New research from the University of Copenhagen, conducted in collaboration with NASA, offers a possible explanation. Scientists analyzed blood samples from 52 astronauts who spent months aboard the International Space Station and found signs that the digestive system begins changing relatively soon after astronauts leave Earth.

“We see changes in astronauts’ blood samples that indicate that the gut bacteria begin to ferment protein to a greater extent than usual within weeks after the astronauts arrive in space, and this change continues until they are back on Earth,” says joint first author and associate professor at the Department of Nutrition, Exercise and Sports, Giorgia La Barbera.

Gut Bacteria Shift Toward Protein

Protein fermentation occurs when bacteria in the intestines begin breaking down protein rather than fiber because available dietary fiber has already been used. The process is not unique to astronauts and can also occur in people on Earth.

The researchers suspect that microgravity may slow the movement of food through the digestive tract. If food remains in the intestines longer, bacteria have more time to turn to protein fermentation.

“The lack of gravity in space probably causes food to move more slowly through the intestine, and this fits with the fact that we are seeing signs of increased protein fermentation. This may also help explain constipation in astronauts,” says co-author of the article Henrik Roager, who is also an associate professor at the Department of Nutrition, Exercise and Sports.

Gut Changes Could Affect the Brain

The findings came from an investigation of metabolites in astronauts’ blood. Metabolites are small molecules circulating through the bloodstream that can provide clues about how the body is functioning.

By analyzing these molecules, researchers can learn about processes ranging from diet and metabolism to what is happening inside the intestines.

The results could matter beyond digestion because changes in the gut can influence other parts of the body, including the brain.

“We know that products of protein fermentation [are] often associated with negative health consequences, such as kidney damage, potential effects on mood or reduced ability to focus, among others,” says Lars Ove Dragsted, senior author of the study and professor at the Department of Nutrition, Exercise and Sports.

Why This Matters for Missions to Mars

The findings could become especially important as space agencies prepare for longer missions to the Moon and eventually Mars. Astronauts on those journeys may spend far more time away from Earth’s gravity than crews currently stationed aboard the International Space Station.

“When we plan for longer journeys in space – for example to Mars countermeasures may be needed to mitigate negative effects of space travel on the intestines,” says Giorgia La Barbera.

Researchers say several approaches might help reduce these digestive changes.

“Our findings can inform potential interventions – either directly through increasing dietary fiber, supplementation with prebiotics or other types of treatments that promote peristalsis and decrease gut transit time. This would help reduce protein fermentation and thus provide a healthy gut environment and avoid negative health consequences,” adds Henrik Roager.

Peristalsis refers to the coordinated muscle contractions that move food and waste through the digestive system. Supporting that movement could shorten the amount of time material remains in the intestines and reduce the opportunity for excessive protein fermentation.

The Findings Could Also Help Patients on Earth

The work may also have implications far beyond spaceflight.

People who spend extended periods confined to bed often experience constipation as well. Researchers suggest that slowed intestinal movement in these patients could potentially produce similar changes in gut fermentation.

“You can use this knowledge to help bedridden patients. They also experience constipation and likely also have increased protein fermentation that may aggravate health conditions,” concludes Lars Ove Dragsted, senior author of the paper.

What Is Protein Fermentation?

Protein fermentation happens when bacteria in the gut begin breaking down protein after they no longer have enough fiber available.

The process is more likely when food remains in the intestines for an extended period, even if a person does not feel noticeably constipated.

As bacteria digest protein, they produce various metabolites. These molecules can enter the bloodstream and travel throughout the body.

Some can also influence the brain through the gut-brain axis, the two-way communication system linking the digestive tract and the brain.

Previous research has associated metabolites produced during protein fermentation with effects on the brain, including anxiety and reduced ability to concentrate.

Why the Results Were Unusually Clear

Research on blood metabolites can be difficult to interpret because metabolism differs considerably from person to person.

The unusual nature of the astronaut data helped researchers overcome some of that variability and strengthened confidence in the pattern they observed.

“The samples we have analyzed come from a total of 52 different astronauts, on different missions and across many years. This is very different from how you normally do a study. So, it is quite impressive that our method has been able to show so clearly that there is consistently more protein fermentation,” says joint first author and assistant professor Jan Stanstrup.

