indoor air quality

More consumer and industrial products may emit volatiles that now are known to be harmful, including furniture, passenger cars, and industrial trucks. Interest in detecting and measuring the gaseous pollutants is growing in order to create effective responses to reduce or eliminate health risks. 

Air pollution is a serious public health issue worldwide with airborne hazardous substances such as volatile organic compounds (VOCs), particle matter (PM), and nitrogen dioxide linked to a wide range of adverse health conditions. According to the World Health Organization (WHO), “the combined effects of ambient (outdoor) and household air pollution cause about seven million premature deaths every year, largely as a result of increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer and acute respiratory infections.” There’s also an economic impact of air pollution. Related illnesses and loss of life cost billions of dollars in healthcare services globally. And while we might think that air pollutants are only present outdoors, we’re more exposed to them – and they’re more potentially dangerous – indoors, where higher concentrations of volatile organic compounds (VOCs) produced by paint and furniture are a major concern. The COVID-19 pandemic has only heightened our awareness of the air we breathe. With recent medical research showing that viruses may be transmitted by attaching themselves to airborne particles, indoor air quality (IAQ) monitoring is becoming even more important. As we turn to VOC sensors for IAQ monitoring, it’s important to note that not all VOC sensors are equal. The current crop of low-cost VOC sensors are primarily total VOC sensors. Generally based on electrochemical or metal-oxide transducers, total VOC sensors provide a “grayscale” image of IAQ, which doesn’t differentiate among different gases. This limits people’s ability to make informed decisions regarding the level of threat to their health, since not all VOCs are equally hazardous and don’t require detection at the same concentrations. In addition, total VOC sensor technologies don’t support PM detection. This forces end-device designers to either add a module of optical sensors or switch to a completely different system. While optical sensors provide excellent performance, particularly for PM detection, they’re much more expensive, as well as more complex, bulky and power-hungry. This makes them ill-suited to resource-constrained portable devices where cost, size and power are at a premium. FBAR-based IAQ sensors emerge The shortcomings of available technologies for IAQ sensors has boosted the development of alternative solutions that provide better performance – in terms of both sensitivity and selectivity – as well as greater versatility, lower cost and smaller size. Acoustic sensor technologies featuring the latest advancements in film bulk acoustic resonator (FBAR) sensors are emerging as a leading candidate. Sensitivity is important in VOC detection because certain hazardous compounds, such as formaldehyde, are dangerous at very low concentrations. As highly sensitive devices, FBARs are a MEMS equivalent of a weight scale, but instead of detecting kilograms or grams, they can sense femtograms, which are just one-quadrillionth of a gram each. FBARS work by putting a thin film piezoelectric material into a mechanical resonance through application of an AC electric signal (GHz range) to a pair of electrodes on either side of the film. This resonant frequency is sensitive to the mass attached to the electrode surface. Whenever the mass attaches to the active area of the sensor, it produces a frequency shift, and this shift is proportional to the mass attached on the surface. Another major benefit of this approach is selectivity, which allows a device to distinguish between different target molecules or species. By placing a layer of material on the sensor –which is the functionalization layer – FBAR sensors display high selectivity on targeted materials. This allows the consumer to distinguish among different VOCs instead of just measuring a mixture of VOCs that vary in toxicity. Unlike older IAQ sensing technologies, FBAR sensors support functionalization layers comprised of different materials, from metal oxides and polymers to more exotic options such as carbon nanotubes and graphene. This increased versality makes it easier to use FBARs for a variety of applications, ranging from gas sensors to medical sensors. Sorex Sensors’ FBAR sensor in a 3mm x 3mm ceramic package FBAR technology is a perfect match for IAQ. In addition to high sensitivity and selectivity, it enables the manufacture of very small arrays, and it’s low-power, all of which make it a good choice for small portable devices. Plus FBAR technology is CMOS-compatible, so FBAR sensors can be made using standard MEMS processes and combined with integrated circuits fabricated using standard CMOS processes, making them cost-effective. With its origins at the University of Cambridge in the UK, Sorex Sensors is leading the commercialization of FBAR devices for sensing applications. After releasing our first product in 2019 – a standalone FBAR sensor and a development kit that can be used for particle monitoring – we’re preparing to release an FBAR sensor array that detects five different gases later this year. By offering specific functionalization layers for different targets, our new FBAR technology will provide a level of selectivity that other silicon-based sensors can’t achieve – particularly in light of FBAR’s low power consumption and very small size (less than 5mm x 5mm). In addition, the sensor’s targeted gases will include one of the main headaches in the IAQ space, formaldehyde, which is especially carcinogenic and is widely found in the varnishes used in furniture. We’re planning for this new iteration of our FBAR technology to help our customers sense in color rather than in grayscale – providing a level of granularity that’s unmatched in IAQ sensing. Check out the latest news about Sorex Sensors on our website and on LinkedIn. About the Author Mario de Miguel Ramos, Ph.D., is the co-founder and CEO of Sorex Sensors Ltd, a spin-out of the University of Cambridge, the University of Warwick, and the Universidad Politecnica de Madrid (UPM). Founded in 2017, the company focuses on the development of highly sensitive mass sensors based on film bulk acoustic resonators (FBARs). Dr de Miguel Ramos has been working in the field of FBARs for a decade. Prior to founding Sorex Sensors, he worked as a postdoctoral research associate at the Electronic Devices and Materials (EDM) Group at the University of Cambridge. He holds a master’s in Telecommunications Engineering and a Ph.D. in Electronic Engineering Systems, both from UPM. Sorex Sensors is a member of MEMS Sensors Industry Group®(MSIG), a SEMI technology community that connects the MEMS and sensors supply network in established and emerging markets to enable members to grow and prosper. Visit us today.

