This brief was developed using Microsoft Copilot and edited by Charlotte Sutcliffe, Duke undergraduate research assistant; for full text and references see
Marcus, M. (2025). Testing above the limit: drinking water contamination and test scores. Retrieved from https://doi.org/10.1016/j.jpubeco.2025.105313
Background:
This study investigates the causal impact of drinking contaminated water - specifically violations of the Total Coliform Rule (TCR) - on student academic performance in North Carolina. Coliform bacteria, often containing fecal contamination, can cause gastrointestinal illness, which may impair students’ ability to concentrate and retain information. The research focuses on two types of violations: Acute (requiring immediate public notification) and Monthly (requiring notification within 30 days). Using detailed student-level test score data linked to residential and school addresses, the study isolates the effects of exposure to contaminated water during the school year and testing periods.
Findings:
Exposure to Monthly coliform violations during the school year - when public notification is often delayed - significantly reduces math test scores by approximately 0.038 standard deviations. This effect is comparable to a $822 reduction in per-student school spending, or a one-third standard deviation drop in teacher quality. The impact is not observed for Acute violations, likely due to timely public notification enabling avoidance behaviors such as switching to bottled water. The negative effect persists into the following academic year, suggesting long-term consequences for student learning. The study finds no significant impact on reading scores or behavioral outcomes like suspensions. Importantly, the effect is not driven by increased absences alone; rather, it likely reflects diminished comprehension and retention due to subclinical illness. The results are robust across multiple specifications, including controls for weather, air pollution, socioeconomic factors, and regional shocks. Placebo tests using future violations confirm the validity of the causal interpretation.
Takeaways:
This research provides the first causal evidence that drinking contaminated water - specifically when public notification is delayed - harms student academic performance. The findings underscore the broader educational and economic costs of environmental health hazards and highlight the importance of timely public communication in mitigating these effects. Policymakers should consider the educational spillovers of environmental regulations when conducting cost-benefit analyses. Ensuring rapid notification and improving water infrastructure could yield substantial benefits in student achievement and long-term earnings, particularly in disadvantaged communities disproportionately affected by water quality violations.