In a groundbreaking study published in Science Advances, an international team of researchers has unveiled how everyday personal care products, such as lotions and fragrances, can significantly disrupt the delicate chemical oxidation field generated by humans indoors. This newly characterized human oxidation field arises primarily from the interaction between ozone—a reactive molecule commonly found in outdoor air that infiltrates indoor environments—and oils on our skin, notably squalene. The formation and dynamics of hydroxyl radicals (OH), which dominate this oxidation field, have far-reaching implications for indoor air quality and human exposure to chemical species.
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
