A heightened concern for the environment, combined with increasing sophistication in tracking and modelling air currents, has led to the realization that local air and water may be contaminated by pollutants emitted many miles away. Indeed, this realization has resulted in inter-regional and international tensions regarding air pollution. For example, much of the acid rain problem in the northeastern United States is a result of SOx emissions from coal-fired power plants in the Midwest. Similarly, the United States and Canada have a long-standing (and occasionally heated) debate over the issue of which country is exporting its acid rain to the other, while the Scandinavians have been making similar.
