This $65 Air-Quality Sentinel Fights Smog With Negative Ions—And It Never Sleeps

In the haze-choked cities of northern Thailand, where winter temperature inversions trap fine particulate matter at breathing level for weeks on end, a team of researchers at Naresuan University has built something that does more than watch the pollution roll in. Their prototype, called Cleara-Air, is a closed-loop system that senses dangerous PM2.5 concentrations, decides on its own when the air has become unhealthy, and automatically switches on a negative-ion generator and ventilation fan to fight back—all while streaming every measurement to a live, map-based dashboard that anyone with a browser can open. The system, described in a study published in Frontiers in Sustainability, was put through fifteen repeated closed-room trials, and in every one of them the sensing-processing-communication-actuation chain fired exactly as programmed.

The stakes could hardly be higher. According to figures cited in the study, air pollution is now linked to as many as 8.1 million deaths globally each year, and fine particulate matter—particles smaller than 2.5 microns, roughly one-thirtieth the width of a human hair—is among the most insidious culprits. Too small to be seen with the naked eye, PM2.5 penetrates deep into the lungs and crosses into the bloodstream, driving respiratory disease, cardiovascular damage, and lung cancer. Thailand’s problem is compounded by a lethal cocktail of sources: vehicle and industrial fuel combustion, the seasonal burning of rice straw and agricultural residues, forest clearing in the north, and household burning of wood and fossil fuels. In provinces such as Chiang Mai, Lamphun, and Phitsanulok, calm winter winds and temperature inversions—cold air trapped beneath warm—lock the dust in the lower atmosphere, sometimes for days.

What separates Cleara-Air from the growing crowd of low-cost air-quality monitors is that it refuses to be a passive observer. Most existing IoT platforms, the researchers note, focus on data collection, visualization, or machine-learning forecasts. Cleara-Air closes the loop. At its heart sits an ESP32 microcontroller wired over the I2C protocol to a Sensirion SPS30 laser-scattering sensor, which measures PM1.0, PM2.5, PM4.0, and PM10 continuously. The controller compares every reading against an operational set-point of 37.5 micrograms per cubic meter—Thailand’s 24-hour PM2.5 standard, a number Thai citizens already recognize as a health warning. When the concentration climbs above that line, the ESP32 sends a HIGH logic signal to two GPIO pins, driving a 4-channel 5V relay that switches on a 220-volt AC ventilation fan and a carbon-brush negative-anion generator. When levels fall back below the threshold, the actuators shut off to save energy.

The actuation mechanism itself is a piece of applied electrostatics. Negative ions released at high voltage from the generator’s discharge tip attach to suspended particles and alter their surface charge, increasing the probability that particles collide and agglomerate. Fine dust that would otherwise linger in the breathing zone clumps into larger, heavier aggregates that settle out of the air more rapidly, while the fan distributes the ion cloud through the room. The team deliberately chose a carbon-brush generator over a conventional corona-discharge module to reduce the potential for ozone formation—a well-known byproduct of traditional ionizers and a genuine safety concern. The researchers are careful, however, to state that no physiological benefits of negative ions are claimed, and that direct ozone concentrations were not measured in this study.

Every fifteen seconds, the ESP32 packages PM1.0, PM2.5, and PM10 readings together with the device’s ON/OFF status and transmits them over Wi-Fi to a PostgreSQL database via HTTP POST. From there, the data flow into a responsive Web-GIS application built with PHP, HTML5, CSS3, JavaScript, and the Leaflet mapping library, which displays sensor locations, real-time concentrations, historical trends, and actuator status on any modern browser—computer, tablet, or smartphone. The database schema links four tables: Devices, which stores installation coordinates; Sensor_Logs, which records measurements and ionizer status as time-series entries; Command_Logs, which captures remote-control commands; and Users. The architecture is entirely open-source, from PostgreSQL 18.1 and Arduino IDE 2.3.10 to QGIS 3.40.6, and the total hardware bill comes to roughly $65, with the SPS30 sensor at $20.91 and the ESP32 board at $11.75.

