Rethinking indoor air chemistry
People generate their own oxidation field and change the indoor air chemistry around them
Peer-Reviewed PublicationIMAGE: COMPUTER MODELLING OF THE OH REACTIVITY (LEFT) AND OH CONCENTRATION (RIGHT) AROUND HUMAN BODIES IN A TYPICAL INDOOR SITUATION WHILE PEOPLE SITTING AROUND A TABLE view more
CREDIT: UC IRVINE
People typically spend 90 percent of their lives inside, at home, at work, or in transport. Within these enclosed spaces, occupants are exposed to a multitude of chemicals from various sources, including outdoor pollutants penetrating indoors, gaseous emissions from building materials and furnishings, and products of our own activities such as cooking and cleaning. In addition, we are ourselves potent mobile emission sources of chemicals that enter the indoor air from our breath and skin.
But how do the chemicals disappear again? In the atmosphere outdoors, this happens to a certain extent naturally by itself, when it rains and through chemical oxidation. Hydroxyl (OH) radicals are largely responsible for this chemical cleaning. These very reactive molecules are also called the detergents of the atmosphere and they are primarily formed when UV light from the sun interacts with ozone and water vapor.
Indoors, on the other hand, the air is of course far less affected by direct sunlight and rain. Since UV rays are largely filtered out by glass windows it has been generally assumed that the concentration of OH radicals is substantially lower indoors than outdoors and that ozone, leaking in from outdoors, is the major oxidant of indoor airborne chemical pollutants.
OH radicals are formed from ozone and skin oils
However, now it has been discovered that high levels of OH radicals can be generated indoors, simply due to the presence of people and ozone. This has been shown by a team led by the Max Planck Institute for Chemistry in cooperation with researchers from the USA and Denmark.
"The discovery that we humans are not only a source of reactive chemicals, but we are also able to transform these chemicals ourselves was very surprising to us," says Nora Zannoni, first author of the study published in the research magazine Science, and now at the Institute of Atmospheric Sciences and Climate in Bologna, Italy. "The strength and shape of the oxidation field are determined by how much ozone is present, where it infiltrates, and how the ventilation of the indoor space is configured," adds the scientist from Jonathan Williams' team. The levels the scientists found were even comparable to outside daytime OH concentrations levels.
The oxidation field is generated by the reaction of ozone with oils and fats on our skin, especially the unsaturated triterpene squalene, which constitutes about 10 percent of the skin lipids that protect our skin and keep it supple. The reaction releases a host of gas phase chemicals containing double bonds that react further in the air with ozone to generate substantial levels of OH radicals. These squalene degradation products were characterized and quantified individually using Proton Transfer reaction Mass Spectrometry and fast gas chromatograph-mass spectrometry systems. In addition, the total OH reactivity was determined in parallel enabling the OH levels to be quantified empirically.
The experiments were conducted at the Technical University of Denmark (DTU) in Copenhagen. Four test subjects stayed in a special climate-controlled chamber under standardized conditions. Ozone was added to the chamber air inflow in a quantity that was not harmful to humans but representative of higher indoor levels. The team determined the OH values before and during the volunteers' stay both with and without ozone present.
In order to understand how the human-generated OH field looked like in space and time during the experiments, results from a detailed multiphase chemical kinetic model from the University of California, Irvine were combined with a computational fluid dynamics model from Pennsylvania State University, both based in the USA. After validating the models against the experimental results, the modeling team examined how the human-generated OH field varied under different conditions of ventilation and ozone, beyond those tested in the laboratory. From the results, it was clear that the OH radicals were present, abundant, and forming strong spatial gradients.
“Our modeling team is the first and currently the only group that can integrate chemical processes between the skin and indoor air, from molecular scales to room scales,” said Manabu Shiraiwa, a professor at UC Irvine who led the modeling part of the new work. “The model makes sense of the measurements — why OH is generated from the reaction with the skin.”
Shiraiwa added that there remain unanswered questions, like the way humidity levels impact the reactions the team traced. “I think this study opens up a new avenue for indoor air research,” he said.
