Indoor Air Quality

Indoor air quality has been equated to CO2 levels for Demand Controlled Ventilation (DCV) applications in an effort to improve building health and reduce energy costs. The benefits of healthy and “green” buildings are well known today and progress in controlling ventilation rates to optimum values is still being made. One area that has received significant study and attention is the measurement of indoor air quality pollutants.

The traditional measurement of CO2 levels is often seen as limiting when compared to the total amount of volatile organic compounds (VOCs) present in the air that have a detrimental effect on the human perception of air quality. These indoor VOCs are hydrocarbons that originate mainly from bio-effluents (odors from human respiration, perspiration and metabolism) and vapors generated from building materials and furnishings. There are thousands of unique VOCs that may be present in indoor air that affect the air quality. The table below lists some of the more common VOCs and their source.

It is generally understood that the root cause of indoor air quality problems lies with the presence of these VOCs. Unfortunately, it has been difficult to accurately measure VOCs due to the lack of suitable VOC sensing devices. Early VOC sensors suffered from long-term stability problems, drift and an output signal that was difficult to define and apply in a reliable way.

CO2 sensors have long served as an adequate air quality indicator with a defined range ppm output signal that is easy to set thresholds to. The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) is generally used in DCV applications because minimum ventilation rates are clearly defined based on occupancy and CO2 sensors are then generally used to determine occupancy.

This system has worked for DCV system designers due to it’s straight-forward design, predictable results and energy saving results. However, the control of indoor air quality based on CO2 levels alone is not optimal because it ignores other air contaminants that are often present. On-demand ventilation should react to all sources of contamination, not only CO2.

The IAQ Sensor

The Indoor Air Quality Sensor uses an advanced MEMS metal oxide semiconductor sensor to detect poor air quality. The sensor reacts quickly to detect a broad range of VOCs such as smoke, cooking odors, bio-effluence, outdoor pollutants and from human activities. The sensor captures all VOC emissions that are completely invisible to CO2 sensors.


Extensive studies and research have shown that there is direct correlation between CO2 levels and VOC levels and the Air Quality Sensor has been calibrated to provide a “CO2-equivalent” ppm measurement value, thereby achieving full compatibility to existing HVAC CO2 ventilation standards. The sensor also includes control algorithms that correct sensor drift and aging and therefore provides a long-term consistent DCV solution while overcoming the deficiencies of CO2 measurement by detecting the true root-cause of ventilation demand, VOCs. The IAQ sensor emulates the human perception of air quality much more than a CO2 sensor and even detects odorless, potentially hazardous substances such as carbon monoxide.

The CO2-equivalent sensor output value was developed over a period of several years to allow the IAQ sensor to be optimized for Demand Controlled Ventilation applications. The long-term IAQ sensor performance was monitored in various locations including offices, cafeterias, schools, production facilities, apartments and homes in direct comparison to infrared-absorption CO2 sensors. The data shows consistent results between measured CO2 values and the IAQ CO2-equivalent values and also highlight the poor air quality events detected by the IAQ sensor that the CO2 sensor misses. A sample chart showing CO2 measurements vs. IAQ measurements is shown below.

IAQ Sensor Features

  • Measures total VOCs
  • Direct correlation to CO2 levels
  • High sensitivity and fast response
  • Stable long-term operation
  • 0 to 2000 ppm CO2 output signal
  • LCD to display air quality information
  • Internal menu for easy setup
  • Analog stepped output for damper control
  • Linear output for logging and control
  • Selectable 0-5 or 0-10 Vdc signal
  • Relay output with adjustable setpoint
  • Optional resistive temperature sensors

For more information about Greystone’s line of Indoor Air Quality sensors (AQDT & AIR41) click here.