How the Study Was Conducted

The researchers examined how human metabolism changes during spaceflight by measuring metabolites in astronauts’ blood.

Their analysis indicated that protein fermentation in the intestines increases when astronauts leave Earth and enter microgravity. The blood samples also revealed smaller changes associated with fish and caffeine consumption.

Rather than searching only for predetermined substances, the researchers used a so-called non-targeted approach. This allowed them to analyze the broad range of metabolites in the blood and identify which ones changed during spaceflight.

The samples came from three studies conducted aboard the International Space Station. Even though only small quantities of blood were available, researchers were able to perform additional analyses and extract more scientific information from the existing studies.

The collaboration between researchers at the University of Copenhagen and NASA also highlights the international scientific cooperation made possible by the ISS.

NASA researchers Sara R. Zwart and Scott M. Smith contributed to the study. University of Copenhagen researchers Giorgia La Barbera, Jan Stanstrup, Henrik M. Roager and Lars Ove Dragsted also contributed.

The simplest assumption is that beyond our cosmic horizon, there is simply more universe. More stars. More galaxies. More AI-generated cat videos. It is similar to standing somewhere on Earth and assuming that beyond your visible horizon, there is still more Earth.

But how large is the entire universe, including everything beyond what we can see?

The frustrating answer is that we may never know.

The Limits of the Observable Universe

The boundary of the observable universe is not merely the farthest distance our telescopes can currently reach. It represents a deeper limit on what information can ever reach us. The universe contains a finite amount of information that can become accessible to us, even if we waited for an arbitrarily long time.

Beyond that boundary, we are left to infer and speculate.

One possibility is straightforward: the universe is infinite. Space simply continues forever with no endpoint.

But another possibility is that the universe is finite.

That immediately raises an obvious question. How can something be finite without having an edge?

Earth provides the answer.

The surface of our planet has a finite area, yet someone traveling along that surface never encounters a boundary where the ground suddenly ends. Yes, Earth has an edge if you leave its surface in the third dimension and head into outer space, but that misses the point. The two-dimensional surface itself is finite and has no border because it curves back around.

How Geometry Reveals Curvature

We do not need to leave Earth to prove that its surface is curved. Mathematics gives us several ways to detect curvature while remaining entirely on the surface.

Triangles provide one example. On a perfectly flat plane, the three interior angles of a triangle add up to 180 degrees. Thank you, Euclid.

But imagine drawing enormous lines between three widely separated cities on Earth. The resulting triangle would not behave like one drawn on a sheet of paper. Its interior angles would add up to more than 180 degrees because the surface beneath it is curved.

Parallel lines offer another clue. On a flat plane, parallel lines never meet. On a curved surface, however, lines that begin parallel can eventually converge.

Imagine that you and I begin at different points along the equator and travel straight north. Eventually, our paths meet at the North Pole. Neither of us turned toward the other. The surface of Earth curved beneath us.

Astronomers can perform similar geometric tests on the universe itself.

Ancient Light Tests the Shape of Space

One of the most powerful tools comes from the cosmic microwave background, or CMB.

This ancient radiation was released when the early universe cooled enough for light to travel freely through what had previously been a hot, dense plasma. Scientists understand much of the relevant plasma physics (we have a decent understanding of plasmas here on Earth), which allows them to predict what patterns should appear in this ancient glow.

Those predictions include tiny temperature variations scattered across the CMB.

And those variations are exactly what astronomers observe.

More importantly, scientists can calculate how large those temperature patches should appear. If space were strongly curved, the light traveling toward us across billions of light years would follow altered paths. That distortion would change the apparent size of the patterns.

Astronomers can therefore compare the observed size of the CMB features with the size predicted for different cosmic geometries.

The features appear at essentially the expected scale for flat space.

That is why scientists say the universe appears to be geometrically flat.

So is the mystery solved? Does a flat universe automatically mean an infinite universe?

Not quite.

Flat Does Not Necessarily Mean Infinite

Imagine trying to measure the curvature of Earth while confined to your own neighborhood. The area would look nearly flat. Small triangles would behave almost exactly as Euclid predicted, and short parallel paths would not noticeably converge.

