Ground-level ozone (or smog) is formed when nitrogen oxides (NOx) and volatile organic compound (VOC) gases react with sunlight, particularly in the warm summer months. Once formed, ozone targets the respiratory system, aggravating asthma, increasing susceptibility to respiratory illnesses like pneumonia and bronchitis, and contributing to permanent lung damage. It can also damage forests, reduce the productivity of agricultural crops, and lead to the decay of monuments and buildings. The Ozone Transport Commission (OTC) was established under the Clean Air Act Amendments of 1990 to help states in the Northeast and Mid-Atlantic region meet the National Ambient Air Quality Standard (NAAQS) for ground-level ozone. Historically, ozone control strategies have focused on VOC emissions. In recognition of the important role of NOx in ozone formation and transport, the OTC has focused much of its efforts on regional NOx reduction strategies. The OTC developed an unprecedented, multi-state cap and trade program to control NOx emissions and address regional transport of ozone. This market-based program, called the NOx Budget Program, sets a regional 'budget' (or cap) on NOx emissions from power plants and other large combustion sources during the 'ozone season' (from May 1st through September 30th).