Air pollutant impacts occur either in relation to:
exposure to toxic pollutant concentrations in the atmosphere (direct effects)
accumulated doses by deposition of pollutants from the atmosphere (indirect effects).
Direct toxicity is typically associated with high concentrations for short periods of time, representing acute exposure. By contrast, the indirect effects are generally a result of long term chronic exposure.
Assessment of the direct toxicity to air pollutants is generally assessed by comparing measured pollutant air concentrations with "critical levels", which are set for a range of air pollutants. Critical levels have been quantified for sulphur dioxide, oxidised nitrogen, ozone, ammonia and cloud water droplets.
(In contrast, the deposition of pollutants is assessed using "critical loads".)
In principle, direct toxicity of air pollutants may be manifest in terms of different impacts to the indirect long-term effects. Classic symptoms of direct pollutant toxicity include leaf tip damage and speckly lesions, as well as decreasing in the vitality and loss of sensitive species, such as lichens. By contrast, the indirect effects are usually manifest by a long term change in plant species competition, for example coupled to a change in soil nutrient status.