National Trends. The amounts and types of emissions change every year. These changes are caused by changes in the nation's economy, industrial activity, technology improvements, traffic, and by many other factors. Air pollution regulations and emission controls also have an effect. The National Air Pollutant Emission Trends report summarizes long-term trends in emissions of air pollutants and gives in-depth analysis of emissions for the current year. The report also discusses emission evaluation and prediction methodologies.