Engineering the closed loop was not without its traps. Because the 220-volt fan is an inductive load, switching the relay off generates electromagnetic interference that can freeze the sensor’s I2C bus—a failure mode the team calls an I2C hang. Rather than inserting crude delay commands, the firmware uses the millis() function so the processor continuously checks its own status, with a 1,000-millisecond I2C timeout and an error counter: if the sensor fails to respond three times in a row, the microcontroller performs a hard reset within three seconds, clears the bus, and resumes operation without interruption. It is the kind of unglamorous resilience engineering that determines whether a real-world device survives contact with mains electricity.

The experimental evaluation was refreshingly blunt. In fifteen independent trials conducted in a closed room, the researchers deliberately elevated particulate concentrations by introducing powdered flour roughly fifteen centimeters from the sensor inlet. Each trial followed the same scripted sequence: continuous measurement, detection of a threshold crossing, relay activation of the fan and ionizer, transmission of time-stamped data and device status to PostgreSQL at fifteen-second intervals, visualization of the updated status on the Web-GIS interface, and finally actuator deactivation once the reading fell back below the set-point. Database records from the trials capture the drama in miniature—one entry shows the ionizer switching on the instant a particulate event pushed PM2.5 to 75 micrograms per cubic meter, with a later record marking its shutdown. The repeated tests demonstrated that the programmed sequence operated reproducibly under the tested conditions.

The researchers are equally candid about what the trials do not prove. The study evaluates system functionality, not PM2.5 removal efficacy. There was no untreated control condition, no particle-decay analysis, no comparison with established filtration technologies, and no co-location of the SPS30 with a reference-grade instrument—so measurement bias, error metrics, and humidity-dependent uncertainty remain unquantified. Ozone was not directly measured, formal electrical safety certification was not performed, and the bare-comparator threshold rule could cause rapid relay cycling as concentrations fluctuate around the set-point; the team recommends adding a hysteresis dead-band or minimum on-time before any operational deployment. The laboratory tests also used unencrypted HTTP on a local server, meaning secure HTTPS, authentication, and credential protection must precede any field installation.

Conceptually, the team frames Cleara-Air across five dimensions of urban informatics: urban sensing, urban data, urban computing, urban science, and urban applications. The SPS30 provides the sensing layer; time-stamped, georeferenced PostgreSQL records form the data layer; the ESP32 edge controller and server-side processing constitute the computing layer; GIS and Web-GIS supply the spatial representation; and the integrated platform delivers the application. The researchers position the prototype primarily as system-level urban environmental informatics infrastructure rather than a city-scale analysis—one unit cannot characterize how pollution varies across a province. But the framework is designed to scale: a distributed network of calibrated devices with standardized siting criteria and quality-control flags could eventually support investigations of intra-urban PM2.5 variability, population exposure, environmental inequality, and spatially targeted planning interventions.

For a medium-sized regional city in a data-constrained Global South context—Phitsanulok Province, with its roughly 74 percent average relative humidity and patchwork of mountains and low plains, was selected as the target environment—the appeal of a $65, open-source, self-acting air-quality guardian is obvious. Municipalities that cannot afford dense networks of reference-grade monitors could deploy Cleara-Air-class devices to raise public awareness, automate localized mitigation, and feed decision-ready information to policymakers. The next validation phases, the authors say, will include sensor calibration against reference instruments, replicated treatment-versus-control chamber experiments, direct ozone monitoring, and long-duration evaluation of communication reliability and energy consumption. If those hurdles are cleared, the humble flour-dust trials of this prototype may be remembered as the moment urban air-quality informatics stopped merely watching the smog—and started doing something about it.

Subject of Research: IoT and Web-GIS closed-loop system for PM2.5 monitoring and automated ionization actuation

Article Title: Development and experimental evaluation of an IoT-based closed-loop system for PM2.5 monitoring and automated electrical ionization actuation

Article References: Development and experimental evaluation of an IoT-based closed-loop system for PM2.5 monitoring and automated electrical ionization actuation. (n.d.). https://doi.org/10.3389/frsus.2026.1939212

Image Credits: AI Generated

DOI: 10.3389/frsus.2026.1939212

Keywords: Development, experimental, evaluation, IoT-based, closed-loop, system, monitoring, automated, electrical, ionization, actuation, scientific research