What effect, if any, does exercising in air pollution have on my workout performance?

 A by-product of extremely complex chemical reactions, air pollution can negatively impact an individual's aerobic exercise capacity in several ways. The magnitude of air pollution's effect is at least partly related to the ability of pollutants (e.g., carbon monoxide, ozone, sulfur dioxide, etc.) to infiltrate your respiratory system.
When you are not engaged in physical activity, your respiratory tract serves as both the major route of infiltration for air pollutants and the major barrier to their penetration. For example, during normal inspiration, the mucous membranes in the nose are very effective in removing large particulate matter and highly soluble gases from the air that reach the lungs. On the other hand, during aerobic exercise you tend to breathe through your mouth, the natural air filtration process of the body becomes less efficient, resulting in more pollutants reaching the lungs. Once in the lungs, many of these pollutants subsequently diffuse into the bloodstream and circulate through the body. Since the respiratory tract is the largest surface area of the body to come in contact with air pollutants, many of the adverse effects of air pollution occur in that region. Among the many negative physiologic effects of air pollution are the following:

• Bronchial vasoconstriction (i.e., the airway openings become smaller), which results in increased airway resistance.
• The lungs lose some of their diffusing surface area either as a result of destruction of the alveoli (i.e., the functional units of the lung) or as a by-product of an increased level of mucus secretion.
• Oxygen transport capacity is reduced. Less oxygen enters the bloodstream (via the pulmonary system), resulting in an inadequate supply of oxygenated blood to the exercising muscles.

The net result is that these negative physiologic effects of air pollution can cause a substantial decrease in your physical performance.