Lesson 5 – Indoor Air Quality: When Ventilation Is Not the Solution
Lesson Objective
Understand why increasing outdoor air does not always solve indoor air quality complaints, and how many IAQ problems are caused by sources, distribution, and operation rather than ventilation rate alone.
The Ventilation Reflex
When occupants complain about odors, headaches, or “bad air,” the first reaction is often to increase outdoor air. While ventilation is important, it is not a universal solution. In many cases, adding more outdoor air treats the symptom while leaving the real problem untouched.
IAQ Is About Contaminants, Not Just Air Volume
Indoor air quality depends on what pollutants are present, where they come from, and how they are distributed. Ventilation only dilutes contaminants; it does not remove their sources. If the source remains active, complaints will continue regardless of airflow increases.
Example 1 – More Air, Same Smell
An office increases outdoor air to address odor complaints. Energy use rises, but the smell remains. Investigation later reveals that the source is a small pantry with poor exhaust and frequent cleaning chemicals. The issue was local source control, not overall ventilation rate.
Outdoor Air Quality Matters Too
Outdoor air is not always clean. Traffic, nearby construction, or industrial activity can introduce pollutants into the building. Increasing outdoor air under these conditions can worsen indoor air quality instead of improving it.
Example 2 – Fresh Air That Isn’t Fresh
A building near a busy road increases outdoor air intake during peak hours. Occupants complain of headaches and irritation. The outdoor air contains high levels of pollutants, and filtration is inadequate. More air made the problem worse.
Ventilation Effectiveness and Air Distribution
Even when the correct amount of outdoor air is supplied, poor air distribution can prevent it from reaching occupants effectively. Short-circuiting between supply and return, stagnant zones, and poorly placed diffusers reduce ventilation effectiveness.
Example 3 – Correct Numbers, Wrong Results
Ventilation calculations meet code requirements, but occupants in meeting rooms complain of stuffiness. Measurements show that fresh air never reaches the occupied zone due to poor diffuser placement and short-circuiting to returns.
Source Control: The Forgotten Strategy
The most effective way to improve IAQ is often to control or isolate pollutant sources. This includes local exhaust, material selection, equipment segregation, and proper maintenance. Source control reduces the load before ventilation is even needed.
Example 4 – Printer Room Problems
A large printer area causes odors throughout the office. Increasing outdoor air does little. Adding local exhaust and sealing the room resolves the issue with minimal impact on the main HVAC system.
Energy and IAQ Trade-Off
Increasing ventilation significantly increases energy consumption, especially in hot and humid climates. If ventilation is used blindly as the primary IAQ solution, energy penalties can be severe without guaranteed improvement in occupant satisfaction.
Key Insight
Ventilation is a tool, not a cure. Without understanding pollutant sources and air distribution, increasing outdoor air often creates new problems instead of solving existing ones.
Key Takeaway
When facing IAQ complaints, do not ask “How much more air do we need?”
First ask “What is the contaminant, and where is it coming from?”
Reflection
In your projects, how many IAQ problems were solved by source control—
and how many were treated only by adding more outdoor air?
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