Methodological Issues in Studies of Air Pollution and Perinatal Outcomes

A workshop to discuss methodological problems affecting perinatal environmental epidemiological studies.

September 5, 2007
Sheraton Hotel, Mexico City, Mexico

Workshop Rationale

In the past decade numerous studies have been conducted to investigate associations between air pollution and perinatal health outcomes, including low birth weight, preterm delivery, and infant mortality. These studies are important links to setting air quality standards that generally have not considered perinatal outcomes. Recent syntheses of these studies have concluded that there is likely an adverse effect of air pollution on perinatal outcomes and that the evidence for this adverse effect is strongest for respiratory-related infant mortality outcomes. Although there have been several studies published examining low birthweight and birthweight, the evidence is inconsistent but not inconsequential. Fewer studies have examined preterm delivery.

Inconsistencies in the findings arise from many aspects of the study designs and available data. Most studies have examined particulate matter, but the measured component varies from total suspended particulates (TSP) in earlier studies to particulate matter measured less than 10 µg/m3 (PM10) and finer particles, PM2.5 in later studies. Other pollutants examined in one or more studies include carbon monoxide (CO), ozone (O3), sulfur dioxide (SO2), and the nitrates (NOx). Some studies examine trimester of exposure, with some finding more potent effects earlier in pregnancy and others identifying later exposures as more harmful. Examining simultaneous exposure to multiple pollutants has been hindered by data availability; published studies are not comparable because of different sets of pollutants. Regional and demographic differences in study populations also may contribute to the disparate findings; regional differences in pollution components and mixtures as well as underlying population differences in access to care and susceptibility could lead to variations in study conclusions.

This workshop discussed these and other methodological problems affecting perinatal environmental epidemiological studies. In particular, this workshop focused on crafting a research agenda that will aid in the synthesis of future studies. In addition to the questions below, three cross cutting themes were considered in each of the sessions:

Given data limitations, what can do use with existing data and methodologies to address some of the uncertainties?
Using data from different parts of the world, can we identify collaborative analyses to evaluate whether differences in published findings arise from differences in analytic strategies or intrinsic/geographic differences?
Are there vulnerable populations that can be identified – and should regulatory agencies and public health educators target interventions toward specific groups?

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Workshop Format

The workshop was divided into four broad topic areas, with introduction and conclusion sessions at the beginning and ending. The beginning session introduced the topics generally and defined the goals of the workshop. Each of the four topic sessions began with a summary of the available research related to the topic, and was followed by a discussion of the strengths and weaknesses of the current approaches and suggestions for future work and synthesis.

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Workshop Questions

Confounding and Effect Modification: maternal and demographic factors

What is the role of demographic and maternal factors on the association between pollution and birth outcomes?
Are there susceptible populations with increased vulnerability?
What are the barriers to identifying susceptible populations?
What are the key opportunities and next steps in study design that would help evaluate the intersection between air pollution and demographic variations in adverse pregnancy outcomes.

Spatial variation

What are the best methods for exposure assessment and evaluation for varying types of geographically defined data?
What are the barriers to examining geographic variation?
What are some of the key opportunities or study designs that would help evaluate regional and geographical variation?
Is there evidence that effects of pollutants on birth outcomes vary geographically?

Multiple pollutants

What are the best methods for exposure assessment and evaluation for multiple pollutants?
What are the barriers to examining multiple pollutants?
What are some of the key opportunities or study designs and assignment methodologies that would help evaluate role of exposure to multiple pollutants?

Temporal variation

How can we disentangle variation in exposure over time and the impact of exposure on particular times during pregnancy?

What is the role of seasonal variation in exposure on pregnancy outcome?
How can we identify particular times during pregnancy that are most vulnerable to exposure?

What are the best methods for exposure assessment and evaluation for windows of exposure?
What are the barriers to examining windows of exposure?
What are some of the key opportunities or study designs that would help evaluate windows of susceptibility?

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