Introduction
We first connected with Ethan Brooke last year while integrating AirGradient‘s outdoor air quality data into Local Haze, and we’re thrilled to reconnect with him this year to share his insights on transparency, trust, and the science of the air we breathe.
Ethan is the publisher of The Air Quality Index newsletter, the flagship publication of BreatheSafeAir, where he rigorously evaluates sensors and smart home air technology – digging into the data to determine what these devices actually measure versus what they claim to protect. He has established himself as a leading independent voice in sensor, air purifier, and respirator testing, and today he’s turning that same scrutiny toward the questions our community asks most: what can you trust, and why does it matter?
Originally from New Zealand, Ethan moved to South Korea in 2016, where his experience with regional air quality challenges inspired him to create detailed, accessible resources on air pollution and protective equipment. What began as articles documenting his research evolved into BreatheSafeAir, launched in 2020 as a rigorous testing ground for air quality devices. He has conducted hands-on testing of over 50 air quality monitors and built a community of readers seeking measured, evidence-based information about tools for monitoring and improving the air that they breathe.
Thank you for joining us today, Ethan – we are excited to have you join us!
Measuring What Matters
You’ve now reviewed over 50 air quality monitors, and your reviews consistently highlight the gap between what these devices measure and what matters for real health protection. The air quality monitoring space is increasingly crowded with low-cost sensors that only report PM2.5 concentrations.
What aspects of air quality that matter for health but aren’t currently being captured by the devices most people have access to, and why?
This is a great question, and it brings to mind an experience I recently had at the ASIC conference in Los Angeles. At the conference, I met Aaron Collins, who is creating a CPC (Condensation Particle Counter) that can measure ultrafine particles – particles smaller than 0.1 μm or 100 nm. Aaron took one of his OpenCPC devices (shown below at left), and I took the upcoming AirGradient Go (shown below at right), a device that uses a popular consumer-grade PM sensor (the Sensirion SPS30), to the LAX sign to measure the airborne particles as planes flew overhead. The results were eye-opening.
The CPC often recorded 60,000 – 100,000 particles per cubic centimetre, while the PM2.5 concentration was only around 5μg/m³ (the PM0.3 particle count was also only around 1000). Many people assume they can estimate UFP count with PM1 or PM2.5, but this isn’t true as PM2.5 concentrations are usually not correlated with ultrafine particle counts, as these can remain airborne for very long periods of time, meaning that even if your PM monitor shows 0, you may still be breathing thousands of ultrafine particles. Attention is only now beginning to shift towards UFPs, but we all know they can’t possibly be good for our health. Exactly how harmful they are, we don’t yet know.
As for why consumer devices don’t currently monitor UFPs, it simply comes down to price. While OpenCPC is the most affordable device in its category, it’s still 10x more expensive than even a premium consumer-grade air quality monitor. I also think that gases like NO2, O3, and even CO are not commonly monitored – at least not at low concentrations. Again, this is a cost issue. An electrochemical sensor that monitors these pollutants at common indoor concentrations is often over USD $100 for the individual sensor alone, making it quite cost-prohibitive.
Some of them also require frequent calibration, which means they’re just not feasible to include in an affordable air quality monitor. However, all of them are harmful to health. While CO sensors are affordable at alarm levels, low but consistent levels can still be detrimental to health, and measuring them affordably is very difficult.
Measuring Where It Matters
Historically, air quality has been monitored by Governmental organizations at a small number of outdoor locations. With solid-state sensors, monitoring air quality is now feasible both indoors and at many more outdoor locations. Recent efforts to develop portable “air quality dosage meters” have yielded promising advancements, notably the Atmotube PRO 2 (shown below, at left). This device represents a substantial evolution over earlier models like the Plume Flow (shown below, at right) and traditional industrial solutions.
Do you see these “air quality dosage” monitors as the monitors of the future?
