Something in the air
There’s a story “Pollution can cause lung problems in unborn baby – research” in the Herald, which I’m not convinced by, but the reasons are relatively subtle.
The researchers compared levels of traffic-related air pollution exposure for different pregnant women, and looked at the lung function of the children at age four and a half (press release). The story gets the name of the main pollutant (nitrogen dioxide) wrong in two different ways, but is otherwise a good summary. It’s all correlation, but weaker associations than this are fairly reliably estimated for short-term exposures to air pollution. Long-term exposure is different, and that’s what’s interesting.
Studies of short-term effects of air pollution compare the number of people dying or going to hospital on days when pollution is high to the number on days where pollution is low. That is, the comparisons of pollution are for the same people and for the same air pollution monitors. There are a fairly limited selection of other factors that could explain the association — the main ones being related to weather.
Studies of longer-term effects compare people with high exposure to pollution and people with low exposure to pollution. Actually, they don’t quite do that, because air pollution monitoring is expensive in labour and equipment. They compare people with high estimated exposure and low estimated exposure. Since we’re comparing different people, any factor that affects health and also affects where people live could cause a bias, and it’s very well established that poorer people tend to get exposed to more pollution, at least in cities. Also, since we’re comparing different air pollution monitors, there can be biases from how representative the monitors are of the local area.
These problems mean that it’s much harder to be confident about effects of longer-term air pollution exposure, even though these effects are likely to be bigger than the short-term ones. Fortunately, we don’t need to be sure of these effects in setting public policy. The main source of the pollution is traffic, and there are other independent reasons why we want to have fewer cars burning less fuel.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
Thanks for the interesting response to the article in the Herald. There is already quite a lot of evidence that NO2 has a detrimental effect on lung development, so it is possible that what the researchers found was real. I’ve seen the study mentioned in more serious scientific news than the Herald. It is true though, that like most air quality research it relies on a lot of assumptions, generalisations and correlations.
However, what I wanted to respond to was your final assertion: “The main source of the pollution is traffic” as its not correct. While I certainly do not want to downplay the health effects of vehicle emissions, vehicles are not, except in a few very specific places such right as beside busy urban roads, the major source of pollution in New Zealand, most of the time.
Unfortunately, you don’t define ‘pollution’ (did you mean just NO2, which was one of the two pollutants studied, or did you mean all pollutants?) and it’s not clear from the context if you meant traffic was the main source of pollution globally, or just here in NZ? Assuming you meant just in NZ, the 2012 Health and Air Pollution in NZ report (www.hapinz.org.nz) is the only major study on the causes and effects of pollution in NZ. It used exposure to fine particulates (PM10) as the proxy for ‘pollution’. It identified domestic fires as having the single largest effect of the anthropogenic sources of air pollution and by quite some margin. Using social costs as the unit of measurement it said 56% of the effects from anthropogenic emissions were from home heating while only 22% were from vehicles. So if you are looking at all of NZ and all pollutants the assertion that vehicles are the ‘main source’ of pollution is not correct.
The HAPINZ summary report does go on to say: Domestic fires dominate the health impacts associated with anthropogenic air pollution in every location across New Zealand, except the Auckland region (most particularly in the TLA of Auckland City where motor vehicle health impacts are nearly twice those of domestic fires). However, not being able to robustly assess NO2 exposure means that the results of this update most likely under-estimate the health impacts of motor vehicle-related air pollution.
So, although the HAPINZ report suggests NO2 might be important in NZ, we do not have enough data to show that the main source for NO2 is transport.
10 years ago
That’s a good set of questions.
For NOx and benzene, I think traffic is the main source in NZ, as in the published paper. For particulate pollution it depends on your metric — wood smoke is, as you say, higher across NZ and especially in Canterbury and Otago, but on a population basis I’d guess traffic is still dominant. I don’t have really good data, though.
There’s also moderately good evidence from North America that traffic pollution is more harmful than wood smoke. When I was in Seattle this was a big issue: our particulate air pollution had a lot of wood smoke and we didn’t see the cardiovascular effects that people saw on the East Coast or in the industrial midwest, and neither did our colleagues in Vancouver. It looked as though wood smoke had substantial effects on asthma, but less impact on deaths.
So, in answer to the questions. The research was about NOx and benzene, which really are dominated by traffic in the areas studied. The basic issue is still present, though. Acute air pollution studies are within-person, within-monitor, and are relatively resistant to confounding. Studies like this one are between-person, between-monitor, and are much more susceptible to confouding.
The susceptibility to confounding doesn’t mean the results are wrong. It does mean that we can’t be as confident about them.
10 years ago