The Science of Biofilm Formation and How It Affects Airflow

For a long time, I assumed airflow problems in buildings were mechanical. A fan slowed down. A filter clogged. A duct design failed. These explanations felt logical and visible. When air felt heavy or stale, the solution seemed straightforward. Clean the system. Replace the filter. Increase ventilation.

What I did not realize was that one of the most influential factors affecting airflow was something I could not see at all.

Biofilm

Biofilm is not dirt in the traditional sense. It is not dust that settles on a surface and waits to be removed. It is a living structure formed by microorganisms that attach themselves to moist surfaces and begin to grow together. Once established, biofilm becomes highly resistant to removal and quietly alters how air systems behave.

Understanding biofilm changes how we think about airflow and indoor air quality.

Biofilm formation begins when microscopic organisms encounter a surface that provides moisture and nutrients. HVAC systems create ideal conditions for this process. Coils, drain pans, and internal surfaces frequently experience condensation. Air carries organic particles. Temperature remains stable. Over time, microorganisms settle, attach, and begin producing a protective matrix.

This matrix acts like glue. It anchors the organisms to the surface and traps additional particles from the air. What starts as an invisible layer slowly thickens into a structured film that changes the physical properties of the surface itself.

In HVAC coils, this has serious consequences.

Airflow relies on clear pathways. When biofilm forms on coil fins, it narrows the gaps air must pass through. Resistance increases. The system must work harder to move the same volume of air. Even a thin layer of biofilm can significantly reduce heat transfer efficiency and restrict airflow.

What makes this particularly challenging is how subtle the process is. Biofilm does not appear suddenly. It develops gradually. Airflow reduction happens slowly. Energy use increases incrementally. Occupants feel discomfort without obvious cause.

The building does not fail. It quietly underperforms.

Biofilm also changes how air behaves once it passes through the system. As air moves across contaminated surfaces, it picks up microbial byproducts, odors, and fine particles released from the biofilm structure. These contaminants reenter occupied spaces repeatedly, even when filters are present downstream.

This is why buildings with clean filters can still experience poor air quality.

Filters stop what comes in. Biofilm affects what the system generates internally.

Traditional cleaning methods often focus on visible debris. Dust is removed. Surfaces appear clean. But biofilm is not easily eliminated by surface cleaning alone. The protective matrix shields microorganisms from mechanical removal and many chemical treatments. Once established, biofilm regrows quickly after cleaning, especially in moist environments.

This cycle explains why airflow issues often return shortly after maintenance. The root cause remains active beneath the surface.

The science behind biofilm also explains why energy efficiency declines over time. As airflow resistance increases, fans consume more power. Heat exchange becomes less effective. Systems run longer to achieve the same temperature results. What appears to be an aging system is often a biologically burdened one.

From a human perspective, the impact is just as important.

Restricted airflow leads to uneven temperature distribution. Some areas feel stuffy while others feel overcooled. Humidity control becomes inconsistent. People experience fatigue, headaches, and difficulty concentrating. These symptoms are rarely traced back to biofilm, yet the connection is real.

Healthy buildings depend on predictable airflow. Biofilm introduces variability. It turns precise systems into uncertain ones.

What makes biofilm particularly problematic is that it thrives in exactly the places people rarely inspect. Deep within coils. Inside drain pans. Along internal surfaces hidden from view. By the time it is detected, it has often been influencing airflow for months or years.

This is why modern building science increasingly treats biofilm as an active system factor rather than a maintenance nuisance. Addressing airflow effectively means addressing biological activity at its source.

Prevention becomes more powerful than removal. Limiting microbial growth reduces biofilm formation before it can restrict airflow. Keeping surfaces inhospitable to biofilm changes the long-term behavior of the system.

When biofilm is controlled, airflow stabilizes. Energy efficiency improves. Air feels lighter. Systems operate closer to their original design intent. Occupants may not know why the space feels better, but they notice the difference.

The science of biofilm reveals an important truth about indoor environments. Not all problems are mechanical. Some are biological. And the smallest organisms can have the largest impact on how a building breathes.

Airflow is not just about moving air.

It is about what the air touches along the way.

When those surfaces remain clean and biologically balanced, airflow becomes effortless again. And when airflow is effortless, indoor spaces begin to feel the way they were meant to.