While I think that being able to monitor your personal exposure with a portable device is invaluable, there are inherent trade-offs with such devices compared to both regulatory-grade and consumer-grade stationary monitors. So, while I think devices like the Atmotube PRO 2 are very useful, I think they’re part of a hybrid approach or a multi-level system. For example, there is no way to include accurate electrochemical sensors at an affordable price point yet (think NO2, which you might be exposed to if you commute along busy roads), and portable devices often have slow sampling rates to conserve battery life. They’re fantastic tools, but I firmly think they’re part of a larger toolkit.
With that said, we are getting closer and closer to creating portable exposure monitors with few trade-offs. For example, the Bosch BMV080 and SenseAir S12 are enabling monitors to get smaller and consume less power while remaining as accurate as stationary, consumer-grade monitors.
Measurement to Action
Your recent work has grown to include not just reviews, but also educational guides on CO₂ monitors, radon detection, and the science behind different sensor types.
How do you think about the relationship between providing people with quantitative air quality data and helping them actually act on that information to improve their lives?
I try to remember (but sometimes forget when I’m so familiar with these devices) that the number is never the point. Data only matters if it changes what someone does, and a monitor that reads 150 while offering no next step doesn’t protect anyone – if anything, it makes them more anxious. Most of the low-cost sensor movement has quietly assumed that visibility equals empowerment, that you show people the figure and better decisions follow, but that’s not true. A raw number means very little without a frame around it. Is 800ppm of CO2 fine or worrying? Is this reading normal for my kitchen at dinnertime, or is it unusual? The value lives in the comparison and the trend, and I’ve been trying to better explain this in my recent content.
The other half of the equation is whether someone can actually act on what they’re seeing. CO2 is the case I often come back to, because it’s cheaply and directly fixable: open a window, watch the number fall, see the intervention work. That feedback loop is what changes behavior, and it’s often missing for something like outdoor PM2.5, where you can leave a person fully informed and completely powerless. I tend to think of a monitor as a diagnostic tool rather than a solution.
The best outcome isn’t someone anxiously watching graphs forever. It’s someone who needed the device for a while, learned their sources and their patterns, fixed what they could, and reached the point where they only occasionally check the numbers.
Bridging Outdoors and Indoors
Today, there are two separate markets for air quality monitoring: outdoors and indoors. Sometimes, products for both are offered by the same vendor, but the resulting data is treated as entirely separate. As outside air eventually enters and indoor air exits, it would seem there should be opportunities to simultaneously monitor both indoor and outdoor air quality. Examples might include evaluating the performance of indoor filtration systems or simply guiding decisions on when to open and close windows.
Do you foresee a merger of indoor and outdoor air quality monitoring systems in the future?
This was actually one of the bigger themes at the Indoor Air 2026 conference in Singapore. Outdoor air has decades of regulation and dense reference networks behind it, whereas indoor air, which is where we spend the overwhelming majority of our time, is only now getting its first serious regulatory frameworks.
My own feeling is that the merge arrives at the software and data layer because the hardware is very similar, if not the same. You don’t need a new device so much as platforms that ingest an indoor and an outdoor stream and reason across them, which is exactly what many in the Home Assistant crowd already do by pulling a local outdoor monitor and an indoor one into a single dashboard and watching the delta.
However, the key benefit is not just a more aesthetically pleasing graph; it lies in closed-loop control. For instance, if outdoor particulate levels rise, the system can automatically boost the air purifier’s activity. Conversely, if the outside air becomes cleaner and cooler than the stale indoor air, the system may prompt you to open a window. At that point, the measurement stops being the product and becomes the input to a decision that the building quietly makes on your behalf, enhancing your living environment without requiring constant input from you.
To learn more about Ethan’s research and insights
Thank you to Ethan for taking the time to join us and share your insights on air quality monitoring.
To learn more about Ethan’s research, subscribe to the free Air Quality Index newsletter, and explore BreatheSafeAir.
Photo credits:
(Plume) Joshua Goldman/CNET, (Window) Photo by haidong wang on Unsplash
Disclosure:
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