In fact, breathing smoggy air can be hazardous because smog contains ozone, a pollutant that can harm our health when there are elevated levels in the air we breathe. Learn what kinds of health effects ozone can cause, when you should be concerned, and what you can do to avoid dangerous exposures.
What is ozone?
Ozone is a colorless gas composed of three atoms of oxygen. Ozone occurs both in the Earth's upper atmosphere and at ground level. Ozone can be good or bad, depending on where it is found:
Good Ozone. Ozone occurs naturally in the Earth's upper atmosphere-10 to 30 miles above the Earth's surface-where it forms a protective layer that shields us from the sun's harmful ultraviolet rays. This "good" ozone is gradually being destroyed by manmade chemicals. An area where ozone has been most significantly depleted-for example, over the North or South pole-is sometimes called a "hole in the ozone."
Bad Ozone. In the Earth's lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight.
Should I be concerned about exposure to ground-level ozone?
That depends on who you are and how much ozone is in the air. Most people only have to worry about ozone exposure when ground-level concentrations reach high levels. In many U.S. communities, this can happen frequently during the summer months. In general, as ground-level ozone concentrations increase, more and more people experience health effects, the effects become more serious, and more people are admitted to the hospital everyone should be concerned about ozone exposure.
How might ozone affect my health?
Scientists have been studying the effects of ozone on human health for many years. So far, they have found that ozone can cause several types of short-term health effects in the lungs:
Ozone can irritate the respiratory system. When this happens, you might start coughing, feel an irritation in your throat, and/or experience an uncomfortable sensation in your chest. These symptoms can last for a few hours after ozone exposure and may even become painful.
Ozone can reduce lung function. When scientists refer to "lung function," they mean the volume of air that you draw in when you take a full breath and the speed at which you are able to blow it out. Ozone can make it more difficult for you to breathe as deeply and vigorously as you normally would. When this happens, you may notice that breathing starts to feel uncomfortable. If you are exercising or working outdoors, you may notice that you are taking more rapid and shallow breaths than normal. Reduced lung function can be a particular problem for outdoor workers, competitive athletes, and other people who exercise outdoors.
Ozone can aggravate asthma. When ozone levels are high, more asthmatics have asthma attacks that require a doctor's attention or the use of additional medication. One reason this happens is that ozone makes people more sensitive to allergens, which are the most common triggers for asthma attacks. (Allergens come from dust mites, cockroaches, pets, fungus, and pollen.) Also, asthmatics are more severely affected by the reduced lung function and irritation that ozone causes in the respiratory system.
Ozone can inflame and damage the lining of the lung. Some scientists have compared ozone's effect on the lining of the lung to the effect of sunburn on the skin. Ozone damages the cells that line the air spaces in the lung. Within a few days, the damaged cells are replaced and the old cells are shed-much in the way that skin peels after a sunburn. If this kind of damage occurs repeatedly, the lung may change permanently in a way that could cause long-term health effects and a lower quality of life.
Scientists suspect that ozone may have other effects on people's health. Ozone may aggravate chronic lung diseases, such as emphysema and bronchitis. Also, studies in animals suggest that ozone may reduce the immune system's ability to fight off bacterial infections in the respiratory system.
Most of these effects are considered to be short-term effects because they eventually cease once the individual is no longer exposed to elevated levels of ozone. However, scientists are concerned that repeated short-term damage from ozone exposure may permanently injure the lung. For example, repeated ozone impacts on the developing lungs of children may lead to reduced lung function as adults. Also, ozone exposure may speed up the decline in lung function that occurs as a natural result of the aging process. Research is underway to help us better understand the possible long-term effects of ozone exposure.
Who is most at risk from ozone?
Certain groups of people become sensitive to ozone when they are active outdoors, because physical activity (such as jogging or outdoor work) causes people to breathe faster and more deeply. During activity, ozone penetrates deeper into the parts of the lungs that are more vulnerable to injury. Sensitive groups include:
Children. Active children are the group at highest risk from ozone exposure. Such children often spend a large part of their summer vacation outdoors, engaged in vigorous activities either in their neighborhood or at summer camp. Children are also more likely to have asthma or other respiratory illnesses. Asthma is the most common chronic disease for children and may be aggravated by ozone exposure.
Adults who are active outdoors. Healthy adults of all ages who exercise or work vigorously outdoors are considered a "sensitive group" because they have a higher level of exposure to ozone than people who are less active outdoors.
Older adults may be more affected by ozone exposure, possibly because they are more likely to have pre-existing lung disease.
People with respiratory diseases, such as asthma. There is no evidence that ozone causes asthma or other chronic respiratory disease, but these diseases do make the lungs more vulnerable to the effects of ozone. Thus, individuals with these conditions will generally experience the effects of ozone earlier and at lower levels than less sensitive individuals.
People with unusual susceptibility to ozone. Scientists don't yet know why, but some healthy people are simply more sensitive to ozone than others. These individuals may experience more health effects from ozone exposure than the average person.
How can I tell if I am being affected by ozone?
Often, people exposed to ozone experience recognizable symptoms, including coughing, irritation in the airways, rapid or shallow breathing, and discomfort when breathing or general discomfort in the chest. People with asthma may experience asthma attacks. When ozone levels are higher than normal, any of these symptoms may indicate that you should minimize the time spent outdoors, or at least reduce your activity level, to protect your health until ozone levels decline.
Ozone damage also can occur without any noticeable signs. Sometimes there are no symptoms, or sometimes they are too subtle to notice. People who live in areas where ozone levels are frequently high may find that their initial symptoms of ozone exposure go away over time-particularly when exposure to high ozone levels continues for several days. This does not mean that they have developed resistance to ozone. In fact, scientists have found that ozone continues to cause lung damage even when the symptoms have disappeared. The best way to protect your health is to find out when ozone levels are elevated in your area and take simple precautions to minimize exposure even when you don't feel obvious symptoms.
How do scientists know about the health effects of ozone?
EPA has gathered a great deal of information about the health effects of ozone. This information comes from a number of sources, including animal research, studies that compare health statistics and ozone levels within communities, and controlled testing of human volunteers to determine how ozone affects lung function. In these studies, volunteers are exposed to ozone in specially designed chambers where their responses can be carefully measured. Volunteers are prescreened in medical examinations to determine their health status, and they are never exposed to ozone levels that exceed those found in major cities on a very smoggy day.
Though our understanding of ozone's effects has increased substantially in recent years, many important questions still remain to be investigated. For example, does repeated short-term exposure to high levels of ozone cause permanent lung damage? Does repeated exposure during childhood to high levels of ozone cause reduced lung function in adults? Scientists are continuing to study these and other questions to gain a better understanding of ozone's effects.