The problem would not be that Earth is actually flat. Your measuring area would simply be too small compared with the size of the planet.

Something similar could be true of the universe.

Our measurements are limited to the observable cosmos. Within that enormous region, space appears extremely flat. But the universe outside our observable patch could be vastly larger.

It is therefore possible that the universe has curvature that becomes apparent only on scales much larger than we can access (I know that tens of billions of light-years isn’t exactly tiny, but it is compared to how big the universe COULD be.)

If so, space might eventually curve back around itself.

In principle, that could mean traveling continuously in one direction would eventually return you to where you started, much as traveling around Earth can bring you back to your starting point.

In practice, the expansion of the universe places regions beyond our cosmic horizon permanently out of reach, so such a journey remains a theoretical thought experiment.

But things can get even stranger.

The Universe Could Be Flat and Still Wrap Around

A universe can be geometrically flat without being infinite in every direction.

To understand how, picture a flat sheet of paper. Draw triangles and parallel lines on it. Now bend the paper into a cylinder without stretching or tearing it.

Locally, the geometry remains flat. The triangles still behave like ordinary triangles, and the parallel lines still behave like parallel lines. Yet one direction now loops around.

This highlights the difference between geometry and topology.

Geometry describes properties such as curvature. Topology describes the broader way a space is connected.

The observable universe appears geometrically flat. But that does not automatically tell us whether one or more dimensions might wrap around and connect back to themselves while remaining locally flat.

Topology allows for some very strange possibilities.

A Mobius strip can be created by connecting the ends of a strip after introducing a twist. A Klein bottle can be thought of as another unusual connected surface. Cylinders, donuts, Mobius strips, and Klein bottles can all illustrate how spaces with locally flat geometry can have very different overall structures.

In three dimensions, mathematicians have identified 17 distinct topologies that can all remain geometrically flat. One particularly interesting example is Hantzsche Wendt space, which involves repeating structures related to hexagonal patterns.

Scientists Have Looked for Cosmic Loops

Astronomers have searched for evidence that the universe has a closed topology.

One strategy is to look for repeated patterns in the cosmic microwave background. Another is to search for galaxies that might appear in different places on opposite sides of the sky because their light has traveled through a universe that wraps around itself.

So far, researchers have found no convincing evidence that this happens.

Based on everything we can currently observe, the universe appears both geometrically flat and topologically simple. In other words, there is no detected sign that its dimensions loop back on themselves.

But once again, our cosmic horizon creates a fundamental limitation. If the scale of any such looping is larger than the observable universe, we may never be able to detect it.

That means we may never know with certainty whether the entire universe is infinite, finite, curved, or connected in some larger and stranger way.

And then there is the multiverse.

In some cosmological ideas, everything we call the universe, including regions far beyond our observable horizon, might be only one bubble among potentially countless others. Those bubbles could expand away from one another while new regions undergo their own Big Bang-like beginnings.

But that is probably enough cosmic geometry for one day.

Researchers at the University of Florida found that glucosamine use was associated with a greater likelihood that mild cognitive impairment would progress to dementia. Mild cognitive impairment, often shortened to MCI, describes measurable problems with memory or thinking that are greater than expected with normal aging but do not necessarily interfere substantially with everyday life.

The findings come from a large retrospective analysis of patient health records, supported by experiments involving human brain tissue and mouse models of Alzheimer’s disease. The results remain preliminary and will need to be tested in a human clinical trial, but the researchers say they add to growing evidence that disrupted metabolism may play an important role in neurodegeneration.

The study was published in Nature Metabolism.

A Popular Supplement Comes Under Scrutiny

“In the United States, there are about 7 million people living with Alzheimer’s and millions more with related dementias such as Lewy body or frontotemporal dementia,” said senior author Ramon Sun, Ph.D., director of the Center for Advanced Spatial Biomolecule Research and associate director for innovation of UF’s McKnight Brain Institute. “A lot of these people actively take an over-the-counter supplement that could be making their disease progression worse.”

