Janice Nolen, national policy adviser at the American Lung Association, says that rather than dealing with the harmful effects of air pollution, the focus should be on tackling the problem at source.

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The Department of Environment, Food and Rural Affairs said even though air quality in the UK is "generally good, more needs to be done, especially in the cities, to reduce the harmful effects of air pollution".

BBC: Traffic fumes linked to lower birth weight

We can and should control surface pollution as that effects air quality yet there is a huge difference between controlling air quality as that remains constant in cold or warm environments while temperatures constantly fluctuate from day to night, month to month and year to year, in the Western isles of Europe we are having a particularly cool and wet summer.

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Instead they were studying monsoon effects in Mexico and air pollution in New Hampshire.

FORBES: Fact and Comment

The changes could not be explained by other factors - such as mother's age and socioeconomic status or population effects such as changes in air pollution and influenza epidemics.

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They warn that there could be "large" implications because air pollution is "common and may have lasting neurological effects".

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