Particulate matter 2.5 (PM2.5) refers to tiny particles or droplets in the air that are 2.5 microns or less in width. These particles are so small that they can be inhaled and cause serious health problems. PM2.5 particles are a subset of PM10 particles, which are inhalable into the lungs and can induce adverse health effects. The largest PM2.5 particles are about 30 times smaller than a human hair, and several thousand of them could fit on the period at the end of this sentence.

Sources of PM2.5 include dust, dirt, soot, smoke, and drops of liquid. PM2.5 particles can travel deeply into the respiratory tract, reaching the lungs, causing short-term health effects such as eye, nose, throat, and lung irritation, coughing, sneezing, runny nose, and shortness of breath. Exposure can also affect heart and lung function, worsening medical conditions like heart disease and asthma, and increase the risk for heart attacks. Scientific studies have linked increases in daily PM2.5 exposure with higher cardiovascular and respiratory hospital admissions, emergency department visits, and deaths. Long-term exposure to fine particles causes increased mortality from heart disease and may be associated with increased mortality from respiratory diseases and lung cancer.

PM2.5 particles come from many sources, including primary and secondary sources. Primary sources cause particle pollution on their own, such as wood stoves and forest fires, while secondary sources let off gases that can form particles, such as power plants and coal fires. Other common sources of particle pollution can be either primary or secondary, such as factories, cars and trucks, and construction sites.

In conclusion, PM2.5 is a type of particulate matter that is 2.5 microns or less in width and can cause serious health problems when inhaled. It comes from many sources, including primary and secondary sources, and can travel deeply into the respiratory tract, causing short-term and long-term health effects.