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    Smog: Not an Allergen, but an Irritant

    WebMD Feature

    For people with allergies and asthma, sometimes the very air they breathe can be bad for their health. That’s because a variety of pollutants in our air -- collectively called smog -- can aggravate asthma and allergy symptoms, leaving people with these conditions struggling to breathe.

    What is Smog?

    Smog is a type of air pollution that results from a mix of gases and particulates reacting with sunlight. The gases in smog include carbon monoxide (CO), carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen oxides (NO2), and volatile organic compounds (VOCs), as well as ozone. The particulates found in smog can include smoke, dust, sand, and pollen.

    In recent years, air pollution has declined somewhat, but a 2010 report from the American Lung Association says that more than 175 million people -- about 58% of the population -- still live where pollution levels frequently make it difficult for some people to breathe.

    The Particle Problem

    Particulate pollution is created by mechanical processes, like construction and mining, and by chemical processes, like burning fossil fuels. Coal, natural gas, and petroleum products such as gasoline, diesel, and jet fuel are common types of fossil fuels used in the United States.

    Fumes from cars with gas engines are often thought of as the only major source for smog, but particulates from diesel engines that power trains, large trucks, and some busses also contribute to air quality problems. "In recent years, we’ve been able to get CO2 emissions down, but we’ve become increasingly concerned about smaller particulates, especially those from diesel," says James Sublett, MD, managing partner with Family Allergy and Immunology, based in Louisville, Ky.

    In addition to engine emissions, particle pollution can come from other sources, depending on where you live. For example, if you live near a coal-fired power plant, the smog in your area may include sulfur particles generated by the plant.

    No matter what the source is, the smaller the particle, the bigger the risk. That’s because particles less than 10 micrometers in diameter, about one quarter the diameter of a human hair, can be inhaled into the lungs and get into the bloodstream, affecting your breathing, and in some cases, your heart function.

    Particle pollution has many serious negative health effects, but it’s especially bad for people with asthma, children in particular. Studies have shown that increases in particulates in the air lead to more hospitalizations for children who have asthma.

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