CO₂ Isn’t the Problem. What It Reveals Is.

Carbon dioxide (CO₂) is frequently misunderstood in discussions about indoor air quality. It is often described as a pollutant responsible for headaches, fatigue, and poor concentration. In reality, at the concentrations typically found indoors, CO₂ is not directly toxic.

Its importance lies elsewhere.

CO₂ is one of the most reliable indicators of how well an indoor space is ventilated relative to occupancy. Rather than being a primary health hazard, it functions as a proxy for broader air quality dynamics that are otherwise difficult to observe.

How CO₂ Accumulates Indoors

Indoors, CO₂ is generated almost entirely by human respiration. In spaces with sufficient air exchange, exhaled CO₂ is diluted and removed. In spaces with limited ventilation, concentrations increase rapidly.

CO₂ accumulation is influenced by:

• occupancy density

• ventilation rate

• air exchange efficiency

• duration of occupancy

Because CO₂ responds quickly to changes in these variables, it provides near real-time feedback on how a space is performing.

CO₂ as a Ventilation Indicator

Elevated CO₂ levels indicate that fresh outdoor air is not being introduced at a rate sufficient to offset human occupancy. While CO₂ itself may remain within non-toxic ranges, its rise signals a broader issue: air stagnation.

When ventilation is inadequate, other airborne substances are also more likely to accumulate, including:

• fine particulate matter

• volatile organic compounds (VOCs)

• moisture and bioaerosols

For this reason, CO₂ is often described as an indicator of “air freshness” rather than a pollutant in isolation.

Cognitive and Comfort Effects

A substantial body of research shows associations between elevated indoor CO₂ concentrations and reduced cognitive performance, including slower decision-making and impaired concentration. These effects are thought to arise not from CO₂ toxicity, but from the overall degradation of indoor air conditions that CO₂ levels reflect.

Importantly, these impacts can occur well below levels associated with regulatory concern, making them easy to overlook in standard compliance-based assessments.

Why CO₂ Spikes Matter

CO₂ does not increase gradually and uniformly. It rises in response to specific activities and conditions.

Common scenarios include:

• meetings in enclosed rooms

• overnight occupancy in bedrooms

• classrooms and shared workspaces

• home offices with closed windows

Short-term elevations can be significant, even if daily averages appear acceptable. Because many air quality guidelines focus on mean values, these transient but repeated exposures are often underrepresented in reporting.

Patterns Over Thresholds

Interpreting CO₂ as a simple pass–fail metric is misleading. More meaningful insights come from examining patterns:

• how quickly CO₂ rises after occupancy begins

• how long it remains elevated

• how effectively it returns to baseline

• how often these cycles repeat

These dynamics reveal ventilation performance, room suitability, and usage mismatches that static thresholds cannot capture.

What CO₂ Data Enables

CO₂ measurements provide actionable information about indoor environments without requiring complex chemical analysis. They allow:

• assessment of ventilation adequacy

• identification of underperforming spaces

• comparison of different rooms or layouts

• evaluation of operational changes over time

Used correctly, CO₂ data supports evidence-based decisions about ventilation strategies, space usage, and building design.

Understanding CO₂ in Context

CO₂ should not be viewed as a standalone health risk, nor should it be ignored. Its value lies in context. When combined with other environmental measurements, it helps explain why certain spaces consistently feel uncomfortable or cognitively demanding.

In this sense, CO₂ is not the problem to be eliminated, but a signal to be interpreted.

Understanding indoor air quality begins not with symptoms, but with indicators that reveal how spaces actually function.