Smog adversely affects agriculture by reducing crop yields, a consequence a world with 6 billion people cannot easily afford. Air pollution also kills off or weakens plants in the wild, putting additional stress on ecosystems that are rapidly being lost to human development. The most harmful pollutant for plants is ozone. Ozone is a molecule of pure oxygen, but in a form that differs from that usually found in nature. That additional atom is what transforms ozone into a toxin.
Ozone destroys plants, as free radicals of oxygen atoms attack plant tissues.
Ozone can be beneficial-the ozone layer in the stratosphere helps protect us from harmful ultraviolet rays. It is the ozone near ground, produced by the mixture of sunlight and emissions from cars and industry, that is the problem. This accounts for approximately 90 percent of all crop damage due to air pollution. Most of the observable ozone damage to plants occurs on their leaves, where they produce their food through photosynthesis. If enough leaves are injured by ozone, a plant begins to starve.