Carbon monoxide and carbon dioxide are different gases with different monitoring needs. This article focuses on carbon dioxide (CO₂) and how monitoring it can help you investigate ventilation and indoor air quality (IAQ).

According to the U.S. Environmental Protection Agency, people spend about 90% of their time indoors. In occupied spaces, CO₂ levels can change with the number of people in the room, their activity, and the amount of outdoor air being supplied. Monitoring those changes can help you understand how a space is being ventilated over time.

Key Takeaways

• CO₂ can be a useful indicator when investigating ventilation in occupied spaces, but it does not represent every aspect of indoor air quality.
• A reading of 1,000 ppm is not a universal boundary between safe and unsafe indoor air.
• Recording CO₂ over time can reveal patterns that a single spot measurement may miss.

What Is CO₂?

Carbon dioxide is a colorless, odorless gas produced by processes including respiration and combustion. People naturally add CO₂ to indoor spaces as they breathe, so occupancy and ventilation both influence the concentrations you measure.

That makes CO₂ useful for evaluating certain ventilation questions, particularly in occupied spaces such as classrooms, offices, meeting rooms, and gyms. But CO₂ is only one part of IAQ. A CO₂ measurement does not tell you whether other pollutants, such as particulate matter, volatile organic compounds, or carbon monoxide, are present.

Why Monitor Indoor CO₂?

CO₂ measurements can help you investigate how conditions change throughout the day and how those changes relate to occupancy and building operation. For example, increasing CO₂ during occupied periods may help identify when a space needs closer evaluation of its ventilation strategy.

Several factors can influence indoor CO₂ levels:

• Number of occupants
• Occupant activity
• Room volume
• Outdoor-air ventilation and air distribution
• Combustion sources
• Outdoor CO₂ concentration

CO₂ should be interpreted in context rather than against one universal indoor threshold. ASHRAE does not establish 1,000 ppm as a universal indoor CO₂ maximum, even though that number is commonly used as a general reference point.

CO₂ Levels: What Do the Numbers Mean?

Different CO₂ values are used for different purposes. Ventilation indicators, occupational exposure limits, and immediately dangerous concentrations should not be treated as the same kind of threshold.

These occupational exposure values are not recommended everyday targets for offices, classrooms, or other normally occupied indoor spaces. For routine IAQ and ventilation work, interpret CO₂ in relation to occupancy, outdoor conditions, and the question you are trying to answer.

How Can CO₂ Data Logging Help?

A CO₂ data logger records measurements at regular intervals, giving you a time-based view of how conditions change rather than a single reading. That can help you compare occupied and unoccupied periods, identify recurring patterns, and investigate how ventilation changes affect the space.

The HOBO MX1102A records CO₂ from 0 to 5,000 ppm along with temperature and relative humidity. Bluetooth communication provides nearby access to recorded data through HOBOconnect.

Monitoring is the measurement step, not the solution itself. A data logger does not remove pollutants or adjust ventilation automatically, but the data can help you decide when a space needs further investigation or changes to building operation.

CO₂ is only one part of indoor air quality, but tracking it over time can give you useful evidence about how occupied spaces are behaving and where a closer look at ventilation may be worthwhile.