Ambient standards set maximum allowable levels of a pollutant in the receiving medium (air, water and soil). Ambient standards can offer a simple method of establishing priorities since areas (or stream lengths) which comply with the relevant ambient standards are considered to require no further intervention, while other areas may be ranked by the extent to which concentrations exceed the ambient standards. Setting ambient standards requires an explicit agreement on the environmental quality objectives that are desired, and the costs that society is willing to accept to meet those objectives. However, because ambient standards can be set at different levels for different locations, it is possible to use them to protect valuable ecosystems in a way that would not be possible by using emission standards.
It has been usual to establish an ambient standard for a pollutant by reference to the health effects of different levels of exposure, although some countries are moving toward ambient standards aiming for the protection of natural ecosystems. Historically, ambient standards in the industrialized market economies have been continually tightened in the light of medical evidence on the impact of certain pollutants, and as the demand for better environmental quality has increased. In particular, as reductions are achieved in the levels of simple pollutants such as BOD , the focus has moved to the control of less obvious but more persistent pollutants such as heavy metals, PCBs etc., which are accumulative and essentially not biodegradable.
These set maximum amounts of a pollutant that may be given off by a plant or other source. Emissions standards have typically been expressed in concentrations although there is increasing use of load based standards, which reflect more directly the overall objective of reducing the total load on the environment. They may be established in terms of what can be achieved with available technology or by trying to relate the emissions to impacts on the ambient environment.
Technology based standards are based on knowledge of what can be achieved with current equipment and practices. There have been a wide range of different principles used such as 'Best Available Technology' (BAT), 'Best Practicable Technology' (BPT) or 'Best Available Technology Not Entailing Excessive Cost' (BATNEEC). All of these approaches are open to interpretation and are related to establishing what are the highest levels of equipment and performance that can reasonably be demanded from industrial plants.
Alternatively, emission standards can be established by estimating the discharges that are compatible with ensuring that receiving areas around the plant meet the ambient standards that are defined for the pollutant. However, this requires both considerable information on both the sources and the ambient environment and will be variable from area to area.
New Source Performance Standards (NSPS) are specific emission standards in which the standard is only applied to new plants. They are a special form of grandfathering since emissions from existing plants are treated differently from emissions from new plants. Where NSPS are significantly stricter than standards imposed on existing plants and therefore costly, they may have the effect of prolonging the economic life of existing plants -- subject of course to the influence of other economic and technological factors. On the other hand, it is easier for new plants to adopt cleaner processes and to incorporate treatment requirements in the initial design and therefore the costs of well designed NSPS need not be excessive.