Glucosamine is widely sold without a prescription and is especially popular among older adults who take it for joint discomfort and joint health. Because of how commonly it is used, the researchers wanted to know whether it might influence Alzheimer’s disease and related dementias, known as ADRD.

Working with Yi Guo, Ph.D., and Jiang Bian, Ph.D., the team used artificial intelligence to analyze deidentified UF Health records collected from 2012 to 2024.

The researchers focused on patients diagnosed with either ADRD or mild cognitive impairment, or MCI. In both groups, 8% of patients reported using glucosamine. That included 1,896 people with ADRD and 2,750 people with MCI.

Glucosamine Linked to Dementia Progression

After accounting for age, sex and demographics, the researchers found that glucosamine use was associated with a 25% higher likelihood that mild cognitive impairment would progress to dementia.

Among people who already had ADRD, glucosamine use was also associated with a 25% higher mortality risk, meaning a greater likelihood of death during a defined period of time.

The researchers did not observe that mortality association in the MCI group. That difference suggests glucosamine may have a stronger effect once dementia is already established.

Importantly, the health record findings do not show that glucosamine itself causes dementia to progress. Observational studies can reveal associations, but other differences between people who take a supplement and those who do not may contribute to the results.

“The electronic health record data are very provocative,” said Matt Gentry, Ph.D., chair of UF’s Department of Biochemistry and Molecular Biology and a study co-author. “While it’s an association and not proof of causality, it does raise an important clinical question that now deserves much more attention.”

A Metabolic Pathway May Help Explain the Link

The researchers also uncovered evidence of a biological mechanism that could help explain the association.

Their work points to a metabolic pathway involving the attachment of sugar structures to proteins. This process is a normal and important part of cell biology, but the researchers found signs that it becomes excessively active in Alzheimer’s disease.

Sun said that abnormal activity in this pathway could eventually become a target for new treatments.

“Our results suggest that altered metabolism is a significant contributor to Alzheimer’s progression and, in addition, addressing the metabolic defect could be an important complement to approaches focused on Alzheimer’s plaques and tangles,” Sun said.

Plaques and tangles are two of the best known features of Alzheimer’s disease. Plaques are abnormal deposits of a protein called amyloid beta that accumulate between brain cells, while tangles are twisted forms of the tau protein that develop inside neurons. Much Alzheimer’s research has traditionally focused on these abnormalities, but scientists are increasingly studying additional processes that may contribute to the disease.

Mapping Thousands of Molecules in the Brain

A spatial technology developed in Sun’s laboratory helped researchers examine the metabolic changes in much greater detail.

“This technology allows us to examine thousands and thousands of molecules created when the body breaks down food or drugs and to uncover intricate pathways that otherwise would stay hidden,” Sun said.

The team then focused specifically on glucosamine. Glucosamine is a naturally occurring, sugar-related molecule that can cross the blood-brain barrier, the protective boundary that tightly controls which substances can move from the bloodstream into brain tissue.

Once inside the brain, glucosamine can enter biochemical pathways that construct complex sugar structures and attach them to proteins. Commercial glucosamine supplements can be produced from materials including shellfish shells or corn.

According to the researchers, the effects of glucosamine may depend heavily on the biological environment in which it is acting. A healthy brain and a brain affected by Alzheimer’s may therefore respond differently to the same molecule.

The Alzheimer’s brain appeared to be particularly vulnerable to this metabolic pathway.

Mouse Experiments Point to a Possible Mechanism

To explore the mechanism more closely, the scientists studied genetically modified mice.

Glucosamine significantly increased the attachment of sugar residues to proteins inside cells. At the same time, glucosamine-treated mice developed worse deficits in “social memory” — or memory of recognition — compared with the other animals.

When the researchers used a chemical treatment to suppress the sugar attachment process, memory performance improved.

That experiment suggested that excessive sugar tagging could play a direct role in the memory problems seen in the animals rather than simply occurring alongside them.

Human Alzheimer’s Brain Tissue Shows the Same Pattern

The researchers next examined human brain tissue with Stefan Prokop, M.D.

Brain specimens from people with Alzheimer’s disease, provided by the UF Neuromedicine Brain and Tissue Bank, showed significantly more sugar attachment than tissue from normal controls.

