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.

O is for Ozone

Ground-level ozone is another big contributor to the smog problem. This isn't the "good" ozone layer found high in the atmosphere that protects us from UV rays of the sun. Ground-level ozone is a pollutant produced when sunlight reacts with the chemical fumes our cars and industrial plants churn out. It aggravates asthma, irritates the lungs, and makes it difficult to breathe. Long-term inflammation from breathing in too much ground-level ozone can permanently scar lung tissue.

Smog with high levels of ozone is also particularly damaging for people with asthma. Researchers found that during the 1996 Summer Olympics in Atlanta, peak morning traffic decreased by 23% and peak ozone levels also went down, by 28%. What else went down? Emergency room visits for kids with asthma, by a whopping 42%.

Air pollution from high-ozone smog can make existing asthma symptoms worse as well as triggering the onset of the condition in the first place, Sublett says. And the closer you get to it, the worse your symptoms are likely to be. "There have been studies showing that children who live near high-traffic areas, such as expressways, have higher rates of asthma. And the increasing rate of asthma appears to directly correlate with the closer you live to high traffic levels."

Staying Safe from Smog

So what can you do to protect yourself or your child from smog if you have asthma or allergies? Since it's usually not practical for most people to just move to a less-polluted area, here are some tips to try:

• Keep track of the daily air quality index in your area by checking local news reports so you'll know how high the pollution levels will be that day. When the color-coded alert level reaches the orange level, the air is considered to be unhealthy for sensitive groups. People with respiratory conditions such as asthma, especially children, should take precautions. Stay indoors. If you must go outside, keep activity low and take frequent breaks.
• When the air quality index goes past orange and up to the red alert level, the air quality is rated "unhealthy." People with asthma or severe allergies should stay indoors as much as possible and avoid outdoor activity.
• If you must go outside when the air quality index is poor, do it in the morning, before the heat of the day generates more smog and ozone, and avoid exercising outdoors.
• Wear a mask to cover your mouth and nose when you go outside. It can help filter out irritants that aggravate allergy and asthma symptoms.
• Outdoor air pollution can also get inside. Make sure your heat and air conditioning system has a MERV 11 or 12-level filter to screen out particulates. During the spring months, when you're tempted to open the windows, check air quality levels first. If they're high, resist the spring air and use a circulating fan instead.