The report was produced by the European Academies Science Advisory Council, a body set up in 2001 to help national academies such as the UK's Royal Society feed scientific advice into EU policy-making. The work was led by Professor David Fowler of the Centre for Ecology and Hydrology, Edinburgh - one of the UK's foremost experts on air pollution.
The scientists note that EU Member States have made considerable reductions in emissions and are committed to further cuts under a 2001 national emissions ceilings Directive (ENDS Report 319, p 52 ). The EU, and other European and North American countries, have also agreed to reduce emissions under a UN Economic Commission for Europe (UNECE) protocol (ENDS Report 296, pp 44-45 ).
However, the report warns that "recovery from past damage in some important areas is slower than expected and in some cases the environment is expected to deteriorate further."
The first concern is that there has been "little or no improvement" in freshwater ecosystems in some sensitive upland areas which were worst hit by acid rain. The scientists pin the blame firmly on emissions of sulphur and nitrogen compounds from international shipping.
The second main worry is ground-level ozone pollution. Efforts to reduce Europe's emissions of precursors such as oxides of nitrogen and volatile organic compounds have succeeded in reducing peak concentrations - with notable exceptions such as the hot summer of 2003 (ENDS Report 348, pp 8-9 ). However, background concentrations - which are now 2-3 times higher than in pre-industrial times - are increasing steadily at roughly 1ppb per year.
The report notes that annual mean ozone concentrations in much of the Northern hemisphere are now within 10ppb of levels which reduce the yields of sensitive crops, and within 20ppb of levels which affect breathing in sensitive individuals.
"Given the rate of increase," the scientists warn, "it is a matter of a few decades before the mean surface concentration of ozone in some areas of Europe becomes damaging to human health, the health of natural ecosystems and crop yields."
In a background paper, Professor Fowler also warns that reductions in nitrogen emissions "seem to have had little impact on deposition of nitrogen species or on effects such as eutrophication across Europe."
The report calls on the EU to take "immediate steps" to consider how concerted, international action should be taken forward. It calls for controls on emissions of ozone precursors "throughout the countries of the Northern hemisphere", and curbs on emissions of sulphur and nitrogen oxides from international shipping.
However, the report is careful to stress that "growing awareness of the external influences on European air quality should not in any way lessen national commitments to reductions in emissions already agreed."