One important concern with the release of VOCs and NOX in particular is that these gasses are precursors to ground level ozone. In sunlight, VOCs and NOX gasses react with each other to produce ozone. If this reaction happens high in the stratosphere, the ozone serves to block harmful UV radiation from the Earth’s surface. If it happens close to the ground, however, it causes smog, which can harm human respiratory systems. It can also seriously damage plants, reducing
crop yield and making ecosystems more vulnerable to pests and disease. Ozone creation is seasonal. Because the reaction that creates ozone is dependent upon sunlight, ground-level ozone layers rise in the summer, when there is more sunlight.