The established benchmark for assessing indoor air quality is CO2 level measurement. While particulate matter, VOCs, and other contaminants are important, indoor CO2 levels are often used as a proxy for pollutant dilution in densely occupied spaces, indicating fresh air availability.

Indoor CO2 levels typically reflect the amount of fresh air entering a building. Higher CO2 concentrations indoors, compared to outdoor levels (350-450 ppm), indicate lower fresh air exchange. CO2, a by-product of human respiration, accumulates in poorly ventilated spaces. Well-managed indoor spaces maintain CO2 levels between 450-1,000 ppm. Levels above 1,000 ppm often result in complaints about stuffiness and poor air quality and can cause headaches, sleepiness, poor concentration, and attention loss. Extremely high CO2 levels can be life-threatening due to oxygen deprivation.

Monitoring CO2 is crucial for maintaining indoor air quality (IAQ) and preventing the spread of airborne illnesses. In homes, offices, classrooms, gyms, and commercial buildings, elevated CO2 levels can lead to severe health effects if not properly monitored. A common scenario is decreased performance and focus during long meetings in poorly ventilated rooms due to high CO2 levels.

CO2 sensors are essential for indicating the amount of "filtered" air and identifying when more ventilation is needed. Fresh air should be supplied to reduce high CO2 levels. When CO2 sensors indicate high levels, opening windows or doors can quickly restore productivity, focus, and energy.

Energy audits are a common practice in IAQ applications, focusing on energy efficiency and addressing health and comfort concerns. Monitoring CO2 levels in smart homes and commercial buildings can reduce gas and electric bills while improving health and well-being.

For a quick reference, consult the CO2 Classification Guide, which outlines CO2 levels indicating "normal outdoor levels" versus "poor indoor air quality" and the OSHA/ASHRAE recommended comfort standards. This guide helps determine when more ventilation is needed to create healthier environments.

CO2 monitoring systems often need to be installed where access to main power is limited or costly. The placement of CO2 sensors usually involves a compromise between optimal gas mixing locations and accessibility to power and communications infrastructure. Traditionally, CO2 monitors have been designed as small white boxes mounted on the wall near doors. However, more recently, IAQ monitors are being installed in ducting or adjacent to rooms.

Long-term, autonomous operation of CO2 sensors, powered by batteries or energy harvesting techniques, is highly desirable to reduce maintenance costs. Users seek CO2 sensors that can operate independently for many years without intervention.

When discussing CO2 sensors for indoor air quality, Non-Dispersive Infrared (NDIR) technology is typically the most common method of measurement. NDIR technology relies on high-quality spectroscopy using an IR lamp that emits a beam of light. There is an inverse relationship between the amount of CO2 in the gas sample and the amount of IR light detected at the target energy level. Without CO2 in the chamber, all IR light reaches the detector. As CO2 concentration increases, more IR light is absorbed, and less is detected by the IR detector. This precise measurement principle enables CO2 sensors to calculate the approximate concentration of CO2 in the gas sample in parts per million (ppm).