CFCs from years past are only now beginning to affect the ozone layer, and may remain a threat for up to one hundred years. As the ozone layer is depleted, increased levels of UV light reach the earth, impacting human health and leading to greater incidences of skin cancer, cataracts and impaired immune systems. Crops too, are sensitive to UV, and increased exposure can reduce crop yields. The third oxygen atom can detach from the ozone molecule and reattach to molecules of other substances, thereby altering their chemical composition. Some of the major sources of NOx and VOC are waste products emitted by cars, power plants, industrial boilers, refineries and chemical plants.
Metropolitan areas are prone to high levels of ozone, but no region is immune.
But even the smallest amounts of ground-level ozone can be damaging to lung tissue. At the end of the bronchioles are tiny air sacs called alveoli, which fill up and expand like little balloons to put oxygen into the bloodstream. Ozone primarily injures these key oxygen exchangers. Ozone affects people differently, based on their susceptibility. But generally speaking, in the event of an ozone alert, outdoor activities should be scheduled for morning or early evening hours to avoid the mid-day peak when levels are at their highest.