The term "smog" was first used in London during the early 1900's to describe the combination of smoke and fog.  What we typically call " smog"  today is a mixture of pollutants but is primarily made up of ground-level ozone   .      Ozone can be beneficial or harmful depending on where it stays. The ozone staying high above the Earth protects human health and the environment, but ground-level ozone is responsible for the choking, coughing, and painful eyes associated with smog. The ozone in smog also prevents plants growth and can cause widespread damage to crops and forest.      Major smog occurrences often are linked to heavy motor vehicle traffic, high temperatures, sunshine, and calm winds. Weather and geography affect the position and severity of smog. Because temperature regulates the length of time it takes for smog to form, smog can form faster and be more severe on a hot and sunny day. When warm air stays near the ground instead of rising and winds are calm, smog may stay trapped over your city for days. As traffic and other sources add more pollutants to the air, the smog gets worse. Smog is often more severe away from the pollution sources because the chemical reactions that cause smog occur in the atmosphere while the reacting chemicals are being moved by the wind.     Smog is a visible example of air pollution. You can look into the distance during the day to see how much smog there is in the air. In addition, most cities measure the concentrations of pollutants in the air and report the results to the public.      Environmental Protection Agency sets national standards for pollutants. Areas that fail to meet the standards for at least one air pollutant are called "nonattainment    areas. " New measures are being taken by local governments across the country to reduce air pollution in nonattainment areas. These include: banning charcoal barbecues and wood burning in stoves or fireplaces when pollution levels are high; developing programs to encourage carpooling and voluntary "ozone actions" ; limiting traffic in overcrowded areas; expanding or improving public transportation systems;  requiring employers to contribute to employee public transportation costs;  assessing " smog fees" on cars according to the number of miles driven and vehicle emissions  produced; and even buying and breaking up older " super-dirty"  cars. According to the passage, ground-level ozone can   _   .
damage the environment benefit human health protect crops and forest limit the spread of smog