Guidelines for Building Sensors to Measure Indoor Air Quality

Guidelines for Building Sensors to Measure Indoor Air Quality

Sensors Insights of Dr. Christian Meyer

The Importance of Good Air Quality

Maintaining good air quality has become a priority around the world, especially in terms of well-being and living comfort. Bad air quality can lead to fatigue, headache and other negative effects. Continuous monitoring of the indoor air quality can reduce these effects and may be used to trigger an action, such as automating HVAC controls, air purification, or simply opening a window.

Although there is no universal definition for clean air, it has been shown that organic compounds are an essential source of bad air and can act as an indicator of indoor air quality (IAQ). Because of the constant presence of gases in our everyday lives, it has become more imperative to monitor volatile organic compounds (VOCs) in comfort, environmental safety, automation, health, and medical care applications. Depending on the use case and application, a single VOC or the sum of all organic compounds, referred to as the total VOC (TVOC), may also be monitored.

Fig. 1: Typical sources of TVOC inside a home.
Fig. 1: Typical sources of TVOC inside a home.

 

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Implications of VOCs

Problems with indoor air quality, such as the so-called sick building syndrome, have significant health and financial impacts on the community. The Environmental Protection Agency (EPA) estimates that poor indoor air quality (IAQ) affects 33-50% of all commercial buildings in the US and is responsible for over 125 million lost school days and 10 million lost work days each year.

For example, studies have found that increased indoor pollutant concentrations and lower ventilation rates are associated with a statistically significant reduction in perceived mental performance among students, and that controlling pollutant concentrations improved the measured performance of office workers[1]. Reports also show that a building’s indoor air quality significantly affects the prevalence of respiratory disease, allergy and asthma symptoms, and work performance. The effect of indoor air quality on health is dramatic. It has been reported that the median level of VOCs in office buildings may become several times greater than outdoor levels due to outgassing of building and construction materials.

There are several sources of recommendations regarding the TVOC levels that are hazardous to human health. Figure 2 gives a summary for working place threshold safety limits by the German Research Foundation (DFG) and the World Health Organization (WHO) that compares levels from the varying sources for a range of chemicals. In addition, guidelines from the EPA[2] for once-in-a-lifetime or rare, exposure to airborne chemicals are considered.

Fig. 2: Typical VOC chemicals and maximum concentration limits which the governmental institutions in the US (EPA), Germany (DFG), and the World Health Organization (WHO), report.
Fig. 2: Typical VOC chemicals and maximum concentration limits which the governmental
institutions in the US (EPA), Germany (DFG), and the World Health Organization (WHO), report.

All levels are expressed in milligrams per cubic meter (mg/m3) with a usual exposure time of 8 hours, except where a different period is mentioned. Although the concentrations vary from agency to agency, there is a general agreement to keep the TVOC concentrations to a minimum. These values are maximum indoor gas concentrations given for an 8-hour working day and are based on an average 40-hour work week. Now, the question remains: What are typical and safe concentration levels for VOCs in a home?

 

Guidelines for Indoor VOCs

Currently there is no global standard that defines IAQ. Some countries have local approaches and have published studies that give indicators of clean ambient air and its implications. These studies suggest that there is a direct link between the TVOC and poor IAQ, although in most cases, health impacts are not considered. This shows the complexity of air quality effects and how much additional research is needed to bring it to a legislative level.

The most relevant studies, research results, and certifications include the UBA study by the German Environment Agency (UBA), the Well-Building Standard by the WELL Building Institute (IWBI), the EPA’s Indoor airPLUS, and the Indoor Air Quality Management Group of Hong Kong. Some of these publications also include non-VOC related IAQ parameters, such as room climate, humidity, construction materials, radon, mercury, bacteria, and others, which are only relevant for the particular standard’s mission. However, there is no legal commitment to make use of one of the standards mentioned above. An indoor air quality certification may be issued based on the examination result. Focusing on one specific definition for IAQ, such as UBA, will help the end user evaluate the indoor air hygiene.

