Investigating the Aliso Canyon gas blowout disaster and adverse birth outcomes: A quasiexperimental approach
Did the massive Aliso Canyon gas leak disaster cause pregnant women living nearby to have worse pregnancy outcomes - specifically, were their babies more likely to be born underweight or premature?
College of Health researcher(s)
College unit(s)
Abstract
On 23 October 2015, operators at the Aliso Canyon Natural Gas Storage field in Northern Los Angeles reported an uncontrolled natural gas leak. The blowout persisted for 112 days, releasing ~109,000 metric tons of methane into the atmosphere. Elevated air toxics and fine particle pollutant levels were also measured in nearby communities. We used California’s birth records and a quasiexperimental design to assess whether pregnant women living in affected communities during the disaster experienced more adverse birth outcomes than expected. Overall, the prevalence of low birthweight and term low birthweight were 45 to 100% higher than expected among women living in the affected communities whose late pregnancy overlapped with the blowout. The strongest effects were observed among women living directly south and southwest of the facility. Furthermore, we observed a dose-response effect, where the odds of low birthweight were highest among women living closest to the well and attenuated out.
The Aliso Canyon Gas Leak and Birth Outcomes: A Research FAQ
This document addresses key questions about the 2015 Aliso Canyon gas blowout, the largest natural gas leak in U.S. history, and its potential effects on pregnancies in nearby communities. The answers provided are based on the findings of a major scientific study published in the peer-reviewed journal Science Advances, which investigated the disaster's impact on adverse birth outcomes.
What was the Aliso Canyon gas disaster?
The Aliso Canyon gas disaster was an uncontrolled natural gas leak from the Aliso Canyon Natural Gas Storage field in northern Los Angeles. The blowout began on October 23, 2015, and persisted for 112 days before it was permanently sealed.
This event stands as the largest anthropogenic release of natural gas from a single point source ever recorded in the United States. Key facts about the disaster include:
- Massive Emissions: Approximately 109,000 metric tons of methane were released into the atmosphere.
- Unprecedented Rate: At its peak, the leak's emission rate was comparable to that of the entire U.S. oil and gas industry or the daily emissions of 4.5 million cars.
- Community Impact: The disaster had profound effects on the local population, leading to the temporary evacuation of nearly 10,000 people and the closure of two local elementary schools for the remainder of the school year due to health concerns.
In total, hundreds of thousands of people lived downwind (≤10 km) of the leaking well during the blowout.
What chemicals were people exposed to during the leak?
During the 112-day blowout, residents in surrounding communities were exposed to a complex mixture of air pollutants. Monitoring during and after the event identified several key substances:
- Natural Gas Components: The primary pollutants were the main constituents of natural gas, methane and ethane.
- Odorants: Chemicals added to natural gas to give it a detectable smell, specifically mercaptans, were also a major part of the exposure profile.
- Hazardous Air Pollutants (HAPs): Air monitoring detected elevated levels of several HAPs that were co-emitted with the methane. These included volatile organic compounds such as benzene, toluene, ethylbenzene, and xylene (BTEX compounds), as well as n-hexane and styrene.
- Metals in Household Dust: An unusual cluster of metals, including barium and manganese, was found in dust samples collected from inside homes after the leak was sealed. These metals were identified as part of the muds injected into the well in attempts to stop the leak and were later described by public health analysts as a "fingerprint" of intrusion into the homes from the blowout.
Did the Aliso Canyon gas leak affect pregnancies?
Yes, the study found a significant link between exposure to the Aliso Canyon gas blowout and specific adverse birth outcomes, particularly related to birthweight. However, the evidence for other outcomes was not conclusive.
- Low Birthweight: The study's primary finding was that for pregnant women living in the affected area whose late pregnancy (third trimester) overlapped with the disaster, the prevalence of having a baby with low birthweight or term low birthweight was 50% to 100% higher than expected.
- Dose-Response Effect: The risk was highest for mothers living closest to the leaking well. The odds of low birthweight were greatest for those residing within 5 kilometers (about 3 miles) and decreased with distance.
- Preterm Births: Researchers did not find a conclusive link between the disaster and an increase in preterm births. The data for this outcome did not meet a key statistical assumption (the "parallel trends assumption") required by the study's design, meaning a causal link could not be established.
- Infant Sex Ratio: The study noted a secondary finding of a higher-than-expected proportion of male infants born to mothers who were in their early pregnancy during the blowout. This shift in the male-to-female ratio at birth may suggest an increase in exposure-related fetal loss.
How did researchers determine the gas leak's impact on birth outcomes?
Researchers used a robust statistical method known as a quasiexperimental "difference-in-difference" design. This approach is used to estimate the causal effect of a specific event when a randomized controlled trial is not possible.
In simple terms, the methodology involved:
- Defining Groups: Researchers identified an "exposed" group (pregnant women living in the affected communities near Aliso Canyon) and multiple "unexposed" control groups (pregnant women in other communities across California).
- Analyzing Timelines: They analyzed California's official birth records for specific time periods: several years before the blowout, the period during the blowout, and several years after it was sealed.
- Making a Comparison: By comparing the rates of adverse birth outcomes in the exposed group to the rates in the control groups over time, they could isolate the disaster's impact. The core of this method is the assumption that, had the blowout not occurred, the trends in birth outcomes in the affected area would have been similar to those in the unexposed areas. This method allowed researchers to mathematically subtract the "normal" trends in birth outcomes seen elsewhere, thereby isolating the excess number of adverse outcomes that could be attributed specifically to the Aliso Canyon disaster.
What was the most significant health finding of this study?
The single most significant finding was the direct statistical link between exposure to the disaster during late pregnancy and a substantially higher risk of giving birth to a baby with low birthweight.
The prevalence of low birthweight was up to twofold higher than expected for mothers living in the affected communities. This conclusion was strengthened by several key pieces of evidence:
- Geographic Correlation: The effects were strongest in the areas located directly south and southwest of the leaking well. This geographic pattern aligns with prevailing wind directions and air quality measurements taken during the disaster, which showed the highest concentrations of pollutants in those same communities. This geographic evidence is particularly compelling because while residents' self-reported stress levels were consistent across the entire area, the proportion of households reporting gas-like odors was highest in the same south and southwest areas where the birth outcome effects were strongest. This strongly suggests that direct air pollution exposure, not just community-wide stress, influenced the health outcomes.
- Public Health Significance: This finding is critical because low birthweight is recognized as a major risk factor contributing to the global burden of disease. It is associated with health challenges that can exert continued effects throughout a person's life, from infancy into adulthood.
Conclusion
This research provides strong scientific evidence that the 2015 Aliso Canyon gas blowout had measurable and serious health consequences for a particularly vulnerable population: pregnant women and their unborn children. The study's findings on increased rates of low birthweight serve as a critical case study, underscoring the significant public health risks associated with large-scale industrial pollution incidents, especially those occurring near densely populated residential areas.