The human oxidation field constitutes a zone of active chemistry surrounding individuals within indoor spaces, resulting from complex surface-air interactions. Ozone entering indoor environments readily reacts with unsaturated skin oils, producing hydroxyl radicals—highly reactive species crucial to atmospheric chemistry. These radicals not only affect the transformation of volatile organic compounds (VOCs) but also modulate the composition of indoor air in human breathing zones. Given that humans spend approximately 90% of their time indoors, understanding factors that influence this chemical microenvironment is essential for assessing chemical exposure and potential health impacts.

The researchers combined experimental observations with advanced computational models to reveal that personal care products suppress the intensity and spatial reach of the human-generated hydroxyl radical field. Specifically, body lotions act as physical barriers between ozone and skin surface squalene, thereby hindering one of the critical precursor reactions responsible for OH radical generation. This attenuation directly lowers the ambient OH concentration around individuals wearing lotion, reducing the oxidative potential of their immediate indoor environment.

Complementing this physical inhibition, the chemical constituents of fragrances further diminish the oxidation field through chemical reactions. Ethanol—the primary solvent in many perfumed products—serves as a radical sink, reacting rapidly with hydroxyl radicals but not contributing to their regeneration. This mechanism causes a net loss of OH species near fragranced skin, thereby weakening the oxidative capacity engendered by standard skin-ozone chemistry. Such findings suggest a dual mode of suppression: one via physical shielding and the other through chemical scavenging, complicating the chemical dynamics near humans indoors.

The research was conducted under controlled conditions in a climate chamber, where volunteers were exposed to ozone levels representative of the high end of typical indoor environments. Using a sophisticated combination of multiphase chemical kinetic modeling and three-dimensional computational fluid dynamics (CFD), the team simulated the distribution and transformations of reactive compounds around human subjects. This integrated modeling approach enabled the detailed analysis of how various personal care products modulate concentrations of reactive species such as OH radicals and ozone within the human breathing zone.

The experimental and computational synergy revealed nuanced temporal effects of different products. Fragrances exhibited pronounced suppression of OH activity over shorter timescales, consistent with the volatile nature and rapid evaporation of ethanol-based solvents. In contrast, lotions displayed more persistent effects, linked to their slower emission rates and lasting physical presence on skin surfaces. This temporal distinction underscores the complex interplay between product chemistry, volatility, and surface interactions that govern indoor oxidation chemistry.

One of the notable chemical agents implicated in suppressing the OH field is phenoxyethanol, a widely employed preservative found in many skincare products. Phenoxyethanol reacts readily with OH radicals but, like ethanol, does not participate in regeneration of OH via reaction with ozone. Its dual role as a preservative and chemical sink means that common personal care formulations inadvertently modulate indoor oxidative chemistry by capturing reactive radicals, thereby altering the oxidative environment in subtle yet meaningful ways.

These findings carry significant environmental and health relevance. Indoor air quality is dynamically influenced not only by external pollutant infiltration and emissions from materials such as furniture and flooring but also by the self-generated oxidation fields arising from human occupants themselves. The suppression of this oxidative microenvironment by personal care products implies altered transformation pathways of precursor compounds emitted indoors, potentially modifying exposure to secondary pollutants and affecting the formation of semi-volatile organic compounds.

Moreover, because people modify their skin surface chemistry routinely through the use of consumer products, this research highlights an overlooked human factor in indoor atmospheric chemistry. Emissions from housing materials are well-regulated and tested for toxicity; however, the oxidation field generated by humans leads to secondary chemical processes that transform those emissions in the breathing zone. The attenuation of this oxidation field by lotions and perfumes may reduce or alter the formation of transformation products, the toxicity and health implications of which remain underexplored.

The study’s implications extend to the design and evaluation of indoor environments, where integrating knowledge of human oxidative fields and consumer product chemistry can inform ventilation strategies and material choices. Accurate mechanistic modeling frameworks, such as those developed here, offer powerful tools to predict indoor chemical exposures more realistically by accounting for occupant chemistry and product use patterns. Such sophistication may lead to novel interventions aimed at improving indoor air quality and minimizing health risks associated with reactive indoor pollutants.

