What do you love most about summer? In addition to getting some time off from school, many kids love to sleep in a bit and then awake to rays of sunshine streaming in through their windows. There's nothing quite like getting out of bed and stretching in front of the window while the Sun caresses you with warmth.

Of course, if you live in a big city with lots of cars and heavy industry, the sight you awake to might be a bit hazy. For some kids, looking out the window means looking into a cloudy fog that obscures the horizon. What are we talking about? Smog, of course!

Smog is a type of air pollution caused by tiny particles in the air. The word comes from a mixture of the words smoke and fog and was first used to describe the hazy mixture of smoke and sulfur dioxide produced by the burning of large amounts of coal in London in the early 1900s.

Today, smog generally refers to photochemical smog, which is the haze created when sunlight reacts with nitrogen oxides and volatile organic compounds that can be found in fossil fuel emissions from automobiles, factories, and power plants. These reactions create ground-level ozone and tiny particles in the air, known as particulate matter.

Not only does smog reduce visibility and create an unattractive haze on the horizon, it also has negative health effects. For example, smog can make breathing more difficult. This can be especially troubling for people with asthma. Smog also tends to affect children and the elderly.

The effects of smog aren't limited to people, though. Scientists know that smog also affects plants and animals. Smog pollution has been known to damage crops, as well as to cause health problems in pets and farm animals. Smog has also been known to cause corrosive damage to buildings and vehicles.

In the United States, smog is most closely associated with the city of Los Angeles. Southern California's sunny days combined with the emissions of millions of vehicles create the perfect conditions for smog.

Los Angeles is not alone, however. Many other U.S. cities experience smog. Likewise, smog is a problem in large cities around the world. Anywhere there's sunshine and plenty of emissions from industry and vehicles, smog can become an issue.

Certain landscapes can also contribute to smog problems. For example, large cities located in basins surrounded by mountains are more likely to have problems with smog, because smog gets trapped in the valley where it can't be blown away by winds. Los Angeles and Mexico City are two cities known for having smog problems that are exacerbated by their landscapes.

Smog problems have led many countries, including the United States, to pass laws intended to reduce smog. Some of these laws might restrict when factories can release certain types of chemicals into the air. Other laws might limit open burning of trash and other wastes to certain times of day. Environmentalists are always on the lookout for new ways to limit the effects of smog.

Wonder What's Next?

If you want to be prepared for tomorrow’s Wonder of the Day, you need to get a good night’s sleep. Time for bed!