Air pollution linked to low birth weight

Air pollution from road traffic in London is adversely affecting foetal growth and leading to low birth weights; that's the conclusion of a new study out this week from scientists at Imperial College. And what’s very worrying is that the affected mothers were exposed during their pregnancies to pollution levels that were only about half of what is currently set - under EU law - as the safe limit for human exposure. Chris Smith spoke to the leader of the study, Mireille Toledano...

Mireille - We took every every singleton live birth in London during 2006-2010, that’s a five year period, and that means we were looking at 540,000 births. We then estimated the residential exposure of every mother during her pregnancy to various different air pollutants, in particular small particulate air pollution which is mainly a result of vehicle emissions. We then looked at the link between the air pollutants and the birth weight of every baby, and we then saw that for every five microgramme per cubic metre increase in small particulate air pollution there was a 15% increased risk in a mother having a low birthweight baby at term.

Chris - Those are big numbers aren’t they? How did you firstly quantify the amount of pollution that each individual was being exposed to?

Mireille - Across London there are various different pollution monitors. Measurements from those monitors are then mapped and then we are able to map down to a 20 metre by 20 metre grid for our air pollution exposures across the whole of London.

Chris - Are there not lots of other factors that could be playing a role here? For instance, if you live in a deprived area where there is more traffic, you’re much more proximal to a road, you are therefore subject to much worse air. Could you not also be subject to much worse housing conditions in general, a poorer diet, and therefore a higher risk profile for many of the problems that include low birthweight?

Mireille - Yes, absolutely. It is very important to consider the other factors that do play an important role in a mother’s risk for low birthweight baby, and socioeconomic status is one of those. We did take into account deprivation; what we did was adjust for that at the area level, so we assigned to every mother based upon her residential address what her socioeconomic status was for that area. So, if it was a poor neighbourhood we would assign ‘poor neighbourhood,’ and if it was a richer neighbourhood we would have an affluent score for that.

It’s not perfect, but it does a very good job at trying to take into account factors like socioeconomic status, and things that are correlated to it like smoking we did not have individual level information on, but we did our best to take into account that kind of information as much as possible.

Chris - What about noise as well, because when one drives down a road one makes a lot of noise; when you live near a road you’re subject to a lot of noise? We know people who live near Heathrow Airport, on average, have higher blood pressure and a higher heart disease and stroke risk than people who don’t live near Heathrow and other busy airports, so could not noise just account for this?

Mireille - Yes, absolutely. That was a very important question and that’s something that hasn’t been addressed in any previous research study on this kind of scale before. This was the largest study in the world to look at that question as to whether the effects that we are seeing on low birth weight, are they really from the air pollution or could they be from the noise, or from both?

We actually found in our study that although traffic related noise could potentially have that impact, we did not see an independent effect of traffic related noise on low birthweight throughout London.

Chris - So what should the Secretary of State for Transport take away from your study - not that they’re going to read it - but having listened to this programme what message should they take away?

Mireille - I think the key message for policy makers is that the current legal limit set by the EU for small particulate air pollution of 25 microgrammes per cubic metre, the average pregnancy exposures to small particulate air pollution of the women in London is 14 microgrammes per cubic metre. In other words, our current air pollution levels for small particulate air pollution are actually much lower than the legal limits and, therefore, it’s absolutely clear that we have seen adverse health effects at these lower limits and, therefore, our current legal limits are not safe. They are not protecting our pregnant women and they’re not protecting their unborn babies and they must be reduced.