Fig. 3: Maximum VOC levels for compliance with the Good Class per the IAQ Management Group in Hong Kong.
Fig. 3: Maximum VOC levels for compliance with the Good Class per the IAQ Management Group in Hong Kong.
Fig. 4: Guideline values II for VOCs and non-VOCs to be evaluated for IAQ by the German Environment Agency with concentrations that are likely to present a threat to health.
Fig. 4: Guideline values II for VOCs and non-VOCs to be evaluated for IAQ by the
German Environment Agency with concentrations that are likely to present a threat to health.

 

How to Measure VOCs

There are several technologies for detecting gases, especially VOCs. Approaches like chromatography and spectroscopy are state-of-the-art for environmental research, but typically only experts can operate these complex instruments and interpret their data. Small sensors can overcome these disadvantages and offer a wider range of applications. However, only a few gas sensor technologies operate at a level that is acceptable for widespread practical use. The most common are chemiresistors, which detect a chemical reaction on the sensor surface by a resistance change.

Fig. 5: How a MOx sensor works (left) and examples of analog and digital devices.
Fig. 5: How a MOx sensor works (left) and examples of analog and digital devices.

Development of these metal oxide (MOx) based gas sensors began in the 1960s and have been on the market for nearly 50 years. The key is the MOx chemistry itself, next to a well-designed mechanical hot plate (MHP) structure to hold the MOx, and a reliable and reproducible production technology. This well-known technology consumes very little power due to its small size, and high-volume production keeps its prices low. Typical MOx materials are doped with semiconductor materials like SnO2, WO3, TiO2, etc. At elevated temperatures, gas molecules may adsorb on the MOx surface and generate free charge carriers. A resistance change can be measured at the electrodes. Hence, the sensor signal strongly depends on the present atmosphere, utilized materials, production technology, time and temperature. The development of MOx materials is a relatively practical approach and requires years of experience.

 

Sensor Platforms

Although many IAQ sensors are available on the market, finding the right sensor depends on a user’s system requirements.

First, users need to decide between the output mode (analog or digital), interfaces (UART or I2C) and signals (raw values or algorithm output). For easy integration, a digital output is recommended, coupled with a state-of-the-art I2C interface. The user’s MCU will process the gas sensor signals and once the libraries are downloaded, it will give the favored output signal.

Second, when a sensor platform is selected, several parameters should be considered in selecting the best technology for the application. These parameters include sensitivity (also referred to as accuracy) for a specific target gas, selectivity and cross-sensitivities (sensitivity to other non-target gases), stability (signal long-term behavior) and poisoning (resistance against poisoning gases such as siloxanes). Modern system platforms also offer connectivity to IoT systems (e.g. smart windows, air purifiers, HVAC, etc.) and come with firmware updates for the sensor (e.g. offering improved electronic control or low power optimization). Because most IAQ relevant gas are in the low mg/m3 or even µg/m3 concentrations, it is essential to test and evaluate the sensor functionality in the user’s final application.

Fig. 6: An IAQ sensor monitors VOCs and provides gas measurements in a mobile application.
Fig. 6: An IAQ sensor monitors VOCs and provides gas measurements in a mobile application.

The main advantage of a sensor platform to measure indoor air quality is its robustness against ambient influences (temperature and humidity changes), for example. This allows it to work quietly in the background while it analyzes the room hygiene and triggers actions if needed. All these studies on IAQ and technical features have one goal - to improve the quality of life and reduce health effects.

 

About the author

Dr. Christian Meyer is a Product Marketing Manager at IDT. His past roles in the last 15 years included responsibilities in analyzing different gas technologies, as well as sensor development. His background is in Applied Physics and Engineering for Atmospheric Measurements.

 

[1] U.S. Environmental Protection Agency: Indoor Air Quality Tools for School Program, https://www.epa.gov/iaq-schools

[2]Environmental Protection Agency, Acute Exposure Guideline Levels for Airborne Chemicals, 2017.