Air quality monitoring: Key metrics and their importance

As we embark on 2024, TEQOYA now offers customers the unique opportunity to measure the air quality they breathe. Pierre Guitton, as the co-founder of TEQOYA, a startup established in 2015, you have been at the forefront of providing air purifiers incorporating health-safe and energy-efficient technologies such as : ozone-free ionization and e-filtration. You have previously shared on your blog the vast array of components present in the air. Which specific elements does your air quality monitor analyze?

The initial version of our sensor evaluates:

• Humidity Level: Maintaining a recommended level between 40 and 60% is crucial. Low humidity can cause respiratory issues and irritability, as discussed earlier. Conversely, humidity above 60% fosters mold growth, whose spores travel through the air and a leading source of indoor air allergens, potentially leading to allergic rhinitis and asthma. Humidity also promotes the spread of bacteria and dust mites, thereby increasing the risk of respiratory illness ¹
• Temperature: This has been a widely measured aspect of air quality for years, critical for comfort.

It is important to analyze these two criteria together due to their strong interplay. A high humidity level will especially promote the growth of micro-organisms when the temperature is also high, for example.

• Fine and Ultrafine Particles: The PM2.5 indicator is a basic measure of particle pollution in atmospheric air quality, denoting the mass of particles smaller than 2.5 microns per unit volume. Our S1 air quality monitor displays the PM2.5 level and also provides mass concentration values for PM1.0 and PM10 in the AirObserver application. It is essential to remember that finer particles penetrate deeper into our bodies, affecting the respiratory tract and bloodstream. PM1.0 particles are therefore more dangerous, while also being particularly challenging to measure due to their size being less than 1 micron. But let's not stop there!  

Your new air quality monitor introduces a novel assessment methodology with a unique indicator. Can you explain why?

The most common methodology for air quality analysis devices is to measure the mass of particles, as just described. However, we identified this approach as inadequate for accurately assessing indoor air quality. The most harmful particles are often submicron in size, contributing minimally by mass. Thus, an indicator based on mass overemphasizes larger, less dangerous particles and downplays the presence of finer, more harmful ones, introducing a bias in assessing the potential danger of the air.

For instance, with PM2.5 measurements, particles nearer to 2.5 microns over those closer to 0 micron. This disparity is even more pronounced with PM1.0 particles, which are particles ranging between 0 and 1 micron: particles smaller than 0.5 microns constitute 90% of all particles under 1 micron, and sometimes even more! Yet, they are barely identified when mass is the unit of measurement.

To address this, TEQOYA measures and analyzes the particle count, not their mass. This completely changes the approach and provides a much more precise and relevant perspective on the health risks of the air being measured.

Besides a few high-end professional air quality monitors designed for specific uses and quite expensive, the technology available for consumer-grade devices struggles to accurately measure particles smaller than 0.5 μm. How have you taken this factor into consideration?

TEQOYA's mission is to enable everyone to assess and improve the air quality they breathe. The measurement technology we use is the best in the micro-sensor market, and it's also French! We chose the sensor offered by Tera Sensor, which is recognized by the American organization AQMD² as capable of measuring particles as effectively as a professional reference instrument, , but at a cost that's 1,000 times lower! This applies not only to the three usual mass concentrations (PM1.0, PM2.5, and PM10) but also to the number of particles across three size ranges (0.3 to 1 micron, 1 to 2.5 microns, 2.5 to 10 microns).

Even though it's 1,000 times less expensive, it remains cutting-edge technology, which is why our air quality monitor is priced at €149 including VAT. However, we felt it wasn't reasonable to exceed this budget for an individual consumer, so we adopted the following approach:

• We integrated into our air quality monitor the best 'general public' sensor on the market that can provide a reliable particle count. Unfortunately, the technology used by this type of sensor does not allow for the detection of particles smaller than 0.3 microns, particularly ultrafine particles.
• Based on this, we relied on international scientific research to estimate the quantity of ultrafine particles from the measurement of fine particles: about 200 studies have measured the number of particles by size range in a variety of realistic situations, including ultrafine particles smaller than 0.1 microns. Of course, there are differences between these studies, but analyzing them all allowed us to establish a rule for calculating the quantity of ultrafine particles, in terms of magnitude. We complemented this literature review with measurements in our laboratory, comparing the readings from the Tera Sensor with those from a scientific device capable of counting particles down to 7 nanometers in size.
• This estimate of ultrafine particles is then compared with initial quality thresholds set by the WHO in 2021 for these particles, providing our customers with an indicator of their air quality.

The results displayed by our air quality monitor are the outcome of a threefold approach: the high performance of the sensor, scientific adjustment of the results, and comparison with threshold values provided by recognized authorities on air quality and health impacts. .

