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Current exposure to pollution has greater health impact than former exposure, study shows

The health risks of exposure to pollution follow a similar pattern to those from exposure to tobacco smoke, a study has found. It also noted a marked acute rise in risk from living in a polluted area, which then tails off if someone moves to a place with cleaner air.1

The study, published in Thorax, also said that, while overall pollution levels seemed to be dropping in the United Kingdom, the pollution itself seemed to be more toxic now.

Researchers from Imperial College London used historical pollution monitoring data on black smoke and sulphur dioxide taken in 1971, 1981, and 1991, as well as PM10 levels (fine particulates of ≤10 micrometers in diameter) taken in 2001, to estimate the level of exposure of 367 658 people at those four specific times, according to where they were living at the time. The participants were members of the Longitudinal Study, a 1% sample of the England census, which enabled the researchers to track the participants’ health outcomes over time.

Pollution that people were exposed to in 1971 still affected their health 38 years later, the results showed. Every additional unit of 10 µg/m3 pollution that someone was exposed to in 1971 increased their mortality risk by 2% between 2002 and 2009.

However, more recent exposure had a far more marked impact on health: for every additional unit of 10 µg/m3 pollution that someone was exposed to in 2001, it increased their risk of mortality by 24% between 2002 and 2009.

Anna Hansell, lead author and assistant director of the Small Area Health Statistics Unit of the MRC-PHE Centre for Environment and Health at Imperial College London, said that the link between pollution and health followed a similar pattern to the effect of smoking.

“What we think is happening is that there is some waning over time with the effect size,” she said. “You can make some analogy with smoking. Smoking has an acute effect and it has a long term effect.

“We know if you smoke it increases your risk long term, but if you give up smoking there’s a benefit, which for heart disease is probably strongest over a couple of years, and then you have the longer term effects which appear over time.”

Hansell speculated that some tailing off of the effect might also be due to some people, who were more susceptible to the health effects of pollution, having died. Furthermore, the particulates in pollution had changed over the course of the study, she said, and it was likely that each unit of 10 µg/m3 pollution today is more deadly than in 1971.

“We can’t time travel back to the 1970s and look at the particles then, in the same way as the particles now, but we know we have a very complicated mix now,” said Hansell.

John Gulliver, senior lecturer at the MRC-PHE Centre for Environment and Health, said, “Levels of types of air pollution in the UK have reduced dramatically since the start of our study period, with levels of black smoke currently estimated to be around 20% of what they were in the 1970s.” As a result, in 1971 a 52 µg/m3 difference in black smoke levels was recorded between the most and least polluted areas, whereas in 2001 the difference in PM10 was just 6 µg/m3.

The source of pollution has also changed, Gulliver added. In 1971 it came predominantly from coal burning in homes and by industry and so was greatest in industrial areas, whereas transport is now the main source of pollution. And, surprisingly, only about half of this is from exhaust emissions.

“Brake and tyre wear and road abrasion increasingly make up a larger and larger proportion of particles from road transport—it is almost 50% now,” he said. “And it is set to increase because the focus is on exhaust emissions, not on tyres, brakes, and road abrasion.”

Levels of this type of pollutant are influenced by road surface, speed, and the weight of the vehicle, Gulliver said, such that heavier vehicles such as sport utility vehicles have the greatest impact.

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