By Suherman Candra Maholtra 
Scientific understanding of the indoor environment has undergone a paradigm shift following recent research led by the Max Planck Institute for Chemistry, which reveals that humans do not merely occupy space but actively transform the chemistry of the air around them. While it was established in 2022 that humans generate a personal "oxidation field" through the reaction of ozone with skin oils, a new follow-up study has identified a significant disruptor to this natural process: personal care products. Research conducted by an international consortium of scientists indicates that common items such as body lotions and perfumes substantially suppress the production of hydroxyl (OH) radicals, the primary oxidative agents in the air. This suppression has profound implications for how we understand indoor air quality, the transformation of pollutants in our immediate vicinity, and the long-term health effects of the chemical "cocktail" present in modern living spaces.
The Discovery of the Human Oxidation Field
To understand the impact of personal care products, it is necessary to examine the fundamental discovery made by Jonathan Williams’ research group at the Max Planck Institute in 2022. For decades, atmospheric chemists focused on the hydroxyl radical (OH) as the "detergent" of the Earth’s atmosphere. These highly reactive molecules are responsible for breaking down pollutants in the outdoor air, primarily driven by ultraviolet light from the sun. However, because UV light is largely filtered out by glass windows, it was long assumed that OH concentrations indoors were negligible.
The 2022 breakthrough revealed that humans are a mobile source of these radicals. When ozone ($O_3$), a common outdoor pollutant that infiltrates buildings through ventilation, comes into contact with human skin, it reacts with squalene—a natural lipid that accounts for approximately 10% of human skin oil. This reaction produces 6-methyl-5-hepten-2-one (6-MHO) and other carbonyl compounds. These secondary products then react further with ozone in the air to generate hydroxyl radicals. Consequently, every human being is surrounded by a personal oxidation field—a "chemical halo" that reacts with and transforms the gases we breathe before they enter our lungs.
The Role of Personal Care Products in Chemical Suppression
The latest research, published in the journal Science Advances, sought to determine how this delicate chemical balance is affected by the modern habit of applying synthetic substances to the skin. The study, a collaborative effort involving the Max Planck Institute, the University of California, Irvine, and Pennsylvania State University, found that personal care products (PCPs) do not merely add new chemicals to the air; they actively dampen the human body’s natural ability to generate OH radicals.
According to the study, different types of products affect the oxidation field through distinct mechanisms. Fragrances and perfumes, which are often alcohol-based, primarily utilize ethanol. When ethanol is released into the air or sits on the skin, it reacts rapidly with any available OH radicals. Crucially, unlike the reaction between ozone and squalene, the reaction between ethanol and OH does not regenerate more OH radicals. Instead, the ethanol "consumes" the oxidation field, effectively depleting the concentration of these reactive molecules in the user’s immediate breathing zone.
Body lotions present a more complex interference. Researchers identified two primary pathways for suppression. First, many lotions contain phenoxyethanol, a common preservative and antimicrobial agent. Similar to the ethanol in perfume, phenoxyethanol reacts with OH radicals without producing new ones. Second, the physical application of lotion creates a barrier on the skin’s surface. This layer effectively "hides" the natural squalene from the ozone in the air, preventing the initial reaction that kicks off the production of the oxidation field.
Chronology and Experimental Methodology
The findings are the result of years of integrated laboratory work and computational modeling. The experimental phase took place in 2021 at the Technical University of Denmark (DTU) in Copenhagen. To ensure accuracy, the researchers utilized a specialized climate-controlled chamber where four test subjects were monitored under standardized conditions.
The timeline of the research highlights the complexity of the ICHEAR (Indoor Chemical Human Emissions and Reactivity) project:
- 2021: Experimental measurements were conducted in Copenhagen. Ozone was introduced into the chamber at levels representative of high-end indoor concentrations (safe for humans but chemically significant).
- 2022: The initial discovery of the human-generated OH field was published, establishing that humans are a major source of indoor oxidation.
- 2023-2024: Advanced computational modeling was integrated with the 2021 data. This involved the MOCCIE project, where researchers at UC Irvine developed chemical models to simulate ozone reactions with skin, while Penn State researchers applied 3D computational fluid dynamics (CFD) to map the evolution of the oxidation field around the human body.
