Exposure Assessment Challenges

A variety of approaches have been used to estimate exposures and to investigate the associations between environmental chemical exposures and preterm birth. A common approach has been to use employment or location of residence as a proxy for exposure, such as working with pesticides, proximity to a pollution source, or residence in a polluted locality. In those studies, exposure is typically estimated from measurement of the levels of contamination of common environmental media, such as drinking water sources and ambient air. However, such studies usually lack information about individual exposures and confounders, among other limitations.

Different approaches have been used to obtain information on individual levels of exposure. Perhaps the most conventional approach is to obtain self-reported data on exposures collected by survey or interview, but this method has the potential for reporting bias. Alternatively, measurements of the levels of chemical exposure have been used. These typically involve measurement of the levels of selected pollutants or their metabolites in body fluids or tissues, such as maternal blood, umbilical cord blood, and the placenta. However, the tissue or body fluid selected for toxicant level analysis has implications for the study, depending on the distribution of the toxicant in the body during pregnancy and the expected tissue or body fluid target for the action of the toxicant. Information on individual exposures provides potential advantages, such as controlling or adjusting for confounding variables and minimizing exposure misclassification.

The timing of the assessment may also influence the exposure measure. For example, if the toxicant concentration in the tissue changes over the course of gestation, the toxicant concen trations at the time of birth for preterm births may differ from those for term births simply as a function of gestational age. Similarly, if self-reporting or geographical-ecological assessments are used, the results may vary depending on whether the exposure is assessed for the last month of pregnancy, the entire pregnancy, or even for a period before the pregnancy. Co-exposures are common for environmental pollutants and pose another exposure assessment challenge. For example, cigarette smoke contains thousands of chemicals, many with known toxicity. Sulfur dioxide and particulates are typical co-pollutants of air, and many of the fat soluble toxicants are co-pollutants of food. Few studies have addressed potential impacts of co-pollutants in studies of preterm birth.

A major challenge is the assessment of cumulative exposures. Some pollutants are stored in the body, such as lead in bone or the pesticide DDT in fat. Metabolic changes of pregnancy cause increased metabolism of bone and fat tissues, such that toxicants are released from these sites to the blood and general circulation. Moreover, the impact of cumulative exposure may be important for preterm birth, but has been largely unexplored. However, cumulative exposure assessments face many challenges, in part because individuals are often unaware of exposure levels. Consequently, the development of biological markers for estimating cumulative exposure (e.g., k-x-ray fluorescence measurements of lead in bones) is needed for studies to assess impacts of cumulative exposures.

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