Adapt test methods for furniture and building materials
"We need to rethink indoor chemistry in occupied spaces because the oxidation field we create will transform many of the chemicals in our immediate vicinity. OH can oxidize many more species than ozone, creating a multitude of products directly in our breathing zone with as yet unknown health impacts”. This oxidation field will also impact the chemical signals we emit and receive," says project leader Jonathan Williams, “and possibly help explain the recent finding that our sense of smell is generally more sensitive to molecules that react faster with OH.”
The new finding also has implications for our health: Currently, chemical emissions of many materials and furnishings are being tested in isolation before they are approved for sale. However, it would be advisable to also conduct tests in the presence of people and ozone, says atmospheric chemist Williams. This is because oxidation processes can lead to the generation of respiratory irritants such as 4-oxopentanal (4-OPA) and other OH radical-generated oxygenated species, and small particles in the immediate vicinity of the respiratory tract. These can have adverse effects, especially in children and the infirm.
These findings are part of the project ICHEAR (Indoor Chemical Human Emissions and Reactivity Project) which brought together a group of collaborating international scientists from Denmark (DTU), the USA (Rutgers University), and Germany (MPI). The modeling was part of the MOCCIE project based at the University of California Irvine and the Pennsylvania State University. Both projects were funded by grants from the A. P. Sloan foundation.
CAPTION
Not visible, but measurable: an oxidation field is generated around each person in the stainless steel climate chamber at the Technical University of Denmark.
CREDIT
Original publication
The Human Oxidation Field
Nora Zannoni, Pascale S. J. Lakey, Youngbo Won, Manabu Shiraiwa, Donghyun Rim, Charles J. Weschler, Nijing Wang, Lisa Ernle, Mengze Li, Gabriel Bekö, Pawel Wargocki, Jonathan Williams
Science, 1 September 2022
Doi: 10.1126/science.abn0340
JOURNAL
Science
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
People
ARTICLE TITLE
The Human Oxidation Field
ARTICLE PUBLICATION DATE
2-Sep-2022
The human body produces hydroxyl radicals when exposed to ozone in indoor environments
The human body’s exposure to ozone in indoor spaces generates highly reactive hydroxyl (OH), radicals which are largely responsible for the oxidation of most pollutant gases, researchers report. The findings have implications for our understanding of the role of humans in indoor air chemistry and quality. “[The authors] observed that the human body interacts with the indoor environment in an analogous manner to how Earth interacts with the atmosphere,” write Coralie Schoemaecker and Nicola Carslaw in a related Perspective. “Both the human body and Earth are chemical reactors, consuming or producing oxidants and oxidized species in their surrounding atmospheres.” The vast majority of humans spend most of their time indoors – whether at their home or workplace or while traveling between the two – and are exposed to a number of chemicals from various sources, including outdoor pollutants that find their way inside, gaseous emissions from building materials and furnishings, and products of activities such as cooking and cleaning. Moreover, the human body is also a potent mobile source of emissions. Chemical removal of gas-phase pollutants in outside air during daytime is mostly driven by the production of OH radicals, which are formed primarily by the photolysis of ozone by ultraviolet sunlight. However, indoor air quality is much less impacted by this process, as glass windows largely filter out ultraviolet light. While research has shown that some OH radicals can be generated by other means in indoor environments, few studies have evaluated the chemical influence human bodies in these environments. Through a series of experiments, Nora Zannoni and colleagues found that high concentrations of OH radicals are generated when people were exposed to different concentrations of ozone within a climate-controlled, stainless-steel chamber. According to Zannoni et al., squalene in skin oil reacted with to produce 6-methyl-5-hepten-2-one (6-MHO), which was key to establishing this human-induced oxidation field. What’s more, they found that isoprene from human breath and products of its interaction with OH also react with ozone to produce more OH radicals, suggesting that humans are a net source of reactive oxidants indoors.
JOURNAL
Science
ARTICLE TITLE
The human oxidation field
ARTICLE PUBLICATION DATE
2-Sep-2022
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