This report presents the results of measurements of atmospheric pollution from vehicle emissions near the portal of a road tunnel at Heathrow Airport. At the site where measurements were made, the highway runs in a 7m deep cutting, and the air from one of the tunnels is ventilated into this cutting. As expected, the pollutant levels measured on and near the road were high because of the proximity of the tunnel portal. Statistical analyses derived relationships between pollutant levels, traffic and wind speed, which are very similar to those found at other (ground level) sites. This confirms that normal dispersion was taking place, and there was thus no evidence of pollutants accumulating in the cutting. The pollutant levels measured at ground level around the cutting were lower than would have been expected on a flat site carrying the same amount of traffic. This indicated that placing the road in cutting reduced the impact of air pollution on the surrounding area.

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