- 2024: The final results regarding the suppressive effects of PCPs were published, providing a more nuanced view of the "personal cloud" of chemistry.
By combining real-time air measurements from the chamber with these sophisticated simulations, the team was able to quantify exactly how much the OH concentration dropped when subjects applied lotion or perfume compared to their "natural" state.
Supporting Data and Technical Analysis
The data gathered during the study suggests a clear distinction between the temporal effects of different products. Fragrances were found to have an immediate and sharp impact on OH reactivity. Because the volatile organic compounds (VOCs) in perfumes evaporate quickly, they cause a significant but relatively short-lived suppression of the oxidation field.
In contrast, body lotions demonstrated a "persistent" effect. Because lotions are designed to remain on the skin and release their components more slowly, the suppression of the OH field lasted for several hours. The researchers noted that while there are thousands of different formulations on the market, the presence of preservatives like phenoxyethanol is nearly ubiquitous, suggesting that the dampening effect is a common feature of modern skincare.
Professor Manabu Shiraiwa of the University of California, Irvine, emphasized the importance of the modeling component: "We developed a state-of-the-art chemical model that can simulate reactions of ozone with human skin and clothing. This allowed us to see not just that the chemicals were present, but how they were evolving in the millimeters and centimeters closest to the skin."
Official Responses and Researcher Insights
The implications of these findings have sparked a conversation among atmospheric chemists and public health experts. Jonathan Williams, the lead researcher from the Max Planck Institute, points out that the suppression of the OH field is not necessarily "good" or "bad" in a vacuum, but it fundamentally changes our exposure profile.
"If we buy a sofa from a major furniture company, it is tested for harmful emissions before being put on sale," Williams noted. "However, when we sit on the sofa, we naturally transform some of these emissions because of the oxidation field we generate. This can create many additional compounds in our breathing zone whose properties are not well known. Interestingly, body lotion and perfume both seem to dampen down this effect."
Nora Zannoni, the study’s first author and currently a researcher at the Institute of Atmospheric Sciences and Climate in Bologna, highlighted the significance of the findings for indoor air standards. "The application of a fragrance and a lotion together showed that fragrances impact the OH reactivity over shorter time periods, whereas lotions show more persistent effects," Zannoni stated. This suggests that the timing of personal care routines could influence a person’s chemical exposure throughout the day.
Broader Impact and Implications for Human Health
The study’s findings are particularly relevant given that modern humans spend approximately 90% of their lives indoors. The indoor environment is a complex reservoir of chemicals, including emissions from building materials, furniture, cleaning agents, and cooking.
The human oxidation field serves as a "first line of defense" or a "secondary reactor" that modifies these chemicals before they are inhaled. By suppressing this field, personal care products may be inadvertently changing the toxicity or the concentration of the air we breathe. For instance, if a person’s oxidation field is suppressed, they may inhale more of the primary emissions from a new carpet or a piece of furniture (such as formaldehyde or other VOCs) because the OH radicals are not there to break them down. Conversely, the presence of an active oxidation field can sometimes produce secondary organic aerosols or other irritants that might be avoided if the field is suppressed.
This research underscores a critical gap in current health safety testing. Most consumer products are tested in isolation. However, this study proves that the interaction between the human body, the products we wear, and the ambient air chemistry is a dynamic and integrated system.
Future Directions in Indoor Chemistry
The ICHEAR and MOCCIE projects have opened new avenues for architectural design and public health policy. Future research is expected to look at how different clothing materials, age groups (since skin oil composition changes with age), and indoor humidity levels further modulate the human oxidation field.
As we move toward "greener" buildings with tighter seals for energy efficiency, the concentration of indoor pollutants—and the role of humans in processing them—will become increasingly important. The discovery that personal care products act as a "chemical cloak," hiding our natural reactivity from the environment, adds a new layer of complexity to the quest for healthy indoor air.
For now, the international research team suggests that these findings be used to refine indoor air quality models. Understanding that a room full of people wearing lotion and perfume behaves differently than a room of "natural" occupants is essential for designing ventilation systems that truly protect human health. The study serves as a reminder that in the chemistry of the modern world, even the most mundane daily habits can have invisible, yet significant, consequences.