The human oxidation field is a thin but chemically dynamic zone enveloping the body, arising from reactions between skin oils and indoor ozone molecules. Earlier research by the same interdisciplinary team, published in Science in 2022, identified the skin’s natural oils—most notably squalene—as critical reactants that engage with ambient ozone to produce highly reactive chemical species, including hydroxyl radicals. These radicals, known for their potent oxidative properties, catalyze cascades of chemical reactions that transform indoor air pollutants right in the immediate vicinity of a person’s breathing zone.

This newly-characterized microenvironment is influenced by numerous factors, among them emissions from everyday indoor sources such as cooking residues, cleaning agents, cigarettes, paints, upholstery fabrics, and furniture materials. Additionally, outdoor ozone infiltration further supplies necessary reactants. These reactive zones foster complex chemistry which until now was largely overlooked in assessments of indoor air quality and human exposure to airborne contaminants.

In the recent Science Advances publication, the researchers delve into how the application of personal care products alters this oxidation chemistry. Using a combination of advanced multiphase chemical kinetic modeling and computational fluid dynamics simulations, the team demonstrated that body lotions create a physical barrier on the skin surface. This barrier inhibits the reaction between ozone molecules and squalene, effectively suppressing the precursor formation of hydroxyl radicals. Consequently, the intensity of the human oxidation field is significantly diminished.

Moreover, the study found that ethanol, a common solvent in many fragrances, functions as a chemical sink for hydroxyl radicals. By reacting with and neutralizing these reactive species, ethanol reduces the overall concentration of hydroxyl radicals near the skin, further weakening the oxidation field. This dual mechanism reveals not only a physical but also a chemical mode by which personal care products modulate immediate human-environment chemistry.

Manabu Shiraiwa, UC Irvine professor of chemistry and co-corresponding author, spearheaded the development of the comprehensive chemical kinetic model, capturing multiphase interactions between skin secretions, airborne oxidants, and reactive intermediates. Collaborators at Penn State contributed fluid dynamic modeling to spatially resolve concentration gradients of reactive chemical species around the human form indoors. Together, these tools enabled unprecedented visualization and quantification of the human oxidation field’s behavior in realistic indoor settings.

According to Shiraiwa, “Our approach uniquely integrates skin surface chemistry with indoor atmospheric conditions to simulate the nuanced formation and transformation of reactive chemical species near humans.” This holistic modeling, combining chemical kinetics and fluid dynamics, advances understanding of how subtle chemical processes at the skin interface propagate into broader indoor air chemistry.

The implications of this research are far-reaching. Many indoor environments are tested for emissions from consumer products and furnishings to regulate exposure to harmful compounds before they reach occupants. However, these tests rarely consider the reactive transformations initiated by the human oxidation field upon contact with such emissions. The study points out that when a person interacts physically with furniture or objects, their skin chemistry can catalyze the generation of numerous secondary compounds whose health effects and chemical properties remain poorly understood.

Jonathan Williams, lead author and head of organic reactive species research at the Max Planck Institute, highlighted how this oxidation field modifies indoor air composition: “Even after emission testing, once a person is sitting on a sofa, the oxidation field generated by their skin actively transforms many of those materials, creating new compounds in their breathing zone. Notably, both body lotions and perfumes appear to attenuate this effect significantly.”

This insight compellingly suggests that everyday consumer products—beyond their cosmetic or olfactory roles—can modulate indoor chemical environments and human exposure risks. By mitigating the formation of reactive species, such products could potentially influence human health outcomes related to air quality, respiratory stress, and chemical sensitivities.

The research was conducted under the Indoor Chemical Human Emissions and Reactivity (ICHER) project, an international collaborative endeavor encompassing institutions across Denmark, Germany, and the United States. Computational modeling efforts were coordinated by the Modelling Consortium for Chemistry of Indoor Environments (MOCCIE) at UC Irvine, led by Shiraiwa. Funding was secured through grants from the Alfred P. Sloan Foundation, underscoring the strategic importance of deciphering indoor air chemical processes.

As indoor living continues to dominate human activity patterns globally, this work delivers critical new perspectives on how human-derived chemistry interacts dynamically with the built environment and the products within it. It establishes a foundation for future exploration into mitigating exposure to potentially harmful oxidation products and improving indoor air quality through informed use and design of personal care and furnishing materials.

Given the ubiquity of fragrances and lotions in modern life, the findings also open avenues for innovative product formulations aiming to harmonize personal care with environmental health. This intersection of chemistry, human biology, and indoor air science presents a frontier ripe for further interdisciplinary research and public health advancements.

Collectively, these findings redefine our understanding of the immediate chemical landscape surrounding the human body indoors. They invite a paradigm shift from viewing emissions and pollutants as static entities to recognizing the human occupant as an active chemical participant capable of modulating their personal air microenvironment in complex and consequential ways.

Subject of Research:
The influence of personal care products on the chemical dynamics of the human oxidation field and its impact on indoor air quality.

Article Title:
Personal care products disrupt the human oxidation field

News Publication Date:
21-May-2025

Web References:
https://www.science.org/doi/10.1126/sciadv.ads7908
https://www.chem.uci.edu/~mshiraiw/MOCCIE.html

References:

• Published article in Science Advances, May 21, 2025
• Previous related study in Science, 2022

Image Credits:
Not provided.

Keywords

Chemical compounds, indoor air chemistry, human oxidation field, hydroxyl radicals, skin-ozone interaction, personal care products, body lotion, fragrances, indoor pollutants, computational fluid dynamics, multiphase chemical kinetics, indoor environments

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