Definition:
Substances released into the atmosphere in the form of a solid, gas, or liquid droplets that may cause harm, damage, discomfort, or irritation to people, other living organisms, or the environment. These substances may occur naturally or originate from man-made sources.
Natural Sources: 
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forest fires
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dust storms
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volcanoes
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sea spray
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vegetation
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soil
Man-Made Sources:
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auto emissions
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road dust
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insecticide dust
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power generation
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industrial processes
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construction/demolition
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waste disposal
Facts:
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On average, an adult will breathe in about 125 gallons of air each hour
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The smallest particles will remain in the air for several hours up to several weeks and can be carried by air currents for hundreds of miles
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Precipitation aids in the removal of air pollutant particles
Reactions:
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coughing
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irritation of eyes, nose, throat
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shortness of breath
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headache, fatigue
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more severe respiratory symptoms possible for those with increased sensitivity or if in an environment with elevated levels of pollution
Particle Size:
There is a great range in particle size of the various air pollutants. Heavy particles are most common near busy roadways and industrial areas. These particles usually fall in the range of 2.5 - 10 microns. Fine particles, less than 2.5 microns are found in higher concentrations in areas with smog, smoke, and haze.