A FRESH analysis of a classic pollution study has vindicated its conclusion
that city-dwellers in Europe and the US are dying young because of microscopic
particles in the air.
Most of the concern about particulate pollution began in 1993 with the
publication of the Harvard “Six Cities” study, which identified particles with a
diameter of less than 10 micrometres (PM10) as a threat to public health. A team
of researchers at the Harvard School of Public Health in Boston, led by Douglas
Dockery, compared death rates and pollution levels in six American cities by
following more than 8000 adults for up to 16 years.
They found that the death rates increased in almost direct proportion to the
level of particulate pollution. People living in the most polluted
city—Steubenville, Ohio—had a 26 per cent risk of dying young
compared with residents of the cleanest city, which was Portage, Wisconsin (
New England Journal of Medicine, vol 329, p 1753).
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A larger study by the American Cancer Society in 1995 tested these findings
by following 550 000 adults over seven years. Once again, there appeared to be a
strong link between death rates and particulate pollution. Critics of these
studies argued, however, that other differences between the cities—such as
poverty—might be responsible for the different death rates.
So the Health Effects Institute, an independent research organisation in
Cambridge, Massachusetts, funded jointly by industry and the US government’s
Environmental Protection Agency, spent three years re-analysing the data and
testing dozens of different explanations for the results. They controlled for
factors such as education, ethnicity, income levels and the availability of
health care, as well as differences in other pollutants, temperature and
humidity.
But the re-analysis broadly confirmed the original conclusion. “For the most
part, the inclusion of these additional [factors] did not alter the
association,” says team leader Daniel Krewski of the University of Ottawa. “We
were very surprised and relieved, actually,” says Dockery. Adrian Pope of the
Brigham Young University in Provo, Utah, who led the American Cancer Society
study, hopes the results will end the controversy.
The HEI study suggests that tiny particles with a diameter of less than 2.5
micrometres, or PM2.5, are more dangerous than PM10. Most of the PM2.5 fraction
is caused by by-products of combustion, which may contain more carcinogens.
Currently the US sets air-quality standards for both PM10 and PM2.5. Europe
only has a standard for PM10, but the European Commission is due to review its
particle pollution standards. Roy Harrison of the University of Birmingham, who
advises the British government on particulate air pollution, says separate
monitoring is unnecessary because, in Britain at least, PM2.5 levels rise and
fall with PM10 levels.
But Tim Brown of Britain’s National Society for Clean Air says researchers
need to know more about how particle composition—and not just
size—affects health. Brown asks: “Are all particles equally dangerous?”
