How Air-Quality Sensors Detect Pollution

by | Jul 15, 2026

Air pollution is usually invisible. Smoke may be easy to recognize, but many harmful particles and gases cannot be seen or smelled. Air-quality sensors help make these pollutants measurable by converting a physical or chemical change into an electrical signal.

Different sensor types use different methods. Some shine light through the air, while others measure chemical reactions, electrical resistance, or the absorption of infrared energy. A monitor may contain only one sensor or combine several sensors with temperature, humidity, data storage, and wireless communication.

Understanding the basic technologies helps explain both what air sensors can reveal and why their readings sometimes differ.

Light-Scattering Particle Sensors

Many consumer and community air monitors measure particulate matter using light scattering. A small fan or pump draws air into a chamber containing a light source, usually a laser or light-emitting diode.

When airborne particles pass through the beam, they scatter some of the light. A detector measures the scattered light and uses it to estimate the number and size of particles moving through the chamber. Software then converts this information into measurements such as PM1, PM2.5, or PM10.

These sensors are useful because they provide rapid readings and can show how particle levels change from minute to minute. They may detect increases caused by wildfire smoke, cooking, traffic, construction, dust, or industrial activity.

However, the amount of light scattered depends on more than particle size. Particle shape, color, density, and chemical composition can all affect the result. Humidity is another major influence because some particles absorb water and grow larger in moist air. The sensor may interpret these swollen particles as a higher particulate concentration even though the actual amount of dry material has not increased.

Optical sensors are therefore excellent for identifying patterns and sudden changes, but their estimated particle mass may require correction or comparison with a reference instrument.

Electrochemical Gas Sensors

Electrochemical sensors are commonly used to measure gases such as carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide, and hydrogen sulfide.

Inside the sensor is a small cell containing electrodes and an electrolyte. When the target gas enters the cell, it participates in a chemical reaction at an electrode. This reaction produces an electrical current related to the amount of gas present.

Electrochemical sensors can be small, sensitive, and energy efficient. They are used in personal exposure monitors, portable safety instruments, community monitoring stations, and some indoor air-quality devices.

One limitation is cross-sensitivity. A sensor designed for one gas may also react to another gas with similar chemical behavior. For example, some ozone and nitrogen dioxide sensors have difficulty separating the two pollutants without additional filters, sensors, or mathematical corrections. Temperature, humidity, sensor age, and contamination can also change the response.

Metal-Oxide Semiconductor Sensors

Metal-oxide semiconductor sensors are sometimes described as surface-reaction or surface-oxidation sensors. They contain a heated surface made from a metal-oxide material. Oxygen and other gases interact with this surface and change its electrical resistance.

When pollutants such as carbon monoxide, nitrogen dioxide, ozone, methane, or volatile organic compounds reach the sensing material, oxidation or reduction reactions alter the flow of electricity through the sensor. Electronics translate that change into a signal.

These sensors are inexpensive, durable, and capable of responding to many gases. Their broad sensitivity can be useful for detecting that an air-quality change has occurred.

That broad response is also their main weakness. A metal-oxide sensor may react to several gases rather than identifying one specific chemical. Humidity and temperature can strongly influence the signal, and the heated surface requires more power than some electrochemical sensors. Manufacturers may use filters, multiple sensing materials, and computer algorithms to improve selectivity.

Infrared Carbon Dioxide Sensors

Carbon dioxide is commonly measured using a nondispersive infrared, or NDIR, sensor. Carbon dioxide molecules absorb infrared energy at particular wavelengths.

An NDIR sensor directs infrared light through an air sample and measures how much reaches a detector. When more carbon dioxide is present, more energy at the selected wavelength is absorbed. The instrument uses this reduction to calculate the gas concentration.

NDIR sensors are widely used in schools, offices, greenhouses, laboratories, and heating and ventilation systems. Indoor carbon dioxide is often monitored as an indicator of ventilation and human occupancy, although it does not represent every aspect of indoor air quality.

Photoionization Detectors

Photoionization detectors, commonly called PIDs, are used to detect many volatile organic compounds. These are carbon-containing chemicals that can evaporate from fuels, solvents, paints, cleaners, adhesives, and industrial processes.

A PID uses an ultraviolet lamp to give energy to molecules in the air. Chemicals with sufficiently low ionization energies lose electrons and form charged particles. The detector collects these charges and produces an electrical signal.

PIDs respond quickly and are valuable for locating leaks or identifying changes in total VOC levels. However, they generally cannot determine which individual chemical is present without additional analysis. Different VOCs also produce different responses, so calibration affects how results should be interpreted.

Sensors Are Only Part of the Monitor

An air-quality monitor also needs an air inlet, power supply, electronics, software, and a method for recording or transmitting data. Temperature and humidity sensors are often included because environmental conditions affect other measurements.

Calibration is equally important. Sensor readings can drift as components age or become dirty. Comparing a sensor with a trusted reference monitor under real operating conditions can reveal consistent errors and support mathematical corrections.

Low-cost sensors generally do not replace regulatory monitoring stations, which use carefully controlled equipment and quality-assurance procedures. Their strength is providing frequent, local measurements across many locations. When selected carefully and interpreted with their limitations in mind, air sensors can help people identify pollution sources, observe trends, evaluate ventilation, and better understand the air around them.

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