pollutants have short-term health effects, reduce visibility, interfere with recreational activities, affect crop growth, and present their own set of problems for accounting. In many cases, the substances emitted are precursor emissions; that is, they react chemically in the atmosphere with other substances to form the substance that is ultimately damaging to humans or ecosystems. There are also complex nonlinearities because the formation of the damaging substance depends on the level of precursor emissions, weather conditions, and the presence of other substances with which the precursor emissions react. All of these processes vary on an hourly, daily, and seasonal basis. Emissions, concentrations, and impacts of damaging substances also vary spatially, and there may be important threshold effects as well. Above all, there is the "weed syndrome"—the fact that the same substance may be beneficial or harmful depending on where it is, how much of it there is, the time and duration of exposure, and what organism is absorbing it. Virtually every substance on earth, from water to plutonium, can be an economic good or an economic weed depending on the circumstances.