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How is Indoor Air Quality Measured?

 

For all the right reasons, improving indoor air quality has become a concern for many. However, when do we know the measures being taken to improve the IAQ are actually working? Or when we need to take further measures? 

Many methods are used to extract the level of indoor air quality, specifically measuring common symptoms found indoors. This includes; humidity, CO2 and VOCs.

You can use monitors to check on your building’s air quality to provide a safer and healthier environment. By monitoring air quality, you can also stop the negative consequences of indoor air pollutants. 

Common sensors include: 

  • Humidity sensors

  • CO2 sensors

  • VOC sensors

 

Humidity Sensors

What is Humidity?

Humidity is the concentration of water vapour present in the air. Common sources of excess moisture indoors include the overuse of a humidifier, long showers, running water for other uses, boiling or steaming in cooking, plants, and drying clothes indoors. Also, a tight, energy-efficient building holds more moisture inside.

Humidity Sensor

A humidity sensor is an electronic device that measures the humidity in its environment and converts its findings into a corresponding electrical signal. Humidity sensors vary widely in size and functionality; some humidity sensors can be found in handheld devices (such as smartphones), while others are integrated into larger embedded systems (such as air quality monitoring systems).

Humidity sensors can be divided into two groups, depending on the method used to calculate the humidity. Relative humidity, or RH, is calculated by comparing the live humidity reading at a given temperature to the maximum humidity for air at the same temperature. RH sensors must therefore measure temperature to determine relative humidity. Contrastingly, absolute humidity (AH) is calculated without reference to temperature.

 

CO2 Sensors

What is CO2?

CO2 is a natural constituent of the air we breathe; it is a colourless, odourless and non-flammable gas produced by metabolic processes (such as respiration) and by the combustion of fossil fuels. This is a result of both external atmospheric CO2 and internal production from the presence of people. 

The average outdoor air concentration of CO2 is in the order of 300 to 400 ppm.  Indoor levels are usually higher due to the CO2 exhaled by building occupants. Indoor combustion appliances, particularly gas stoves, can also increase CO2 levels.

CO2 Sensors

A carbon dioxide (CO2) sensor is a device used to measure carbon dioxide gas concentration in the atmosphere. It is measured using “parts per million” (ppm) and is measured through different technologies, including;

  1. Nondispersive Infrared (NDIR): monitors the absorption of infrared light at a specific wavelength (4.3 μm), a wavelength at which CO2 has very strong absorption. If the infrared light is absorbed, then CO2 is present, whereas non-absorption indicates a lack of CO2.
  2. Photoacoustic Spectroscopy: Subjects a sample to pulses of electromagnetic energy that are tuned specifically to the absorption wavelength of CO2. The CO2 molecules within the sample will absorb and generate pressure waves via the photoacoustic effect with each energy pulse. These pressure waves are then detected with an acoustic detector and converted to a usable CO2 reading through a computer or microprocessor.
  3. Electrochemical Carbon: dioxide sensors measure electrical current to determine how much CO2 is in the air. When CO2 enters the sensor, it chemically reacts within a polymer surface, resulting in an electrical charge. The type and amount of electrical charge are then used to determine how much CO2 is present.

 

VOC Sensors

What are VOCs?

Volatile organic compounds (VOCs) are emitted as gases from certain solids or liquids. VOCs are emitted by a wide array of products. Examples include: paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings, office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics and craft materials including glues and adhesives, permanent markers, and photographic solutions.

VOC Sensors 

As the name suggests, VOC sensors detect surrounding volatile organic compounds. VOCs typically come from gases that emanate from solid or liquid compounds. This can be residual paint fumes or gases from solvents or fuels. Commonly, people can detect or identify if they’re near VOCs by smell.

There are three main types of sensors used to detect VOC levels in the air:

  1. PIDs (photoionization detectors) break down compounds into positive and negative ions using ultraviolet light to identify VOCs. These sensors can analyze and detect a vast array of chemicals, including methylene chloride.
  2. FIDs (flame ionization detectors) detect hydrocarbons in various industries. A hydrogen flame interacts with hydrocarbons to produce ions. Alerts sound when any changes in ion levels are detected.
  3. MOS (metal oxide semiconductor sensor) sensors use a delicate film to detect compounds in the surrounding atmosphere, such as benzene, ethanol, and toluene. These sensors can operate in low humidity.

 

Symptoms of Poor IAQ 

Another easy way to measure indoor air quality is by looking around your home, office or workspace and looking for common signs of poor IAQ. Signs commonly include:

  1. Mould 
  2. High Humidity 
  3. Odour (stale smell)