PM2.5 refers to fine particulate matter that is less than 2.5 micrometers in diameter, which can be inhaled deeply into the lungs and potentially enter the bloodstream. This type of air pollution originates from various sources, including vehicle emissions, industrial processes, and natural events like wildfires. Due to its small size, PM2.5 can cause significant health issues and environmental effects, making it a critical concern in discussions about air quality and pollution management.