The technical foundation of the dispute lies in two biomarkers. Ethylene is a simple gaseous molecule that humans both produce internally and inhale from the environment, including from combustion sources and ripening fruit. Inside the body, a fraction of inhaled ethylene is converted by cytochrome P450 enzymes into ethylene oxide, a more reactive epoxide that can bind to hemoglobin and DNA. The specific biomarker used to track this chemistry is N-(2-hydroxyethyl)valine, abbreviated HEV, an adduct formed when ethylene oxide reacts with the N-terminal valine residue of hemoglobin proteins. Because red blood cells circulate for roughly four months, HEV provides an integrated record of ethylene oxide exposure over time. Lin and colleagues compiled a database of breath ethylene measurements and, using physiologically based pharmacokinetic modeling, sought to estimate how much endogenous ethylene oxide production contributes to total HEV burdens in the general population.

What Kirman and Bus highlight in their correspondence is a fundamental analytical problem embedded in that database. Breath ethylene concentrations reported across the literature are extraordinarily variable, and the variability appears to track with the measurement technology rather than with genuine differences between study populations. Studies employing laser-based photoacoustic spectrometry, a technique that detects gas absorption of laser light, reported mean ethylene concentrations of roughly 61 parts per billion with a standard deviation of 130 ppb, an enormous spread. By contrast, studies relying on gas chromatography reported a mean of approximately 20 ppb with a standard deviation of 16 ppb, far tighter and consistently lower. Even within the subset of studies that Lin and colleagues designated as high confidence, mean ethylene levels hovered around just 0.5 ppb, orders of magnitude below the values coming from the laser-based literature.

This is not a new concern, and Kirman and Bus invoke a striking historical precedent. More than two decades ago, Berkelmans and colleagues, working with online laser photoacoustic detection, observed that endogenous ethylene production rates derived from laser-based studies were significantly lower than published values based on gas chromatography, an inconsistency they attributed to fundamental differences between the two analytical approaches. When Berkelmans’s team attempted to fit a physiologically based pharmacokinetic model to their laser-derived data, they found they had to modify measured physiological and biochemical parameters to make the model work, indicating that the laser measurements were inconsistent with the well-established understanding of ethylene toxicokinetics built on gas chromatography. Kirman and Bus argue that this methodological fault line has not been resolved, and that Lin and colleagues’ database largely inherits the problem rather than correcting it.

The PBPK modeling itself is the second pillar of the critique. Physiologically based pharmacokinetic models are mathematical descriptions of how a chemical moves through the body, incorporating blood flow, tissue partitioning, metabolic rates, and ventilation. The model used by Lin and colleagues traces back to the foundational work of Filser and Klein, who developed a toxicokinetic model for inhaled ethylene and ethylene oxide across mouse, rat, and human species, and to earlier work by Csanády and colleagues, who modeled the formation of 2-hydroxyethyl adducts with hemoglobin and DNA from both exogenous and endogenous sources. Running the model with the breath ethylene data, Lin’s team concluded that endogenous production pathways account for less than 20 percent of total HEV in nonsmokers, leaving more than 80 percent unexplained. Kirman and Bus point out that this large unexplained fraction is itself a signal that something in the exposure reconstruction may be missing, whether it be analytical bias in breath measurements, unrecognized internal sources, or contributions from pathways not captured by the model.

Exogenous exposure, meaning ethylene and ethylene oxide inhaled from outside sources such as ambient and indoor air, is the other candidate explanation for the unaccounted adduct burden. Here, Kirman and Bus note that available air monitoring data, including large-scale studies such as Health Canada’s Windsor Exposure Assessment Study, indicate that environmental contributions to total ethylene oxide body burden are expected to be small for the general population. This conclusion aligns with their own prior publications, including a 2021 comprehensive review characterizing total ethylene oxide exposure from endogenous and exogenous pathways and a 2025 assessment of background exposures in the United States that questioned theoretical health risks for populations living near industrial sources. The implication is that the apparent gap between modeled endogenous production and measured HEV cannot simply be closed by invoking ambient air pollution, contrary to some popular narratives about ethylene oxide risk.

The broader context makes this technical disagreement consequential beyond the laboratory. Ethylene oxide is used industrially to sterilize roughly half of all medical devices in the United States, and the Environmental Protection Agency’s recent regulatory actions targeting sterilization facilities have relied on risk estimates derived from inhalation unit risk values. If a substantial fraction of the population’s hemoglobin adduct burden derives from endogenous biology rather than industrial emissions, then the margin between background internal exposure and levels associated with elevated cancer risk is narrower than many assume, and any risk assessment that fails to account for endogenous background risks mischaracterizing the true incremental danger of industrial emissions. Kirman and Bus have argued elsewhere that recognizing endogenous ethylene oxide formation is essential for a coherent risk management framework, since regulatory limits that ignore the body’s own production of the chemical cannot meaningfully protect public health in proportion to actual incremental exposure.

They also point to emerging molecular dosimetry work that helps interpret HEV data. Recent studies by Liu and colleagues on hemoglobin adduct formation in mice exposed to ethylene oxide, and Lin’s own companion work integrating PBPK modeling with tobacco biomarkers to interpret HEV levels in the U.S. population, demonstrate that smoking contributes substantially to measured adduct burdens, since tobacco smoke contains both ethylene and ethylene oxide directly. In nonsmokers, however, the origin of the remaining adduct burden remains contested. Classical biomonitoring studies from the 1990s and 2000s, including molecular dosimetry work by Walker, Fennell, Upton, and Swenberg in rodents exposed to ethylene oxide, established the dose-response framework that still underpins modern interpretation, and those studies consistently emphasized the importance of distinguishing background from incremental exposure when translating adduct measurements into cancer risk.

Neither the correspondence nor the original analysis settles the question of how much endogenous metabolism contributes to human ethylene oxide body burdens, but the exchange underscores a lesson that resonates across exposure science: the quality of a risk assessment is bounded by the quality of the exposure data fed into it. When two analytical technologies applied to the same biological matrix differ by more than an order of magnitude, as laser-based photoacoustic spectrometry and gas chromatography evidently do for breath ethylene, downstream models, no matter how sophisticated, inherit that uncertainty. Kirman and Bus contend that progress requires reconciling the methodological discrepancy, validating breath ethylene measurements against gas-chromatographic benchmarks, and ensuring that PBPK models are fit to data consistent with established toxicokinetics. Until that reconciliation occurs, estimates of how much ethylene oxide in the average person’s blood comes from industry versus biology will remain contested, and the regulatory debate over one of the world’s most widely used sterilants will continue to be fought on contested ground.

Subject of Research: Human exposure to ethylene and endogenous ethylene oxide assessed through breath biomarkers and pharmacokinetic modeling

Article Title: Regarding ethylene exposure and endogenous ethylene oxide levels in humans (Lin et al., 2025)

Article References: Regarding ethylene exposure and endogenous ethylene oxide levels in humans (Lin et al., 2025). (n.d.). https://doi.org/10.1038/s41370-026-00967-3

Image Credits: AI Generated

DOI: 10.1038/s41370-026-00967-3

Keywords: ethylene, ethylene oxide, hemoglobin adducts, PBPK modeling, breath analysis, biomonitoring, exposure science, risk assessment, photoacoustic spectrometry, gas chromatography, environmental epidemiology, Regarding