An interesting validation of our method is this: the maximum value defined for clean air in the PM+ measurement is very close to the maximum particle count per unit volume set for the ISO9 class of the ISO 14644-1:2015 standard on cleanrooms. This ISO9 class is actually indicative of clean outdoor air.

The PM+ indicator, with its measurement unit 'number of particles per unit volume,' offers a much more refined assessment of the actual toxicity of the particles present in the air. It highlights the significance of submicronic particles, particularly ultrafine particles, and provides a more accurate quantification of the need for an air purifier and its effectiveness where necessary.

You just mentioned that beyond the 'raw' measurement values, your air quality monitor also offers 'good quality/poor quality' thresholds. What criteria do you use to establish these thresholds? 

Once again, we have taken a unique approach: we chose not to calculate a 'global air quality index.' While such indexes exist, we believe that they are often misunderstood because they do not correspond to any concrete, universally understandable benchmarks. Instead, we opted to use quality thresholds for each measured parameter : humidity, PM2.5, and the number of particles, which we refer to as PM+. Each of these indicators, therefore, has its own thresholds for transitioning from one colour to another, from green to mauve, indicating air quality from good to severely degraded.

These threshold values are also based on scientific and medical consensus:

• For PM2.5 particle mass density, we adhere to the World Health Organization (WHO) guidelines. ³
• For the number of particles per unit volume (PM+), we use reference values provided by the WHO in its latest recommendations published in 2021 (2).
• For humidity, we apply thresholds from E.M. Sterling's study, published in 1985 by ASHRAE (the American standard-setting body), which are now widely accepted and link humidity to the risk of spreading mould, bacteria, dust mites, and respiratory illnesses. To provide our users with a comprehensive alert system, we assign an 'overall colour' to the air quality, which corresponds to the worst colour among the individual indicators. This way, whether there's a problem with combustion particles (like outdoor pollution seeping indoors) or a humidity issue that poses a risk of microorganism and allergen development, the user will be informed.

Your air quality sensor measures fine particles, temperature and humidity, but not CO2, formaldehyde, or VOCs (Volatile Organic Compounds). Why were these choices made?

There are two different reasons for these choices.

CO₂ is the quintessential indicator of confined spaces: its concentration helps assess the risk of accumulating humidity and indoor pollutants (VOCs, particles, microorganisms, etc.). It affects our cognitive performance (refer to our article on Air Quality and Productivity) . and can trigger asthma attacks in enclosed environments at levels above 1000 ppm (0.1%).

However, its danger is relatively controlled; for example, it does not cause cancer. It is also relatively easy to determine if CO2 levels are too high in a home and to remedy it: simply follow WHO and ADEME recommendations to ventilate by opening windows at least twice a day, as we suggest in our documentation.

CO2 sensor technologies exist but are expensive for reliable measurement. True to our approach of offering only quality, we chose not to integrate a CO2 measure rather than include a poor quality one, the only condition to keep our monitor's price within an acceptable range.

Regarding formaldehyde and VOCs more broadly, our guiding principle remains the same: to offer technically sound and reliable products at accessible budgets. Currently, VOC measurement does not effectively distinguish between different types of pollutants and does not assess mixed pollutant scenarios. Therefore, we are developing an air quality monitor that will track their concentration, but its commercial release will take some time. We need to advance the technology to offer the reliability our customers expect from us. That's why we dedicate 20% of our revenue to innovation!

How can an individual customer, without any scientific background, practically make sense of all these calculations?

There is no need for customers to understand the detailed calculations and threshold values to use our air quality monitor at home! We have implemented a colour code system, ranging from green for good air quality to purple for unhealthy air. The overall indoor air quality displayed by the monitor always reflects the worst performance level among all the measured criteria.

Here is a summary of the measurement units and thresholds, as previously defined, on which the visualization of your air quality by your S1 monitor is based. Air Quality Levels Indicated by Your S1 Monitor:

Small Glossary of the Various Devices Making Up TEQOYA's Air Measurement and Purification System:

• Sensor: A device that analyzes air characteristics and counts particles.
• AirObserver Platform: An application where you can access all data from your sensor (including historical data), as well as information on outdoor air quality.
• Monitor: A device that manages your air quality; it uses data from the indoor air quality sensor, combined with outdoor air quality information, all available via AirObserver. This feature also allows you to program your air purifier and monitor your air quality.
• Air Purifier: A device that removes pollutants from the air. TEQOYA air purifiers use par ozone-free ionization and e-filtration. and are available in various versions, depending on the size of the space and the activities conducted there. 

Sources

1. E.M. Sterling's study published in 1985 by ASHRAE
2. AQMD: The South Coast AQMD is the regulatory agency responsible for improving air quality in large areas of Los Angeles, Orange, Riverside, and San Bernardino counties, including the Coachella Valley. The region houses over 17 million residents, comprising 44% of California's total population
3. WHO Air Quality Guidelines - Summary of Guidance on Particle Number.