Classical air pollution problems caused by very high concentrations of sulphur dioxide (SO₂) and London-type smog have decreased to acceptable levels in most parts of Europe. Nevertheless, there are still a number of potential ecological threats such as acidification and eutrophication of terrestrial and aquatic ecosystems, increased tropospheric ozone (O₃) concentrations and stratospheric ozone depletion, as well as greenhouse effects and human health problems caused by aerosols. Reactive atmospheric nitrogen species contribute to all these phenomena.

Tropospheric O₃ is an all pervasive air pollutant and a challenging problem to present and future world food, fiber and timber production and conservation of natural communities, including their species diversity. Plant response to ambient O₃ exposure can be acute (visible foliar injury) or chronic (reductions in growth and yield, with or without injury symptoms).

The exchange of gaseous pollutants (e.g. O₃) between the atmosphere near the ground and the phytosphere is controlled by complex interactions of meteorological and biological characteristics. Any response of a plant to gaseous pollutant exposure is a consequence of the effective concentration of the active species in the tissue (effective dose, given as mass of the active species per mass of the receptor or mass of the respective organ) and the repair activity of the organ. The latter is a function of other factors modifying the effect, e.g. simultaneous presence of other pollutants or biotic interferences:

plant response = f (effective dose, repair capacity)

Repair capacity is the maximum possible repair effort. The effective dose depends on the amount of molecular diffusion of the respective gaseous pollutant into the leaf interior (uptake, absorbed dose) and is a function of the amount of detoxification:

effective dose = f (absorbed dose, detoxification capacity)

Detoxification capacity is the maximum detoxification capability. Repair and detoxification capacities are determined by the genotype of the respective plant, and depend on factors such as the stage of development and the nutritional status.

One of the basic rules of toxicology is that dose-response relationships can only be established if the effective dose or at least the absorbed dose of a stressor is known. Because the effective dose of a gaseous pollutant (e.g. O₃) at the site of the receptor cannot be determined, measuring or modelling the absorbed dose of a gaseous pollutant is the only sensible tool for establishing dose-response relationships.

The exchange of O₃ between the atmosphere and phytosphere (flux) is governed by the ambient O₃ concentration, the turbulent conductivity of the lower atmosphere, and the sink properties of the plant/soil system. Based on that principle, there is a very strong and concerned movement among the scientists within the UN-EU to arrive a flux-based O₃ air quality critical level to protect vegetation.