Lesson 2 – Common Indoor Air Contaminants and Sources
Indoor air quality problems are often discussed in general terms, but effective ventilation design starts with understanding what we are actually trying to control.
This lesson introduces the most common indoor air contaminants, their sources, and why their behavior matters before applying any ventilation calculation procedure.
What Is an Indoor Air Contaminant?
An indoor air contaminant is any substance present in the air that can negatively affect occupant health, comfort, or performance when its concentration exceeds acceptable levels.
From an HVAC perspective, contaminants are important not because they exist, but because of how they accumulate and how they can be diluted or removed.
Main Categories of Indoor Air Contaminants
Indoor air contaminants can be grouped into a few broad categories. Understanding these categories helps clarify why ventilation is often used as a control strategy.
- Human-related contaminants (bioeffluents, CO₂)
- Building material emissions (VOCs)
- Process-related contaminants (printing, cleaning activities)
- Outdoor-origin contaminants entering the building
- Moisture-related contaminants (mold, microbial growth)
Occupants as a Major Contaminant Source
Occupants are often the dominant source of indoor air contaminants in offices, classrooms, and residential buildings.
For this reason, many ventilation requirements are directly linked to the number of occupants rather than floor area alone.
CO₂ as an Indicator, Not a Pollutant
Carbon dioxide is often misunderstood as a harmful contaminant. In typical indoor environments, CO₂ itself is not the primary concern.
Instead, CO₂ is commonly used as an indicator of ventilation adequacy. Higher CO₂ levels generally suggest insufficient outdoor air relative to occupant-generated contaminants.
This distinction is critical and will become especially important when discussing the Indoor Air Quality Procedure in later lessons.
Building Materials and VOC Emissions
Many indoor air contaminants originate from building materials, furnishings, and finishes.
- Paints and coatings
- Adhesives and sealants
- Carpets and flooring
- Furniture and composite wood products
These contaminants are typically released at higher rates during early occupancy and gradually decrease over time.
Ventilation alone may not always be the most effective solution for controlling these sources, highlighting the importance of source control strategies.
Why Contaminant Sources Matter in Ventilation Design
Ventilation calculations are often performed without explicitly considering contaminant sources.
When source strength is ignored, ventilation rates tend to increase as a precaution, which can lead to higher energy consumption and system oversizing.
Understanding where contaminants come from helps engineers evaluate whether increased outdoor air is truly necessary or whether alternative control measures are more effective.
Key Takeaway
Indoor air contaminants originate from occupants, materials, activities, and outdoor air.
Effective ventilation design begins with understanding these sources, not with airflow calculations.
Reflection Question
In your past projects, which contaminant sources were most often implicitly addressed through ventilation rather than explicitly controlled or reduced?
Pause here and reflect before moving on. You may find it helpful to write down a few thoughts.