The Birth-Weight Pollution Paradox

Maxim Massenkoff asks a very good question. If pollution reduces birth weight as much as the micro studies on pollution suggest, why aren’t birth weights very low in very polluted cities and countries? Figure 1, for example, shows birth weights in a variety of highly polluted world cities. The yellow dashed and blue lines show “predicted” birth weights extrapolated from the well-known Alexander and Schwandt “Volkswagen study” which looked at the effects of increased pollution in the United States. Despite the fact that every one of the highly-polluted cities is much more polluted than the most polluted US city, birth weight is not tremendously lower in these cities. Indeed, there is no obvious correlation between birth weight and pollution at all.

Similarly, US cities were more polluted in the past but were birth weights lower in the past? Figure 2 shows a number of US cities which were two to three times more polluted in 1972 (right side of diagram) than 2002 (left side of diagram). Yet, birth weights do not appear lower in the more polluted past and certainly do not follow the extrapolated birth weight-pollution predictions from the micro literature.

Massenkoff looks at a variety of possible explanations. One possibility, for example, is culling. Perhaps in highly polluted areas there are more miscarriages, still births or difficulty conceiving with the result that the observed sample of births is highly selected. There is some evidence that pollution increases miscarriages and stillbirths but these tend to be correlated with lower birth weight–a scarring effect rather than a culling effect. In addition, the effect of pollution on miscarriages and stillbirths also appears to be bigger on a micro level than on a macro level. That is, these rates aren’t massively higher in high pollution countries.

Another possibility is that pollution isn’t that bad and, in particular, not as bad as I have suggested. As a good Bayesian, I update, but for reasons I have given here, it’s not justifiable to update very much.

I assume, as I always do, that there are some overestimates in the micro literature for the usual reasons. But, more fundamentally, my best guess for the birth-weight pollution paradox is that weight is one of the easiest margins on which the body can adapt and compensate. Even in poor countries there are plenty of calories to go around and so it’s relatively easy for the body to adjust to higher pollution, on this margin. Indeed, weight is known as a variable that creates paradoxes!

Micro studies on weight and exercise, for example, show that exercise reduces weight. But looking across countries, societies, and time we don’t see big effects–indeed, calorie expenditure doesn’t vary much with exercise! Importantly, notice that the micro-estimates are correct. If you increase physical activity for the next 3 months, holding all else equal (which is possible for 3 months), you will lose weight. However, the micro estimates are difficult to extrapolate to permanent, long-run changes because there are complex, adaptive mechanisms governing weight, calorie consumption and energy expenditure.

The exercise paradox doesn’t mean that exercise isn’t good for you–the evidence on the benefits of exercise is extensive and credible. In the same way the birth-weight pollution paradox doesn’t mean that pollution isn’t harmful–the evidence on the costs of pollution is extensive and credible. In particular, it’s going to be much harder to adapt to pollution for heart disease, cancer, life expectancy and IQ than for weight. 

I am always impressed with papers that present big, obviously-true facts that most people have simply missed. Massenkoff is becoming a leader in this field.