THE air in Britain鈥檚 cities is often much dirtier than the official figures
show. The technique used to measure PM10s鈥攑articles with a diameter of
less than 10 micrometres鈥攃an underestimate the total by more than a third.
The thicker the smog, the greater the error.
Last week, the British government changed the way it presents information
about air pollution. Under the old system, air quality was classified into four
bands鈥攆rom 鈥渧ery good鈥� to 鈥渧ery poor鈥�. The new system describes air in
terms of the amount of pollution, which it puts into four categories鈥攆rom
鈥渓ow鈥� to 鈥渧ery high鈥�.
The new system also paints a gloomier picture by changing the boundaries
between the bands. In the winter of 1996-97, for instance, only one day had
鈥渧ery poor鈥� air quality. The new classification would have rated 14 days as
having 鈥渧ery high鈥� air pollution. For PM10s, 鈥渓ow鈥� air pollution now means a
level of less than 50 micrograms per cubic metre; 鈥渧ery high鈥� means more than
100 micrograms per cubic metre.
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Yet even the new system underestimates the health threat from particulate
pollution, say leading scientists in the field. The problem lies in the design
of the PM10 sensor favoured by the British government.
In traditional methods for measuring PM10s, air is sucked through a filter.
The level of particle pollution is then calculated from the increase in the
weight of the filter. But Britain鈥檚 automated PM10 monitoring system uses an
instrument called the tapered element oscillating microbalance (TEOM). It also
sucks air through a filter, but rather than weighing the filter directly, it
calculates changes in weight by recording changes in the frequency of vibrations
of a crystal incorporated into the device.
The two techniques can produce very different results. 鈥淚f you stand the
standard gravimetric instrument in Oxford Street side by side with the TEOM, you
will find a high level on the gravimetric instrument and a lower level on the
TEOM,鈥� says Steve Smith of King鈥檚 College, London.
In a paper in Clean Air (vol 27, p 70), Smith and his colleagues
show that at low levels of pollution鈥攁round 20 micrograms per cubic
metre鈥攖he instruments tend to produce similar readings. But as the smog
becomes thicker, the results diverge. When 100 micrograms per cubic metre was
recorded on a gravimetric instrument, the TEOM reading was 64 micrograms per
cubic metre. Researchers at the Harvard School of Public Health in Boston have
found similar discrepancies (Journal of the Air and Waste Management
Association, vol 47, p 682).
Smith accepts that the TEOM has advantages, notably its ability to make
automated readings every 15 minutes. 鈥淚 don鈥檛 want to knock the TEOM,鈥� he says.
However, he believes that the calibration with other methods should be
improved.
Part of the explanation for the difference, says Smith, is that the TEOM
operates at 50 掳C鈥攚hich is high enough to vaporise some particles.
However, John Bower, chief air quality consultant at AEA Technology in
Harwell, Oxfordshire, the company which operates the government鈥檚 air pollution
monitoring network, maintains that the TEOM produces 鈥渃onsistent and
high-quality results鈥�. He says that a study the AEA carried out for the European
Commission shows that the TEOM results are in 鈥減retty good agreement鈥� with other
techniques.
But Bower concedes that PM10s are not as easy to measure as gaseous
pollution, partly because they consist of a varying cocktail of different types
of particle. 鈥淲hat you measure depends on how you measure it,鈥� he says.