CO2 Monitoring: The Indicator That Reveals Ventilation Problems

Carbon dioxide is a remarkably useful indicator of indoor air quality—not because CO2 itself is particularly harmful at typical indoor levels, but because elevated CO2 reliably signals inadequate ventilation. When ventilation is poor, CO2 accumulates alongside moisture, pollutants, and pathogens.

The Science Behind CO2 Monitoring

Where Indoor CO2 Comes From

Humans exhale approximately 200ml of CO2 per minute at rest, more during activity. This constant production means occupied spaces always have elevated CO2 compared to outdoor air:

• Outdoor air: ~400-420 ppm (parts per million)
• Well-ventilated indoor space: 600-800 ppm
• Adequately ventilated: 800-1,000 ppm
• Poorly ventilated: 1,000-2,000 ppm
• Very poor ventilation: 2,000+ ppm

Why CO2 Indicates Ventilation Quality

CO2 acts as a tracer gas. When occupants produce CO2 at a roughly constant rate, the steady-state concentration depends on the ventilation rate. Higher ventilation = more dilution = lower CO2.

This makes CO2 a proxy measurement. You're not primarily concerned about CO2 itself—you're using it to infer ventilation effectiveness.

What CO2 Levels Tell You

Below 800 ppm

Good ventilation. Fresh air is being supplied adequately:

• Moisture from breathing is being diluted
• Other pollutants (VOCs, particles) are also being removed
• Airborne pathogen concentration is reduced

800-1,200 ppm

Acceptable but worth monitoring. Typical of many occupied spaces:

• May indicate opportunities to improve ventilation
• Acceptable for short occupancy periods
• Consider whether vulnerable occupants are affected

Above 1,200 ppm

Ventilation is inadequate for the occupancy level:

• Moisture from breathing is accumulating
• Other pollutants are likely elevated too
• Stuffiness, drowsiness reported by occupants
• Increased risk of airborne disease transmission

Above 2,000 ppm

Poor ventilation requiring action:

• Immediate ventilation improvement needed
• Occupants may experience headaches, difficulty concentrating
• Condensation risk significantly elevated

The Link to Damp and Mould

Moisture Production

Humans produce moisture as well as CO2—through breathing and perspiration. A sleeping adult produces approximately 40g of water vapour per hour. When ventilation is inadequate to remove this moisture, humidity rises.

The Correlation

High CO2 levels typically correlate with:

• Elevated humidity: Moisture accumulating alongside CO2
• Condensation risk: More moisture means more condensation on cold surfaces
• Mould growth conditions: Sustained high humidity favours mould

Monitoring CO2 therefore provides an indirect indicator of damp risk—especially overnight when windows are closed and bedrooms are occupied.

Practical Applications

Identifying Problem Properties

CO2 monitoring reveals which properties have ventilation issues:

• Properties with blocked vents or sealed trickle vents
• Over-occupied homes producing more moisture than ventilation can handle
• Failed mechanical ventilation systems
• Properties where behaviour prevents adequate air change

Verifying Ventilation Interventions

After installing or upgrading ventilation:

• Compare before and after CO2 levels
• Confirm that interventions have improved air change rates
• Identify installations that aren't performing as expected

Supporting Tenant Conversations

Data makes discussions easier:

• "Your CO2 levels show the bedroom isn't getting enough fresh air"
• "Opening the trickle vent reduced overnight CO2 by 40%"
• Objective basis for advice rather than assumptions

CO2 Monitoring in Practice

Where to Monitor

Focus on occupied spaces with potential ventilation constraints:

• Bedrooms: Long occupancy periods, often poorest ventilation
• Living rooms: High occupancy during evening hours
• Kitchens: Moisture production plus occupancy

When to Monitor

CO2 patterns vary through the day:

• Overnight: Bedrooms peak during sleep with windows closed
• Evening: Living rooms peak during family time
• Daytime: Often lower when properties are unoccupied or windows open

Continuous monitoring captures the full picture; spot checks may miss peak periods.

Interpreting Patterns

Look for:

• Peak levels: How high does CO2 get overnight?
• Recovery time: How quickly does it fall when occupancy ends?
• Baseline: Does CO2 return to near-outdoor levels?
• Correlation: Does high CO2 coincide with high humidity?

Beyond CO2: Indoor Air Quality

While CO2 is a useful proxy, comprehensive air quality monitoring includes:

• Volatile Organic Compounds (VOCs): Off-gassing from materials, cleaning products
• Particulate Matter (PM2.5): Dust, combustion particles
• Temperature: Thermal comfort and condensation risk
• Humidity: Direct moisture measurement

Each metric tells part of the story; together they provide a complete picture of indoor environmental quality.

Key Takeaways

• CO2 reveals ventilation effectiveness—it's a proxy for air change rate
• Above 1,200 ppm suggests inadequate ventilation for the occupancy level
• High CO2 correlates with moisture accumulation and condensation risk
• Continuous monitoring captures patterns that spot checks miss
• Data supports better conversations with tenants about ventilation