Combined with the mouse experiments and health record analysis, the findings suggest that abnormal metabolism may be more than a secondary consequence of Alzheimer’s disease. The researchers reported that it could instead contribute to the disease process itself.

“Proteins are the cell’s molecular machines, and many of them need sugar tags added in just the right way to fold correctly, travel to the right place and do their jobs,” Gentry said. “What we found in Alzheimer’s is that this sugar-tagging system appears to be overactive. The Alzheimer’s brain is adding too many of these sugar structures, and this seems to contribute to the disease rather than protect against it.”

The results raise a potentially important question about glucosamine use among people with cognitive impairment or dementia, but they do not yet establish that people should stop taking the supplement. A controlled human clinical trial will be needed to determine whether glucosamine directly accelerates Alzheimer’s progression and, if so, which patients may be most vulnerable.

The researchers examined how depression, smoking, obesity, sleep problems, and other health conditions are connected with difficult-to-treat rheumatoid arthritis.

Rheumatoid arthritis is a chronic autoimmune disease in which the immune system mistakenly attacks the joints, leading to pain, swelling, and stiffness. In Hungary alone, it affects tens of thousands of people. Most patients respond well to treatment, but 6-28 percent fall into the so-called “difficult-to-treat” group because they do not achieve lasting remission despite therapy.

According to the studies in Nature Reviews Rheumatology and The Lancet Rheumatology, these additional health problems may do more than simply occur alongside rheumatoid arthritis. They may also contribute to keeping symptoms active.

A Vicious Cycle of Pain, Mood, and Sleep

Pain and depression, for example, can reduce physical activity, contribute to weight gain, and worsen both sleep and mood. Those changes can then intensify pain and make daily activities more difficult, creating a difficult-to-break “vicious cycle.”

The researchers also developed a new model that could improve care for patients whose rheumatoid arthritis remains difficult to manage. Under the widely used “treat-to-target” approach, doctors regularly track measurable indicators of disease activity. If inflammation is not reduced enough, treatment is adjusted, such as by increasing the medication dose or switching to another drug.

The team argues that this same approach can also serve as a kind of “early warning system.” If measurable signs of inflammation improve but symptoms remain, that may indicate the pain or fatigue is not being caused solely, or perhaps at all, by ongoing inflammation.

Looking Beyond Inflammation

“When target values improve but the patient still suffers from pain and fatigue, it is worth taking a step back. In such cases, instead of automatically prescribing more medication, doctors should look for what is maintaining the symptoms – whether it is chronic pain syndrome, depression, sleep disorders, or obesity,” said Dr. György Nagy, head of the Department of Rheumatology and Immunology at Semmelweis University.

The researchers say they have seen this approach improve outcomes in difficult-to-treat patients. In many cases, it may also strengthen the doctor-patient relationship by focusing treatment more closely on what is actually driving the patient’s symptoms.

A Model With Growing International Influence

The team’s model has gained substantial international attention. Publications that introduced the concept of “difficult-to-treat” disease and the related treatment strategy have already been cited more than a thousand times by other researchers.

The definition is now used worldwide not only for rheumatoid arthritis, but also in discussions of other diseases.

AI Could Help Personalize Treatment

The researchers are now looking toward the next stage of the work. Alongside their own studies, they plan to participate in projects that use artificial intelligence to develop more effective treatments for people with rheumatoid arthritis.

“With AI-based pattern recognition, we could identify subgroups among patients, and with the help of these data we could create more effective, almost personalized treatment strategies for them,” explained Dr. Lilla Gunkl-Tóth, PhD student at Semmelweis University and first author of the publications.

Research led by Brandon Boor at Purdue University found that fragrance compounds released by both conventional cleaners and botanical essential oil-based products can rapidly react indoors and generate nanoparticles. If inhaled, some of these extremely small particles can penetrate deep into the lungs.

The researchers say exposure can be reduced by choosing unscented products, improving ventilation with exhaust fans or open windows, and avoiding devices that generate ozone while scented cleaning products are being used.

The researchers presented their results at the fall meeting of the American Chemical Society (ACS) during the “Healthy Indoor Spaces: Bridging the Microbiome and Chemistry” symposium in McCormick Place.

