By Suherman Candra Maholtra 
Recent scientific investigations led by the Max Planck Institute for Chemistry have revealed a significant and previously unknown interaction between common personal care products and the immediate chemical environment surrounding the human body. Building upon a landmark 2022 discovery, which established that humans generate a personal "oxidation field" through the reaction of ozone with skin oils, a new international study has demonstrated that the application of lotions and perfumes substantially suppresses this natural chemical process. These findings, published in the journal Science Advances, suggest that the "cleansing" or transformative capacity of the air in our immediate vicinity—the breathing zone—is being fundamentally altered by the daily use of consumer cosmetics.
The Discovery of the Human Oxidation Field
To understand the implications of the latest research, it is essential to revisit the foundational discovery made by Jonathan Williams’ research group at the Max Planck Institute in 2022. For decades, atmospheric chemists have known that the Earth’s atmosphere cleanses itself using hydroxyl (OH) radicals. These highly reactive molecules, often referred to as the "detergents of the atmosphere," neutralize pollutants by breaking down volatile organic compounds (VOCs). Until recently, it was assumed that such high-energy chemistry occurred primarily outdoors, driven by ultraviolet sunlight.
However, the 2022 study proved that humans are mobile sources of these OH radicals. When ozone (O3) from the outdoor air infiltrates indoor spaces, it reacts with squalene—a natural unsaturated oil that makes up about 10% of human skin lipids. This reaction produces 6-methyl-5-hepten-2-one (6-MHO) and other carbonyls, which then react further with ozone in the air to generate high concentrations of OH radicals. This creates a personal "oxidation field" or a "chemical halo" that follows an individual, constantly processing the air they breathe.
The realization that humans actively change the chemistry of their own personal space shifted the paradigm of indoor air quality research. It suggested that we are not merely passive recipients of indoor pollutants but active chemical reactors.
Methodology: Simulating the Human Environment
The follow-up study was an intensive international collaboration involving the Max Planck Institute for Chemistry, the Technical University of Denmark (DTU), the University of California, Irvine, and Pennsylvania State University. The experimental phase was conducted in 2021 at a specialized climate-controlled chamber at DTU in Copenhagen.
The researchers utilized four test subjects who were placed in the chamber under strictly standardized conditions. To simulate a realistic indoor environment, ozone was introduced into the chamber’s air inflow at levels that are non-harmful to humans but representative of the upper range of typical indoor concentrations (often found in well-ventilated buildings or during high-ozone summer days).
To quantify the OH radicals, which are too short-lived to be measured directly in real-time at such scales, the team employed an indirect method. They measured the individual sources of OH and the overall loss rate of the radicals. By combining these physical air measurements with advanced computational models, the team could map the oxidation field’s strength and spatial extent.
Supporting the physical experiments, Manabu Shiraiwa’s team at UC Irvine utilized a state-of-the-art chemical model to simulate how ozone reacts with skin and clothing. Simultaneously, Donghyun Rim’s group at Pennsylvania State University applied three-dimensional computational fluid dynamics (CFD) to visualize how the oxidation field evolves around a person as they move or sit.
The Impact of Perfumes and Lotions
The study focused on two primary categories of personal care products: body lotions and perfumes. The results were consistent across both categories but driven by different chemical mechanisms.
The Effect of Perfume
When subjects applied perfume, the researchers observed a sharp decrease in the concentration of OH radicals around the body. The primary culprit identified was ethanol, which serves as the base solvent for most commercial fragrances. While ethanol is highly volatile and reacts readily with OH radicals, it does not produce new OH radicals when it reacts with ozone. Consequently, the ethanol in the perfume acts as a "sink," consuming the existing OH radicals in the personal oxidation field without replenishing them.
The Effect of Body Lotion
The application of body lotion resulted in an even more persistent suppression of the oxidation field. The researchers proposed two complementary explanations for this phenomenon. First, many lotions contain phenoxyethanol, a common preservative and antimicrobial agent. Similar to the ethanol in perfume, phenoxyethanol reacts with and consumes OH radicals but fails to generate new ones upon interaction with ozone.
