VOC and NOx are called ozone precursors. Volatile organic compounds are emitted by automobiles and various commercial and industrial sources. Nitrogen oxides are by-products of burning fuel in automobiles and heavy industries.
Ozone is the main ingredient of smog.
Ozone concentrations can vary from time of year and from year to year. Repeated exposure to ozone pollution may cause permanent damage to the lungs. Short-term exposure to ozone can cause chest pains, nausea, throat irritation, congestion, shortness of breath, pain when inhaling deeply, wheezing and coughing. It can worsen bronchitis, heart disease, emphysema, and asthma, and reduce lung capacity. According to the American Lung Association, tests of healthy adults and children during exercise show that even brief exposure to ozone at the minimum air quality standard results in decreased lung function.