# The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings
## 2 Sensors and Monitoring Technologies
### Carbon Dioxide Sensors  
Carbon Dioxide (CO\({}_{2}\)) is a natural component of the air surrounding us and is exhaled by human beings during respiration. The amount of CO\({}_{2}\) people emit varies with activities in a quantitative manner [31]; the rate of CO\({}_{2}\) an occupant generates whilst sitting stationary can be up to 0.27 L/min and this can increase up to 0.53 L/min if the same person was lifting and packing [24]. The average adult's breath contains between 35,000 to 50,000 ppm of CO\({}_{2}\), which is up to 100 times more than the outdoor air [32], and carbon emissions (both CO\({}_{2}\) and carbon monoxide (CO)) can accumulate substantially depending on the number of occupants and insufficient ventilation within the building. Therefore, carbon dioxide sensors are some of the most widely used devices today as they enable the monitoring and control of carbon levels to ensure the health and safety of occupants, enhance energy efficiency, and help in maintaining Indoor Air Quality (IAQ) within buildings, while minimizing excessive air changes resulting in increased energy consumption.  
CO\({}_{2}\) sensors typically measure the concentration of gases in a particular space in units of ppm, and provide useful information regarding the presence, location, count and user activity within a building [33; 34; 35; 36]. The most common type of CO\({}_{2}\) sensors in this regard are NDIR (non-dispersive infrared) devices; sensors which monitor the net increase/decrease of light occurring at the wavelength at which CO\({}_{2}\) absorption takes place--the light intensity is used to measure the concentration of CO\({}_{2}\) in the atmosphere. NDIR sensors are widely used due to their high accuracies (\(\pm\)30-200 ppm), wide measuring range (0-10,000 ppm), durability and reliability. However, these sensors have relatively slow response times (30-100 s) and in some cases, have high power consumptions [37; 38; 39; 40; 41], see Table 1. In addition to CO\({}_{2}\) measurements, CO and volatile organic compounds ((VOC), e.g., ethanol, formaldehyde) measurement sensors have also gained significant attention in recent years as these pollutants are also ever-present in indoor environments [42; 43]. The two most commonly adopted types of CO and VOC sensors are metal oxide semiconductor (MOSFET)-based and electrochemical-based sensors. Metal oxide materials, such as tungsten oxide and titanium oxide (particularly WO\({}_{3}\) and TiO\({}_{2}\)), tend to exhibit high sensitivities to variations in air composition [44] and, in MOSFET sensors, this mechanism is exploited to produce a change in electrical conductivity of the sensor's material. Although MOSFET sensors are highly accurate (\(\pm\)30-100 ppm), have a wide measurement range (400-20,000 ppm) and possess both wired and wireless functionalities, they tend to have higher energy consumptions (as they require an electric heater to maintain the sensing element temperature within the sensor at up to 300 \({}^{\circ}\)C) [4], relatively slow response times (50-60 s) and short life spans. On the other hand, electrochemical sensors (devices which operate in a similar way to fuel cells and produce current outputs when exposed to air pollutants) exhibit faster response times (10-60 s), higher accuracies (\(\pm\)0-30 ppm) and sensitivities, longer life spans and lower energy consumptions [45], but tend to have smaller measurement ranges (0-1000 ppm) when compared with MOSFET sensors (see Table 1).