“Importantly, cleaning removes viruses and bacteria from surfaces, but it can also generate invisible air pollution. There’s no visible dust or smoke in the air, but these particles are forming.” — Brandon Boor

Cleaning Can Create Invisible Nanoparticles

“We showed that indoor ozone reactions with fragrances from cleaning produce nanoparticles that carry a respiratory dose comparable to, or greater than, what you would experience from standing outside along a busy road,” says Boor, an Assistant Professor of Civil and Construction Engineering at Purdue University who studies indoor air quality. “The particles are different in terms of their composition, but the total dose can be higher. You’re not seeing smoke, dust, or haze in the air. Instead, you think the air smells great so it must be clean.”

Boor began investigating how cleaning agents and chemical disinfectants affect indoor environments during the COVID-19 pandemic with his colleague Nusrat Jung, a Purdue Assistant Professor of Civil and Construction Engineering. One feature quickly stood out: many of the products people use to clean and disinfect indoor spaces contain strong fragrances.

“That’s often to create a pleasant smellscape in the indoor space,” says Boor. “But clean air should not smell like highly concentrated citrus fruit. It should not really smell of anything.”

Scientists who study the atmosphere have long known that compounds released by plants can react with ozone. Terpenes such as pinene from pine trees, for example, can participate in reactions that generate tiny airborne particles. Over time, those particles can combine and grow until they become large enough to help seed clouds.

That chemistry proceeds relatively slowly in forests because the amount of terpenes in outdoor air is generally low.

Cleaning Products Release High Levels of Terpenes

Indoors, the situation can be very different. Scented cleaners release terpenes when fragrance compounds evaporate from sprayed droplets or cleaned surfaces.

Cleaning liquids commonly contain compounds including pinene, limonene (lemon), thymol (thyme), and linalool (lavender). Their concentrations during cleaning can be far higher than levels typically measured outdoors. Boor says airborne terpene concentrations inside a room during cleaning can rise to tens or even hundreds of times those found in a forest.

To examine what happens under realistic conditions, the researchers tested scented conventional liquid products as well as botanical-containing disinfectant sprays and wipes inside a model home on Purdue’s campus.

The small house includes a functional kitchen, wood flooring, and a bathroom, allowing the researchers to recreate ordinary household cleaning activities. Their experiments showed that the same basic chemistry responsible for nanoparticle formation outdoors can occur indoors much more rapidly and at much higher concentrations, with potentially important consequences for human exposure.

Billions or Trillions of Particles Can Form

Routine tasks such as mopping floors, spraying countertops, and wiping surfaces with scented products generate billions or trillions of particles, with the total depending on the product.

Most were nanoparticles or ultrafine particles measuring only 1-30 nanometers across. Because particles this small often fall outside the detection range of at-home air quality monitors, people may have no indication that particle concentrations have risen.

The researchers found that ordinary cleaning can temporarily push ultrafine particle levels above those measured outdoors.

Their tiny size is important from a health perspective. Ultrafine particles can settle throughout the respiratory tract and reach deep regions of the lungs. Once there, they can contribute to irritation and inflammation in the respiratory system. Some may also have the potential to enter the bloodstream.

One of the biggest surprises was the speed of the process. Particle formation and growth occurred within just minutes.

“By the time you finish cleaning up an indoor space, you’ve already formed a lot of nanoparticles and inhaled them,” says Boor.

Ozone Can Intensify Indoor Particle Formation

More recently, Boor and Ernest Blatchley, a Professor at Purdue, studied what happens when scented surface cleaners are used at the same time as germicidal far-UV (UV-C) lamps designed to disinfect indoor air.

The combination created particularly favorable conditions for nanoparticle formation.

The lamps interact with oxygen in the air and produce ozone. During experiments in the tiny home, ozone concentrations increased to roughly 20 to 40 parts per billion. Those levels were comparable to, although somewhat below, the outdoor ozone concentrations measured when the experiments were performed.

With both elevated ozone and high concentrations of terpenes present, nanoparticle production became even more intense. That combination raised additional concerns about the amount of particulate matter occupants could inhale.

