Fine particles that are suspended in the atmosphere. Many are of natural origin, e.g. pollen grains, fungal spores, and volcanic dust. Others are injected into the air by human activity, e.g. ploughing of dry soil, burning of vegetation, and the burning of fossil fuels. Small particles can be harmful to health when inhaled, because they are able to penetrate deep into the lungs. Particles smaller than 25 μm in diameter are known as PM-25; these are believed to cause a number of respiratory illnesses. Particles 10μm in size, known as PM-10, are possibly even more injurious.
Subjects: Ecology and Conservation.