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also be some dependence on weather conditions - a separate study
indicates that the pollution to be expected with a wind speed of 15
knots is only one-third of that at 5 knots. We may, however,
eliminate both these variables to a large extent by comparing pollution
on weekdays and on Sundays. From 10 a.m. to 8 p.m. there is
approximately the same number of vehicles on both days, but far
fewer diesel vehicles on the Sunday, and correspondingly lower pollution
which shows conclusively that diesels are responsible.
Layers of Pollution A few additional measurements were made at
a height of six feet, at the edge of the
carriageway at a series of points along the road; these indicate that
there was not a very large change in concentration across the pavement,
and that very similar concentrations are observed for some
distance along the road. Another set of measurements made at
different heights above the ground showed that there was little
difference down to about 4 feet, but that concentrations at a height of
2 feet are about 50 per cent, higher. Some measurements were
also made on the road behind "smoking" vehicles by means of a
motorcycle and these indicated concentrations as high as 10,000 microgrammes
per cubic metre, although 3,000 to 5,000 was more common.
Inhalation Risks It is interesting to note that the amount of
air drawn through the filter paper in one hour
during the survey was approximately 2.5 cubic feet, the equivalent of
71 litres and that an average adult when walking at about 3 miles
per hour inhales at the rate of 25 litres per minute. One of the
hourly smoke stains, therefore, indicates approximately the amount
of smoke which might be inhaled in three minutes by a person
walking up Archway Road.
Conclusion The average smoke pollution on weekdays in
the Archway Road is approximately twelve
times the background pollution of the district measured at ten feet
above ground level. This pollution is shown to result from diesel
vehicles in heavy traffic conditions. A greater concentration is
found below 2 feet from the ground.
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