Why do we need environmental air pollution sensors?Today we need environmental air pollution sensors more than ever to ensure that we have clean and safe outdoor and indoor air. Although federal rules have improved air pollution over the past several decades, more than 110 million Americans still live in counties where air quality is below national standards. An estimated 100,000 Americans die prematurely each year of illnesses caused or exacerbated by polluted air.“Cars and trucks are much cleaner than they were, power plants are cleaner, industrial operations are cleaner,” said Paul Billings, Senior Vice President Advocacy for the American Lung Association. But cleaner air is not clean air.”While scientists have long known that air pollution may exacerbate asthma and other respiratory illnesses, new data suggests polluted air leads to higher COVID-19 higher death rates and brain inflammation that can contribute to dementia and autism.To understand the importance of air quality and how we can apply existing sensors and develop new ones, we look both outdoors and indoors (see Figure 1). Outdoor air quality relates to gaseous and particulate pollutants, defined by the Air Quality Index (AQI). The AQI became a standard based on regional thresholds for a set of key outdoor pollutants: four gaseous pollutants (sulfur dioxide, nitrogen dioxide, carbon monoxide, ozone) and particulates (PMs) of different sizes such as 10 μm (PM10) and 2.5 μm (PM2.5). At present, the AQI is measured using traditional analytical instruments. Despite their high acquisition and maintenance costs, these instruments are the only solution to accurately measure these pollutants in the presence of variable environmental background.Figure 1. Examples of outdoor and indoor air quality markers Indoor air quality (IAQ) is also of growing concern. Formaldehyde, benzene, carbon monoxide, and carbon dioxide are some of the key pollutants with restricted concentration levels in residential, office and industrial buildings. Sources of these and other gaseous pollutants include building materials and equipment, workplace cleansers, and building occupants. Regulatory agencies and building occupants use different methodologies to estimate IAQ using gaseous and particulate pollutant analyzers. These estimates also consider air humidity and temperature that affect indoor air quality. Where are we today with environmental sensors?The top three requirements for modern gas sensors include: the sensor reliability to provide accurate readings in diverse environmental conditions over desired period of use low power, to extend battery life or to eliminate its need, and low cost, to facilitate their ubiquitous deployments. Advances in electronics, microfabrication, and packaging have delivered recent important developments in reducing the power consumption and miniature packaged solutions. Recent R D efforts are also increasing the number of successful gas sensor field deployments for outdoor and indoor air quality monitoring. Figure 2 illustrates three examples of recent developments in gas sensors that meet requirements of diverse customers.Electrochemical sensors from SPEC Sensors were collocated with EPA instruments for monitoring of NO2 and O3 in Chicago’s Array of Things Project. Figure 2A shows that these new cost-effective sensors track well the EPA instruments. Advancements in circuit quality, sampling, enclosure design, and initial calibration/compensation were all essential in achieving these results. While this example clearly demonstrates the usability of these sensors in this particular application, the expectations that low-cost, off-the-shelf sensors will match the performance of EPA reference systems that cost 50x-100x more must be adjusted. A micropackaged sensor suite from Bosch Sensortec includes sensors for total volatile organic compounds (TVOCs), temperature, humidity, and pressure. TVOC measurements are needed according to the guidelines by the German Federal Environmental Agency. To report TVOC, the sensor algorithm tracks the TVOC-related resistance of the metal oxide sensor, corrects sensor resistance for ambient temperature and humidity, and outputs the TVOCs Index of Air Quality between 0 (clean air) and 500 (heavily polluted air) as shown in Figure 2B. A recent GE-developed dielectric excitation scheme of metal oxide sensing materials provided a highly desired and long-awaited calibration stability of sensors for monitoring of fugitive methane gas emissions in all-weather conditions. These sensors were used in several field validation campaigns in Oklahoma, North Dakota, Arkansas, and British Columbia and had stable performance after more than 400 days, as compared to an initial calibration (see Figure 2C). Such stable sensor performance has become possible by switching from the conventional resistive mode of operation of metal oxide sensing elements to the dielectric excitation scheme. Figure 2. Examples of applications of contemporary gas sensors based on different detection principles.