Ozone, a molecule (O3) formed by three atoms of oxygen, is a photochemical oxidant that disrupts photosynthetic and metabolic functions. It appears at ground level by being brought down by vertical winds from the stratosphere during electrical storms and is associated with lightning. Also, ozone is produced by a reaction between sunlight and the products of fossil fuel combustion such as from vehicle exhausts, electric power plants and fossil fuel furnaces. Ozone produced by the latter method is considered air pollution and can be held down by an air inversion. Air inversion occurs when warm air aloft traps cooler air near the ground. Damage by ozone is most severe on warm, clear, calm days when there is a high relative humidity and high air pressure. Injury occurs most in the afternoon and least at night.

Leaf symptoms to ozone exposure are termed "stippling" or "speckling" characterized by numerous tiny dots on the upper leaf surface. On potato, the speckles are usually brown or black but may be bronze, yellow or even white ("bleaching"). In extreme cases, the specks may fuse forming larger bronze areas. Symptoms begin with the oldest leaves and gradually work up into the plant as younger leaves get affected. Young, smaller plants are most susceptible to ozone injury. The degree of injury depends on the amount of ozone in the air and the duration of exposure to it. Heavy exposure can result in symptoms appearing in 24 hours and symptoms progress for 10-14 days after exposure. Due to the tissue collapse induced by ozone, leaves are prone to infection such as by early blight and will become senescent sooner. Yields may be decreased due to the early dying.

Potato is considered very sensitive to ozone, such as alfalfa, barley, edible beans, beets (table and sugar), chicory, grapes, soybean, and many garden vegetables. Among potato, there is a wide range of ozone sensitivity, from susceptible ones such as Norchip and the Norlands to tolerant ones as Russet Burbank. On sensitive varieties, injury can occur with exposure as low as 0.04 ppm ozone for four hours.

PAN: Another photochemical oxidant is PAN or peroxyacetyl nitrate. After ozone, it is the most phytotoxic air pollutant. It is formed similarly to ozone. PAN causes leaves to develop bands or blotches of glazed, bronzed or silvery areas. Pre-mature senescence and defoliation may occur. Exposure to 0.01 to 0.05 ppm for one hour will induce symptoms. Potato is very susceptible as is alfalfa, pinto bean, beets (table and sugar), sunflower and several garden crops. Corn, sorghum and wheat are resistant.