In the northern hemisphere, aviation is responsible for about one-third of the NOx at flight altitude. NOx also contributes to acidification. As the emissions are regional, the effects will become manifest at the regional level. Since the majority of flights take place in the USA and the EU, the acidification caused by aviation can be expected to be higher there. At the local level, NOx is a threat to human health, as are particles, VOC, and SO2.
Noise is probably the most health-damaging emission' at present.
This segment is concerned primarily with the regional and global environment, however, and will not give further consideration to the wide range of local aviation problems. It has an acidifying effect, but this is small compared with the acidifying effect of the NOx emissions. Particles are formed in aircraft engines due to incomplete combustion. These aircraft emissions may make an significant contribution to the greenhouse effect, because of their influence on the formation of clouds and aerosols. The radiative effect of aerosols and their ability to modify cloud properties are strongly influenced by their atmospheric concentrations, which exhibit very major local variations in magnitude and composition.