How to Reduce Exposure While Cleaning

Boor emphasizes that the goal is to help consumers make informed choices rather than discourage cleaning. Cleaning remains important for removing viruses and bacteria from surfaces, but several simple measures may reduce exposure to the secondary pollution produced during the process.

The researchers recommend:

• Choose low-fragrance or fragrance-free products.
• Avoid applying several scented products in the same cleaning session.
• Run exhaust fans or open windows to ventilate the space.
• Do not simultaneously clean surfaces with scented products while using ozone-generating devices, such as far UV-C lamps.

“Importantly, cleaning removes viruses and bacteria from surfaces, but it can also generate invisible air pollution,” says Boor. “There’s no visible dust or smoke in the air, but these particles are forming.”

Boor thanks all the graduate students who have worked with him in the tiny house experiments along with the support of undergraduate students.

The research was funded by a National Science Foundation Faculty Early Career Development Program (CAREER) grant and the Alfred P. Sloan Foundation.

Title Indoor atmospheric nanoparticle formation from scented cleaning products

Abstract Scented volatile chemical products, including surface cleaning agents and botanical disinfectants, are widely used indoors and represent a major source of reactive organic emissions. These products are routinely applied in homes and workplaces for surface cleaning and disinfection to reduce the presence of viruses and bacteria. However, their role in driving indoor atmospheric chemistry and nanoparticle formation remains poorly constrained. This presentation investigates the impact of scented cleaning product use on airborne nanoparticle nucleation, growth, and human exposure to secondary pollutants in indoor environments. Field and laboratory experiments were conducted in controlled residential and office settings using real-time, high-resolution measurements of volatile organic compounds and nanoparticle size distributions extending to the nanocluster aerosol (1-3 nm) regime. Surface cleaning and disinfection activities produced rapid increases in terpene and terpenoid mixing ratios (10-1,000 ppb), often exceeding levels observed in outdoor forested environments. These compounds reacted with indoor oxidants, particularly ozone, to initiate intense nanoparticle nucleation and growth events. Observed nucleation rates (~105 cm^-3 s^-1) and condensational growth rates (up to 300 nm h^-1) exceeded typical outdoor values by orders of magnitude, resulting in transient indoor nanoparticle number concentrations of 10⁵-10⁸ cm^-3. Rapid nanoparticle growth enabled survival to sizes that efficiently deposit throughout the human respiratory system, yielding inhalation dose rates comparable to or exceeding those from primary combustion sources such as traffic emissions. Both conventional and botanical cleaning products generated complex multiphase exposure scenarios involving reactive gases and secondary organic aerosol. These findings identify indoor surface cleaning and disinfection as key drivers of indoor atmospheric nanoparticle formation and highlight the need for improved building ventilation, air cleaning, and product formulation to mitigate exposure to secondary pollutants.

As oxygen levels increased for the first time, enormous amounts of microbial material were buried beneath the seafloor. That burial trapped carbon in rocks and left behind an unusual isotopic signature. For decades, many scientists have interpreted that signature as evidence that Earth’s carbon cycle became dramatically unbalanced on a global scale.

A Famous Signal From Early Earth

Evidence supporting that interpretation has come from drill cores — long cylinders of solid rock pulled from deep underground — recovered from ancient marine sediments in Karelia, Russia, and the Francevillian Basin in Gabon. A new study led by Caltech researchers, however, is raising questions about whether the Russian evidence really records a planetwide event.

“One major debate centers on an unusual carbon-isotope signal that has often been interpreted as evidence of a worldwide environmental change,” says Nivedita Thiagarajan (PhD ’12), a senior scientific researcher at Caltech who works in the lab of John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry and Ted and Ginger Jenkins Leadership Chair of the Division of Geological and Planetary Sciences. “We studied gases trapped in microscopic pockets within rocks from the Zaonega Formation in Karelia, Russia, one of the world’s oldest known fossil oil fields, and found that the carbon-isotope signal at this key site can be explained by local phenomena that occurred in a several-hundred-square-kilometer sedimentary basin rather than across the entire globe.”

Thiagarajan is the lead author of a recent paper published in Geology. The study describes how the researchers reconstructed the sequence of changes preserved in Karelia’s rocks following the first major increase in atmospheric oxygen.

