Given that prematurity and growth retardation are relatively easy to measure in a population, we are able to explore what substances in the environment may influence these outcomes. In general, we are unable to determine why individual infants may be born prematurely or grow insufficiently. What we can do, however, is examine patterns among groups of people to generate hypotheses about why the events occur.
Environmental exposures that are related to personal behaviors or occupations are sometimes the easiest to study. We therefore know that children born to women exposed to tobacco smoke during pregnancy are at higher risk for both prematurity and reduced fetal growth. Workplace exposures (for both fathers and mothers) to substances such as lead or welding fumes are also associated with adverse outcomes.
One method that researchers use to investigate environmental influences on prematurity and reduced fetal growth is to collect samples from the mother's surroundings (e.g. air samples) or from the mother herself (e.g. blood samples) and examine them for evidence of exposure. In this way, scientists have observed patterns between these conditions and exposure to:
- Particulate matter of the type arising from motor vehicle traffic (PM2.5)
- Polycyclic Aromatic Hydrocarbons, which arise from diesel fuel combustion
- Pesticides such as diazinon and propoxur, chlorpyrifos, and DDT
- Polychlorinated biphenyls (PCBs), although some studies have not seen this pattern
The most common approach used by scientists, however, is to infer exposures based on where people live during their pregnancy. The advantage of this approach is that maternal addresses are frequently recorded in birth records, so very large groups of people can be studied for very little cost. This kind of study has greater limitations, however, since people frequently move residences or spend substantial time away from home during pregnancy. Nevertheless, simple residential proximity to sources of pollution, such as hazardous waste landfills or traffic is consistently associated with prematurity and reduced fetal growth.
A related question--what particular substances in air pollution could be contributing to prematurity and reduced fetal growth--is much more difficult to answer. Pollutants nearly always occur in combinations, so it is rarely known whether the compound under study is responsible for the pattern or whether there is a separate substance responsible that may or may not have been measured. A study located in one city or country will involve different mixtures of pollutants from another, and the populations in question may not be vulnerable in the same way.
For all of these reasons, answers about specific pollutants are still a ways off. Judging by a large number of recent studies, the pollutants one could subjectively say are most frequently found to have positive associations with prematurity and reduced fetal growth include:
- Suspended particulate matter (PM)
- Carbon monoxide (CO)
- Sulfur dioxide (SO2)