A review of the effects of urban vegetation on air quality
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Outdoor air pollution has been generally accepted to affect human health. With increasing urban density and high traffic episodes air pollution is expected to rise even more. In order to reduce air pollution in urban environments vegetation is used, because of their capability to help clean the air of pollutants. Trees are effective at capturing significant quantities of pollutants from the air and have the potential to improve air quality, although current removal percentages are very low. As cities grow bigger and buildings get taller, street canyons are formed. In these canyons air pollutants get trapped, limiting dispersion to the atmospheric boundary layer above. Planting trees in these settings could limit the dispersion of air pollutants even further. Coniferous trees are better at trapping pollutant particles than deciduous trees, because they keep their foliage throughout the year and have very high surface areas. Hairy and rough leave surface seem to help with capturing particles. As increases in pollutants with increased tree cover due to decreased dispersion will outweigh the positive effects of trees, it seems clear that street designs are best with less trees. It should be noted that roadside trees are not only placed for their effect on air quality and that aesthetics probably plays a more important role.