This interdisciplinary effort involved collaboration between the Max Planck Institute for Chemistry in Germany, the University of California Irvine, Pennsylvania State University, and the Technical University of Denmark. The combination of experimental chamber studies and state-of-the-art computational modeling provided comprehensive insight into the transient and steady-state chemistry near human skin surfaces under realistic indoor conditions.

Future directions envisioned by the research team include expanding chemical characterization of a broader range of personal care formulations, exploring long-term effects of habitual product use, and integrating human oxidation fields into broader indoor air quality models. Understanding how diverse product chemistries influence oxidative reactivity indoors offers potential pathways to mitigate adverse chemical exposures and enhance chemical safety in everyday living spaces.

This pioneering work thus reframes our understanding of indoor air chemistry by revealing how the very products designed to care for human skin simultaneously intervene in the reactive chemistry of our microenvironments. These findings open a new dimension of chemical-person interactions indoors, with profound implications for exposure science, indoor environmental health, and consumer product formulation.

Subject of Research: Not applicable

Article Title: Personal care products disrupt the human oxidation field

News Publication Date: 21-May-2025

Web References: DOI: 10.1126/sciadv.ads7908

Keywords

Environmental sciences, Chemistry, Indoor air quality, Hydroxyl radicals, Ozone chemistry, Personal care products, Oxidation field, Indoor atmospheric chemistry

Criegee intermediates, fleeting yet highly reactive molecules, emerge primarily when ozone encounters unsaturated hydrocarbons like alkenes airborne in the troposphere. These intermediates are central players in atmospheric oxidation processes, serving as precursors to hydroxyl radicals—sometimes called the atmosphere’s “cleansing agents”—and influencing aerosol formation, which impacts climate forcing and human health. Of particular interest is syn-CH3CHOO, which, due to its relative abundance and reactivity, accounts for a significant fraction—ranging seasonally from 25% to nearly 80%—of all Criegee intermediates present.

Conventionally, atmospheric chemists have held the view that syn-CH3CHOO primarily diminishes through unimolecular self-decomposition, a process by which the molecule breaks down in isolation, forming other species over time. However, cutting-edge research recently published in Nature Chemistry by an interdisciplinary team from the Dalian Institute of Chemical Physics (DICP) has revealed that this paradigm is incomplete. Led by Professors YANG Xueming, ZHANG Donghui, DONG Wenrui, and FU Bina, the team demonstrated that syn-CH3CHOO reacts with water vapor in the atmosphere at a pace roughly two orders of magnitude faster than theoretical models had anticipated.

This finding was grounded in precision experimental work utilizing state-of-the-art laser diagnostic techniques. By producing and isolating syn-CH3CHOO radicals under controlled conditions, the researchers directly measured reaction rates with water vapor at various concentrations and temperatures, noting a striking acceleration that could not be reconciled with prior kinetic predictions. This departure from the expected speed suggested an alternative transition mechanism at play during the molecular encounter.

To unravel this puzzle, the team employed an advanced computational approach—constructing a full-dimensional, 27 degrees-of-freedom potential energy surface guided by the fundamental invariant-neural network methodology. This approach allowed for an unprecedentedly high-resolution simulation of the interaction dynamics between syn-CH3CHOO and water molecules, capturing nuances of molecular behavior inaccessible to simpler models. The subsequent dynamical calculations illuminated a fascinating "roaming mechanism" underpinning the reaction acceleration.

Contrary to a straightforward, minimum-energy path where reactants collide and directly transform into products, the roaming mechanism involves the molecules engaging in a subtle, spatially extended dance, influenced heavily by dipole-dipole electrostatic attractions. Within this entrance channel, syn-CH3CHOO and water vapor do not immediately proceed to reaction but instead explore a region of phase space where long-range interactions guide their trajectories. This roaming allows for more frequent and effective orbital overlaps, thus dramatically enhancing the probability of reaction relative to classical transition state expectations.

