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.