Carbon isotopes — heavier or lighter forms of the element — provide information about the origins of biological material that accumulated billions of years ago. By measuring the relative amounts of these isotopes in drill cores (or carbon-isotope signals), researchers can build a record of ancient environmental change, somewhat like reading the growth rings of a tree.

An unusual carbon signal found in both the Zaonega Formation and rocks from Gabon is known as the Shunga-Francevillian event. Scientists have often pointed to it as evidence that Earth experienced a major global disruption of the carbon cycle around 2 billion years ago.

“Earth, in a way, went crazy during that time interval when oxygen appeared in the atmosphere. What we are trying to assess are the causes and consequences of Earth oxygenation,” explains Aivo Lepland, a researcher at the Geological Survey of Norway (NGU) in Trondheim and a co-author on the study. “This information is archived in the rocks, so, in order to study what happened, you have to study rocks.”

Gases Trapped for 2 Billion Years

To examine the Shunga-Francevillian event from another perspective, the researchers analyzed drill cores stored at NGU. They focused on gases sealed inside microscopic fluid inclusions within samples from the Zaonega Formation that are rich in pyrobitumen.

The Zaonega Formation was once part of a marine sedimentary basin. Pyrobitumen is an insoluble form of organic carbon that develops when buried crude oil or kerogen — a source material for natural gas — is subjected to intense heating deep below the surface.

The project took shape after Lepland arrived at Caltech for a sabbatical. He brought with him a new collection of isotope measurements from gases trapped in Zaonega rocks that had not yet been fully interpreted.

At the same time, Thiagarajan and Eiler had recently finished research measuring isotope ratios in natural gases. That work had helped them develop a broader framework for understanding how natural gas forms.

When the researchers combined the two sets of expertise and data, a different explanation for the ancient isotope signatures began to emerge.

Magma, Methane, and Microbes

The team proposes that a sheet of magma forced its way through layers of marine sediment at the Zaonega Formation, which at the time lay beneath a prehistoric ocean. Heat from the magma warmed sediments packed with organic material.

That heating generated hydrocarbons including methane and propane. The gases then moved upward through the sediment and eventually reached microbes living near the seafloor that consumed methane.

Those microbes produced biomass carrying a light carbon isotope signature, potentially explaining the unusual signal preserved in the rocks.

The temperature evidence supports this scenario. The researchers identified a large thermal gradient, with temperatures reaching approximately 350 degrees Celsius beside the magma intrusion and falling to about 72 degrees Celsius at an ancient seafloor asphalt spill roughly 300 meters higher.

“This chain of geological and biological processes can account for the unusual carbon-isotope signal recorded at the Zaonega Formation,” Thiagarajan says. “It was interesting to see that some of the same signatures that we observe in modern oil and gas basins are also there and preserved in 2-billion-year-old samples.”

The researchers emphasize that they cannot completely rule out contributions from other processes. Even so, their results indicate that the carbon isotope anomaly preserved at Zaonega was driven mainly by events within the local sedimentary basin rather than by a global disturbance.

“Because Zaonega is a reference site for the Shunga-Francevillian event, our findings raise important questions about whether it should be considered a worldwide event,” says Thiagarajan.

Testing the Theory in Gabon

The next step will be to determine whether the same explanation can account for the similar isotope signal found in Gabon.

Researchers plan to analyze samples collected through the GOE-DEEP project, co-funded by the International Continental Scientific Drilling Program. The goal is to test whether local geological and biological processes like those identified in Russia also shaped the Gabonese rock record.

In the summer of 2025, Lepland spent four months in Gabon coordinating the drilling campaign. The newly recovered cores arrived at NGU in February and are scheduled to be sampled later this year by an international research team representing 18 countries.

“Now we can really put things together by doing a similar type of study on the Gabonese rocks to compare the two sites,” Lepland says. “This is how science moves forward.”

The Geology paper is titled “Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia.” Additional authors on the study are Florian Eichinger of Hydroisotop GmbH, a natural isotope analysis laboratory in Germany, and Anthony Prave of the University of St. Andrews in Scotland.