From a broader atmospheric perspective, this implies that the water-induced removal of syn-CH3CHOO could be as significant as its self-decomposition pathway, challenging decades-old assumptions embedded in atmospheric chemical models. Current models, which estimate the atmospheric burden and lifecycle of Criegee intermediates, may therefore underestimate the role of water vapor and overestimate unimolecular decay in governing the atmospheric fate of syn-CH3CHOO.

The implications of these refined insights extend well beyond mere academic curiosity. Accurate predictions of hydroxyl radical budgets and secondary aerosol formation are critical for climate modeling, air quality forecasting, and understanding oxidative stressors affecting ecosystems and human health. By incorporating this faster, water-mediated reaction channel, atmospheric chemistry models can achieve higher fidelity, improving the projections of pollutant lifetimes and transformation products.

Moreover, the newfound roaming mechanism exemplifies the intricate coupling between intermolecular forces and reaction dynamics in weakly bound systems. This suggests that similar long-range interaction-driven processes may be pervasive in other reactive contexts, including combustion systems where hydrocarbon oxidation dominates energy production and astrochemical environments where low-pressure, low-temperature conditions prevail.

The DICP team’s work not only clarifies a specific reaction pathway but also highlights the symbiotic relationship between experimental and computational chemistry. High-accuracy experiments provide essential benchmarks that guide and validate sophisticated theoretical models, while advanced simulations elucidate mechanisms that are challenging or impossible to resolve purely through observation.

In particular, the application of invariant neural network potentials for full-dimensional potential energy surfaces represents a significant step forward for computational chemistry, enabling researchers to tackle complex reactive systems with comprehensive dynamical treatments. This methodological innovation could become a cornerstone in studying other elusive atmospheric and interstellar reactions.

Looking ahead, these insights pave the way for expanded investigations into the reactions of diverse Criegee intermediates with various atmospheric constituents. Analyses of their interactions with other small molecules, such as sulfur dioxide or organic acids, could reveal additional accelerated pathways or unrecognized reaction channels important in haze formation and pollutant transformation.

The discovery of a roaming-mediated acceleration in syn-CH3CHOO and water vapor reactions also invites reconsideration of analogous processes in combustion chemistry. Here, the dynamics of radical intermediates and their interactions with ambient molecules dictate flame stability, emissions, and efficiency. Understanding roaming effects could lead to more accurate control strategies and cleaner combustion technologies.

Astrochemistry stands to benefit similarly. Interstellar clouds and planetary atmospheres, where reactions occur at extremely low temperatures and densities, may host reaction mechanisms dominated by long-range interactions and roaming behavior. Observations and models of molecular evolution in these remote environments can incorporate these mechanisms to enhance accuracy.

Ultimately, the work underscores the necessity of integrating interdisciplinary approaches—melding experimental rigor with computational innovation—to unravel the complexities of chemical reaction dynamics. As atmospheric challenges grow with climate change and pollution, such fundamental advances provide the necessary foundation for informed policies and technological strategies aimed at preserving environmental and public health.

This research marks a milestone in atmospheric chemistry, redefining how key reactive intermediates interact with one of the most ubiquitous components of the atmosphere—water vapor. It reshapes foundational concepts and opens new investigative pathways that promise to deepen our mastery over the chemical intricacies shaping the air above us.

Subject of Research:
Not applicable

Article Title:
Reactivity of syn-CH3CHOO with H2O enhanced through a roaming mechanism in the entrance channel

News Publication Date:
16-Apr-2025

Web References:
https://www.nature.com/articles/s41557-025-01798-9
http://dx.doi.org/10.1038/s41557-025-01798-9

Image Credits:
Credit: Dalian Institute of Chemical Physics (DICP)

Keywords

Atmosphere, Water vapor, Theoretical chemistry