Traffic-related Air Pollution and Hypertensive Disorders of Pregnancy
Traffic-related air pollution, or TRAP, is a type of air pollution that comes from the emissions of motor vehicles that result from fossil fuel combustion, and has been shown to be a major risk factor for cardiovascular disease, including hypertension. TRAP contributes significantly to outdoor air pollution, especially in urban settings. Children are especially sensitive to air pollution, and there is increasing evidence that exposure to TRAP may impact pregnancy outcomes and child development.
Hypertensive disorders of pregnancy complicate more than 10% of pregnancies worldwide and are a leading cause of maternal and fetal illness and death. These disorders are defined by hypertension (blood pressure > 140/90 mmHg), usually during the second half of pregnancy, and range in severity from hypertension alone to multi-organ dysfunction and seizures in the mother. According to the American College of Obstetricians and Gynecologists, mothers with hypertension during pregnancy are more likely to have a pre-term delivery. Their infants are at greater risk for low birthweight and a range of long-term health problems associated with pre-mature birth.
The project and protocol were developed to evaluate two emerging children’s health issues associated with TRAP exposure: hypertensive disorders of pregnancy and neurological disorders. An assessment of TRAP and developmental neurological disorders is currently under review by other organizations and therefore, the NTP evaluation focuses only on TRAP and hypersensitive disorders of pregnancy.
NTP conducted a systematic review of published research on hypertensive disorders in pregnant women and their possible link to TRAP. NTP evaluated the risk of hypertensive disorders by different pollutant measurements of TRAP, such as particulate matter, nitrogen oxides, carbon monoxide, and black and elemental carbon, along with parameters like traffic density and mother’s proximity to main roads.
What did the evaluation find?
Using its standard scale, NTP concluded that TRAP is a presumed hazard for hypertensive disorders in pregnant women. The scale ranges from the highest hazard rating of “known,” followed by “presumed,” then “suspected,” and finally, “not classifiable.”
Q: Why did NTP evaluate studies on TRAP, rather than on all air pollution?
A: Traffic-related air pollution is a significant contributor to ambient air pollution and is a major risk factor for cardiovascular disease, including hypertension. Over the past 10 years, a growing number of epidemiological studies have suggested that TRAP could be linked to hypertensive disorders during pregnancy. NTP conducted a systematic review to evaluate whether exposure to TRAP during pregnancy is associated with hypertensive disorders.
Q: Is hypertension a health risk for all women or just for pregnant women and their babies?
A: Although hypertension can affect women at all phases of life, hypertension in pregnancy presents unique challenges because of its potential long-term adverse effects on both mother and child. Mothers are at an increased risk for developing cardiovascular diseases, stroke, and type 2 diabetes later in life. Infants born to mothers with hypertension during pregnancy are at a higher risk for preterm delivery, low birthweight, and a range of prematurity-related neonatal diseases.
Q: What are hypertensive disorders of pregnancy?
A: Hypertensive disorders of pregnancy refer to a range of clinical conditions, all of which include high blood pressure during pregnancy. The American College of Obstetricians and Gynecologists classify hypertensive disorders of pregnancy into four types: (1) gestational hypertension, (2) preeclampsia/eclampsia, (3) chronic hypertension, and (4) chronic hypertension with superimposed preeclampsia. These disorders can be defined by differences in the timing of the onset of symptoms:
- Gestational hypertension is high blood pressure in the second half of pregnancy.
- Preeclampsia is high blood pressure with protein in the urine or impaired liver and kidney function in the second half of pregnancy. If preeclampsia worsens and causes seizures, it becomes eclampsia – a serious condition for mother and child with the potential to be fatal.
- Chronic hypertension is high blood pressure before pregnancy or early in pregnancy and continues throughout pregnancy.
- Chronic hypertension with superimposed preeclampsia is high blood pressure before pregnancy or early in pregnancy that continues throughout pregnancy and protein in the urine or impaired liver or kidney function.
Q: How did NTP arrive at its evaluation of presumed hazard?
A: The hazard classification of “presumed” is based on evaluations of individual components of TRAP measures and consideration was given to the likelihood of being exposed to multiple TRAP components at the same time:
- Particulate matter (PM2.5): Exposure to traffic-related PM2.5 is associated with development of hypertensive disorders in pregnant women on the basis of evidence that the likelihood of developing preeclampsia increases by 51% for every 10 µg/m3 increase in PM2.5 concentration when pregnant women are exposed during the entire pregnancy.
- Nitrogen oxides (NOX): Exposure to traffic-related NOX increases the likelihood of developing hypertensive disorders of pregnancy when women are exposed during the entire pregnancy period.
- Traffic density and proximity to main roads: Residing in regions of high-traffic density or in close proximity to main roads may be associated with hypertensive disorders in pregnant women on the basis of evidence that pregnant women who live within a quarter mile of a major roadway (e.g., 10,000 vehicles or more per day) or in high-traffic density regions may be at an increased risk for developing hypertensive disorders of pregnancy.
NTP scientists performed a comprehensive literature search and reviewed hundreds of studies with potentially relevant data. Overall, they evaluated 18 human observational studies and one animal study that specifically addressed hypertension during pregnancy and TRAP. Usually, experimental animal data add certainty in the conclusions; unfortunately, the limited number of animal studies that assessed the effects of environmental exposures during pregnancy is a research gap.
|Jul 13, 2012||Federal Register notice requesting information on nomination|
|Apr 16, 2014||NTP Evaluation Concept|
|Apr 16, 2014||Meeting Presentation: NTP Evaluation Concept - Pregnancy Outcomes Associated With Traffic-Related Air Pollution|
|Jun 01, 2016||NTP Protocol|
|Jun 24, 2019||Federal Register notice requesting public comments on draft monograph (PDF HTML)|
|Aug 01, 2019||Literature Search Results|
|Dec 18, 2019||NTP Protocol - Revised|
|Dec 18, 2019||NTP Monograph - Final (Abstract)
Preferred Citation: National Toxicology Program (NTP). 2019. NTP monograph on the systematic review of traffic-related air pollution and hypertensive disorders of pregnancy. Research Triangle Park, NC: National Toxicology Program. NTP Monograph 07. https://doi.org/10.22427/NTP-MGRAPH-7
Meetings & Events
|Apr 16, 2014||NTP Board of Scientific Counselors Meeting||Board of Scientific Counselors||
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