To find out what a hedge along the road contributes to the air quality behind it, TNO took measurements in a number of locations in Amsterdam. Measurements showed a hedge immediately adjoining a road does provide some reduction in NO2 concentrations behind the hedge. For people living at a distance of more than 7 metres, the hedge has no effect. And when it is farther from the edge of the road, there is no measurable effect either.
To discover whether the NO2 reduction is due to the obstacle (hedge) or absorption, a test was conducted with an extra plexiglass screen between the hedge and the edge of the road. From these data TNO concludes that there is no NO2 absorption by the hedge. The measurements also indicate that there is no or hardly any decrease in fine dust particles. The level of these mainly depends on environmental concentrations, not local circumstances.
TNO draws the conclusion that hedges are therefore not effective as reductive measures in locations where air quality standards are exceeded. The study also reveals that spraying streets has hardly any effect on air pollution. Trees may even be counterproductive, where dense foliage hinders distribution of air pollution.