How are fine particles formed?
Some fine particles are released into the air when fuels like coal, natural gas, gasoline and diesel motor vehicle fuel, oil and wood are burned. Others form in the air due to secondary formation of pollutants.

What is particle pollution?
Particle pollution is a mixture of microscopic solids and liquid droplets suspended in air. This type of pollution is made up of a number of components, including acids, organic chemicals, metals, soil or dust particles and allergens.

How small are fine particles?
Fine particles (soot) are invisible to the naked eye. In fact, fine particles are about 30 times smaller than the diameter of the human hair! Fine particles are classified in two major categories:

• PM 2.5, or particulate matter that is less than 2.5 micrometers in diameter.
• PM 10, or particulate matter that is less than 10 micrometers in diameter.

Fine particles are visible when they group together to form smoke or haze.

Does weather impact the formation of fine particles?
Yes, on colder days more nitrates are formed. On warmer days sulfates can be higher due to more coal being burned at power plants from the electricity demand for cooling homes and businesses. Also, while stagnant air plays an important role in allowing the build-up of fine particle pollution over a period of several days, prevailing winds can transport fine particles hundreds of miles in the atmosphere.

Does particle pollution have a season like ground-level ozone?
No. Ground-level ozone depends on the sun and warm temperatures for its formation, while fine particles do not. Actually, it's possible for fine particle levels to exceed ground-level ozone levels in the summertime.