PM2.5 refers to the suspended particles in the air that have an aerodynamic diameter of less than 2.5 micrometers (µm), that is, they are fine particles that are 2.5 micrometers or less in diameter. These particles are considered particularly dangerous due to their small size, as they can penetrate deep into the lungs and enter the bloodstream, causing a range of long-term health problems. PM2.5 is generated from a variety of sources, including the burning of fossil fuels, industry, vehicular traffic, and dust from the ground. It is important to monitor and control levels of PM2.5 in the air to protect public health and the environment.