(A) Outdoor performance of NO2 and O3 electrochemical sensors versus EPA-validated instruments.(B) Calibration results of a BME680 metal oxide gas chemiresistor upon exposures to TVOCs (blue stair-profile) and its ± 15% confidence interval band as the Index of Air Quality.(C) Calibration stability of a sensor with an innovative dielectric excitation scheme implemented for monitoring of fugitive methane gas emissions after multiple uses in diverse field validation campaigns. Key challenges and solutions toward realizing new applicationsIn this era of data-on-demand, environmental sensors could enable countless new applications. Imagine you have a gas sensor conveniently integrated into a smartphone or a watch. You are commuting to work, and your sensor alerts you that the subway station through which you are traveling has very poor air quality. How might this alert affect your behavior? Would you put on a mask, change your commuting route to a twice-longer one, or petition the city? What if you are attending a parade downtown with your asthmatic child, and your device informs you that the air is clean? Would you skip the parade if you knew that your sensor was only 10% accurate? How would you avoid a risk of ending with your asthmatic child in a hospital?Design principles of modern sensors originate in the 20th century for detection of high gas levels from leaks, but they did not anticipate the applications proposed now. By design, existing sensors have only a single output – e.g. resistance, voltage, current, light intensity – that mathematically cannot correct for the sensor instabilities caused by the complex chemical background and variable temperature and humidity conditions. Thus, often these simple sensors perform best when pollution levels are high and when the compound of interest swamps others. As a practical example, there are dozens of gaseous pollutants in ambient air with their toxicity that differ 1,000-10,000 fold. Often, the insufficient reliability and accuracy of existing sensors in the field conditions is a significant bottleneck toward the broad adoption of gas sensors. According to the United States Environmental Protection Agency (EPA), the correlation between readings of low-cost sensors versus reference monitors varies widely from 1% to 80%. The EPA also states that no low-cost sensors meet Regulatory Monitoring requirements, and the World Meteorological Organization emphasizes that “low-cost sensors are not currently a direct substitute for reference instruments, especially for mandatory purposes.” However, we now have the increasing number of examples of reliable operation in complex environments (Figure 2) in addition to important advances in reduced power and size of contemporary sensors. Still, the key challenges to realize new applications are often the lack of required accuracy and reliability of available sensors for new contemplated applications.Is it possible to offer low-cost sensors for at least some applications and some gases with the degree of accuracy approaching more expensive specialized instruments? We, the SEMI-MSIG Device Working Group, are saying: Yes. To deliver on this bold statement, our SEMI community brings new technological solutions to the 100-year old general design of gas sensors.Our next blog What is in the Air will provide details on our activities of SEMI-MSIG Device Working Group to establish standards and new measurement schemes to reduce effects from uncontrolled ambient conditions and to improve stability, limit of detection, and dynamic range of environmental sensors. Also learn how new MSIG members can impact this important working group. The MEMS Sensors Industry Group (MSIG) is a SEMI technology community that enables the MEMS and sensor industry to address common challenges, innovate and accelerate business results.Radislav A. Potyrailo is Principal Scientist, Micro Optoelectronics Gas-Chem-Bio Sensors Systems, at GE Research; Ed Stetter is General Manager at SPEC Sensors, LLC; Ryotaro Sakauchi, is Senior Manager of Business Development at Bosch Sensortec; Merry Smith is a Product Manager and Senior Scientist at C2Sense, Inc.; and Sreeni D. Rao is Senior Director of the MEMS Business Group at TDK Corporation.