If you’ve ever driven along I-5 through the California Central Valley, you likely detected an acrid stench that permeates the air. The noxious odor comes from Concentrated Animal Feeding Operations (CAFOs), industrial-scale meat and dairy production facilities that confine large numbers of animals.
The manure waste generated by these animals – thousands to millions of tons of manure, depending on the facility – emits air pollutants like ammonia, hydrogen sulfide, volatile organic compounds, and particulate matter. Waste can also permeate through soil and leach into nearby waterways.
The odor is hard to ignore, but for the people who live near CAFOs this isn’t just about a nuisance smell: it’s about their health.
An understudied threat during pregnancy
As a reproductive epidemiologist, I particularly worry about potential health effects of such exposures during pregnancy. We know that exposure to air pollution during pregnancy increases the risk of complications such as preterm birth and stillbirth, and CAFOs emit many of the same pollutants scientists hypothesize contribute to adverse pregnancy outcomes when emitted from other sources.
A limited number of studies have shown that high CAFO densities increase risk of poor birth outcomes, including infant mortality and low birthweight. But none of these studies were performed in California, a state with an exceptionally large number of CAFOs, many of which are located near communities and homes.
A data breakthrough
Until recently, studying CAFOs and health outcomes was nearly impossible, because California’s CAFO permit records have substantial gaps and location errors due to limited regulatory requirements. That changed this year when Stanford RegLab researchers published the first near census of California CAFOs, detecting 2,121 facilities using satellite imagery, computer vision, and extensive validation by human reviewers. The dataset captures ~98% of all facilities in the state, including 222 facilities absent from state records.
Our results: living near CAFOs and preterm birth risk
With the newly available CAFO location data, my colleagues and I conducted one of the first studies of birth outcomes and CAFO exposure in California, using records from 800,000 first-time mothers from 2007 through 2011. Birth records are an invaluable source of health information, providing comprehensive, detailed information on nearly all births that occurred in the State.
We examined whether living near CAFOs is associated with higher hazard of spontaneous preterm birth, which is defined as delivery before 37 completed weeks of gestation that occurs without medical intervention. Preterm birth is a serious adverse birth outcome and a major driver of neonatal mortality, long-term morbidity, and healthcare costs.
We also specifically assessed whether wind-direction affects associations, as women living down-wind of CAFOs might experience greater risk because of higher air pollution burden.
Our study reveals several important findings:
1. Proximity to CAFOs matters
Living closer to CAFOs is associated with small increases in hazard of preterm birth. These risks are greatest for more severe forms of preterm birth (born at earlier weeks of gestation) and extend to roughly 25 kilometers from a facility on average. Births with at least one CAFO within ~25 km have 16% higher odds of very preterm birth (birth between 20 and 31 weeks) and 8% higher odds of moderate preterm birth (32-36 weeks) compared with births with no CAFO exposure.
2. Wind direction matters too
Mothers who spend more time downwind of facilities experience higher preterm birth risk compared to those who are not downwind, suggesting that windborne pollution may be an important adverse exposure pathway.
3. Burdens aren’t shared equally
Mothers experiencing the greatest burden of CAFO exposure are more likely to have lower maternal educational attainment, younger age, higher Medi-Cal usage, and higher poverty levels.
Why this matters: importance for public health and future work
Our study adds to a small but growing body of evidence that living near CAFOs may have harmful effects for pregnant women, and is the first to link CAFO exposure to preterm birth risk in California. For any individual mother the added risk of CAFO exposure may be small, but compounded across thousands of births, these risks can translate into substantial numbers of additional babies born prematurely.
Nearly 40% of the mothers in this study – over 320,000 women – lived within 25km of at least one CAFO during pregnancy. Based on this study’s findings, we estimate that this level of exposure corresponds to approximately 460 additional very preterm births and 1,100 additional moderate preterm births.[1]
As an observational study, this work cannot establish that CAFO exposure directly causes these outcomes, but it contributes to growing understanding of the complex environmental factors associated with preterm birth. We hope that this study serves as a starting point for deeper exploration of CAFOs and health risks and for improved monitoring of CAFO locations and pollution emissions in California.
[1] These values are estimated by applying the odds ratios reported in the study to the baseline rates of very and moderate preterm birth.
Authors: Saskia Comess, Joshua Warren, Gary Shaw, Daniel Ho
About the author
Saskia Comess, PhD received her PhD from the Stanford Doerr School of Sustainability and is currently a postdoctoral scholar at the Stanford School of Medicine, where she focuses on biostatistical methods for observational clinical data. She is an affiliate member of the EaRTH Center.
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