PM2.5 sensors, also called dust sensors or particulate sensors, can be used to detect the dust concentration in the surrounding air, i.e., the PM2.5 value. According to aerodynamics, dust with a diameter less than 10μm that can enter the alveolar region is usually called respirable dust. Dust particles larger than 10μm mostly deposit via impact, mostly in the nasopharynx when inhaled, while dust smaller than 10μm can reach the deep respiratory tract. Dust deposited in the alveoli is mostly below 5μm.

PM10 refers to particulate matter in the ambient air with an aerodynamic equivalent diameter of less than or equal to 10μm. PM2.5 fine particles have small diameters, stay suspended in the atmosphere for a long time, disperse over long distances, and often contain large amounts of toxic and harmful substances, thus posing greater health risks. PM2.5 can enter the lungs and bloodstream, and if carrying pathogens, can severely affect the respiratory, cardiovascular, and even reproductive systems.

The working principle of dust sensors is based on light scattering. Microparticles and molecules scatter light when illuminated and absorb part of the light energy. When a parallel monochromatic light beam passes through the measured particle field, it is affected by scattering and absorption around the particles, attenuating the light intensity. The relative attenuation can then be used to determine the relative dust concentration in the field. The light intensity is proportional to the converted electrical signal, allowing calculation of dust concentration.

Dust sensors are designed to detect airborne particles. Inside, an infrared LED and a phototransistor are diagonally positioned, with their optical axes intersecting. When a dust-laden airflow passes through the intersection, dust reflects infrared light, and the reflected intensity is proportional to dust concentration. The phototransistor detects reflected light even from very fine particles like tobacco smoke. The infrared LED emits light, and the sensor receives the reflected intensity, outputting a signal. The signal intensity indicates dust concentration, and two different pulse-width modulation signals distinguish different dust particle concentrations.