What are the health effects of visibility-reducing particles? Visibility standards are based on extinction coefficients, which is a measure of the light attenuation due to both absorption and scattering. Some types of particles such as sulfates scatter more light, particularly during humid conditions. Particles that are the most effective at reducing visibility (per unit aerosol mass) have diameters in the range of 0.1-1.0 µm. Scattering by particulates impairs visibility much more readily. Light absorption by gases and particles is sometimes the cause of discolorations in the atmosphere but usually does not contribute very significantly to visibility degradation. As the number of fine particles increases, more light is absorbed and scattered, resulting in less clarity, color, and visual range. Visibility degradation is caused by the absorption and scattering of light by particles and gases in the atmosphere before it reaches the observer. Visibility reduction is probably the most apparent symptom of air pollution. These fine particles, caused largely by combustion of fuel, can travel hundreds of miles causing visibility impairment. Others are formed in the air from the chemical transformation of gaseous pollutants (e.g., sulfates, nitrates, organic carbon particles) which are the major constituents of fine PM. Some haze-causing particles are directly emitted to the air such as windblown dust and soot.
These particles vary greatly in shape, size and chemical composition, and come from a variety of natural and manmade sources. Particulate matter (PM) pollution impacts the environment by decreasing visibility (haze).
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