, inflates the theoretical airflow to account for this imperfect mixing, ensuring the actual peak concentration anywhere in the room (particularly in the breathing zone) stays below target, not just the theoretical room-average concentration.
Selecting an appropriate K factor requires engineering judgment about source location, supply/return air diffuser placement, and room geometry. A source located directly in the breathing zone with poor air mixing might require K=10, while a well-distributed source in a well-mixed space with good diffuser placement might only need K=3.
When to use local exhaust instead of dilution
For point sources with high emission rates — welding fumes, chemical processes, isolated equipment — local exhaust ventilation captures the contaminant at or very near the source before it disperses into the room. This is almost always more energy-efficient and effective than trying to dilute an entire room to control a concentrated point source, since the required local exhaust airflow is typically a small fraction of what whole-room dilution would need. Reserve dilution ventilation calculations for genuinely distributed sources, and use dedicated local exhaust hood or capture velocity calculations for point sources.
For occupant-driven CO2 and bioeffluent control specifically, use the CO2 calculator, which applies the same mass balance principle but with standardized occupant generation rates per ASHRAE methodology.
Frequently Asked Questions
Q = G / (C_target − C_outdoor), where Q is required airflow, G is generation rate, C_target is the maximum acceptable concentration, and C_outdoor is background concentration. Apply a safety factor (K, typically 3-10) for imperfect mixing. For 50 mg/hr generation with 5 mg/m³ target: theoretical Q = 10 m³/hr, then multiply by K for realistic design airflow. Use this calculator with your specific generation rate and target to get a complete design value with mixing factor applied automatically.
General dilution ventilation reduces concentration throughout the entire space by mixing in fresh air. Local exhaust captures contaminants directly at the source using a hood or enclosure, before they disperse. Local exhaust is almost always more efficient for point sources like welding or chemical processes, requiring far less total airflow. Dilution ventilation suits widely distributed, low-level sources like off-gassing materials. Use dilution calculations like this one only for genuinely distributed sources, not concentrated point emissions.