The volatile organic compounds and nitrogen oxides that form ozone come from both natural and human activities. Industrial development and motor vehicles are not the only source of air pollution. Natural VOC emissions are a much larger part of total VOC emissions than natural NOx emissions are of total NOx emissions. Additionally, isoprene, which accounts for the largest portion of biogenic VOCs, is highly reactive and one of the most effective VOCs in producing ozone.

Natural volatile organic compounds are emitted primarily from forested lands. Natural VOC emissions increase with higher temperatures and intense sunlight. Deciduous trees cause higher emissions than evergreens, but large evergreen forests can have signi ficant emissions.

Farming areas are relatively minor sources of emissions of nitrogen oxides in the Mid-Atlantic region. Soil temperature and land use influence the amount of nitrogen emissions produced by organisms in the soil. Natural emissions of nitrogen oxides are hi gher in agricultural areas than in urbanized areas. As noted on page 25, below, however, natural sources account for only about 3% of the total emissions of NOx in the mid-Atlantic region.

Because of natural emissions of nitrogen oxides and volatile organic compounds, some ozone is present even in the absence of human activities. Scientists estimate that an average natural background ozone concentration is 25 to 45 parts per billion. This is much lower than the current ozone standard of 125 parts per billion. Thus, natural emissions by themselves would not cause unhealthful ozone levels.