Second, the physical presence of the lotion on the skin creates a barrier. By coating the skin, the lotion prevents ozone from coming into contact with the natural squalene in human skin oils. Since the reaction between ozone and squalene is the primary engine for indoor OH production, this physical "masking" effectively shuts down the generation of the oxidation field at its source.
Nora Zannoni, the study’s first author and currently a researcher at the Institute of Atmospheric Sciences and Climate in Bologna, noted the temporal differences between the products. "Fragrances impact the OH reactivity and concentration over shorter time periods due to their high volatility," Zannoni explained. "In contrast, lotions show more persistent effects, consistent with the slower emission rate of organic compounds from these products."
Chronology of the Research Project
The findings are the result of a multi-year effort funded by the A. P. Sloan Foundation under two major projects: ICHEAR (Indoor Chemical Human Emissions and Reactivity Project) and MOCCIE (Modeling Consortium for Chemistry of Indoor Environments).
- 2020–2021: The ICHEAR project brings together scientists from Germany, Denmark, and the USA to conduct chamber experiments at DTU.
- 2022: The team publishes its initial discovery in Science, proving for the first time that humans generate their own OH radical fields.
- 2023: Data analysis focuses on the influence of external variables, specifically personal care products, using the MOCCIE computational models.
- 2024: The follow-up study is published in Science Advances, detailing the suppressive effects of PCPs on human oxidation chemistry.
Implications for Health and Indoor Air Quality
The suppression of the human oxidation field is not merely a laboratory curiosity; it has profound implications for how we understand human health and the air we breathe. Humans spend approximately 90% of their lives indoors, where they are exposed to a complex cocktail of chemicals emitted by furniture, flooring, cleaning agents, and outdoor air pollution.
Jonathan Williams highlights a critical irony in modern consumer safety: "If we buy a sofa, it is tested for harmful emissions before being put on sale. However, when we sit on that sofa, we naturally transform some of those emissions because of the oxidation field we generate."
This "personal chemical transformer" can be a double-edged sword. On one hand, the OH radicals can break down harmful VOCs before they are inhaled. On the other hand, the oxidation process itself can create secondary products—new chemicals whose toxicity and health impacts are largely unknown. By using lotions and perfumes, we are effectively turning off this transformer.
The suppression of the OH field means that the chemicals we inhale in our "breathing zone" remain in their original state rather than being oxidized. Whether this is beneficial or harmful depends entirely on the specific chemicals present in the room. If the oxidation field typically breaks down a toxic substance into a harmless one, suppressing it would increase health risks. Conversely, if the oxidation field transforms a benign substance into a respiratory irritant, suppressing it might be protective.
Analysis and Future Directions
The study underscores a significant gap in current regulatory frameworks for indoor air quality. Most safety standards focus on the primary emissions of products—what comes directly out of a bottle of cleaner or a piece of plywood. Very little attention is paid to the "secondary" chemistry that occurs when those emissions interact with the human body and its self-generated chemical field.
The findings suggest that the "personal cloud" of chemistry—often referred to by researchers as the "Pig-Pen effect," after the Peanuts character—is much more dynamic than previously thought. The fact that common cosmetics can dampen this field suggests that personal habits and lifestyle choices significantly dictate an individual’s "chemical footprint."
Moving forward, the research team suggests that future studies must account for the high variability of personal care products. With thousands of different formulations on the market, the interaction between specific ingredients and the human oxidation field remains a vast, uncharted territory. Furthermore, the study raises questions about other factors that might influence the field, such as age (which changes skin oil composition), diet, and even the type of fabric worn, as clothing can also act as a site for ozone-driven reactions.
As we continue to spend the vast majority of our time in enclosed spaces, understanding the "invisible halo" of chemistry we carry with us—and how we inadvertently switch it off with a spray of perfume or a dab of lotion—will be crucial for the future of public health and building design. The work of the Max Planck Institute and its international partners has opened a new chapter in atmospheric science, one that brings the vast complexity of the Earth’s chemistry right down to the surface of our skin.