"The True Dangers of Air Pollution: Not Just the Lungs, but from Gut Bacteria to Atherosclerosis"

"On days when the air quality is poor, my throat hurts" and "I cough"—when thinking of the effects of air pollution, most people likely first think of the lungs and airways. However, recent research has revealed a deeper route: "Inhaled pollutants can change the 'gut,' and these changes can extend to the 'heart (blood vessels).'"

On December 16, 2025, a news release from UCLA Health reported that ultrafine particles, extremely small particulate matter, can significantly disrupt the gut microbiota in mice, potentially worsening atherosclerosis and metabolic stress.UCLA Health

Key Points of the Study: Do inhaled particles connect to "atherosclerosis" via the gut and liver?

According to the release from UCLA Health, the research team divided mice into groups exposed to "ultrafine particles" and groups exposed to clean filtered air, comparing them under conditions of 6 hours per session, 3 times a week for 10 weeks. The results showed that mice inhaling ultrafine particles experienced significant changes in their gut microbiota, which were strongly associated with increased atherosclerotic plaques in major arteries.UCLA Health

Moreover, the effects are not limited to the gut. In the group exposed to ultrafine particles,

• an increase in **short-chain fatty acids (SCFAs)** in feces

• an increase in the oxidative stress marker **malondialdehyde (MDA)** in the liver

• an increase in the expression of antioxidant genes and endoplasmic reticulum (ER) stress response genes in the liver

were observed as "signs related to metabolism and inflammation."UCLA Health

The same findings are reinforced in the abstract of the academic paper published in Environment International. The paper states that using **ApoE-deficient mice (a model prone to atherosclerosis)**, exposure to ultrafine particles altered the gut microbiota, which was associated with worsened lesions in the brachiocephalic artery and aorta, and changes in SCFAs, liver MDA, and ER stress response genes correlated with changes in the gut microbiota.ScienceDirect

Why does the gut change when "inhaled"?

Intuitively, it seems like "inhalation = lungs" would be the end of the story, but particulate matter can reach the digestive tract through various routes. For example, particles adhering to the airways can be transported to the pharynx via mucociliary clearance and swallowed with saliva and mucus, as previously explained.PubMed

In other words, "inhaling" can also lead to "delivering some to the gut." There exists a vast ecosystem of gut microbiota that can lose balance in response to external stimuli (particles, chemicals, inflammation signals).

What makes this study intriguing is that the changes in gut microbiota serve as a "transit point," connecting oxidative stress/ER stress in the liver and the progression of atherosclerosis in a single line.ScienceDirect

What are ultrafine particles (UFP)? A world "even smaller" than PM2.5

"PM2.5" is familiar in Japan, but ultrafine particles are even smaller, generally referring to particles with a diameter of less than 100 nm (0.1 µm) (this size definition is also used in the EU's new framework).EMEP CCC

The smaller the particles, the more their "number" increases for the same mass, their surface area increases, and their reactivity in the body can change, which is troublesome.

Moreover, as a practical issue, there is a debate that ultrafine particles are less likely to be regulated and monitored separately from PM2.5. The U.S. standards focus on "mass concentration" like PM10 and PM2.5, making it difficult for ultrafine particles to be treated independently, as pointed out by researchers and policymakers.UC Davis Environmental Health Sciences Center
On the other hand, in Europe, it has been reported that the expanded monitoring of ultrafine particles (UFP) is included in the revised framework.European Environment Agency
The WHO's Air Quality Guidelines (2021 edition) also present the concept of "good practice" when numerical targets cannot be set for ultrafine particles and others.World Health Organization

As the mechanisms indicated by research accumulate, the question "Is it enough to only look at PM2.5?" becomes more realistic.

What changes when the perspective of "gut→liver→vessels" increases?

It's important not to misunderstand that "balancing gut bacteria can beat air pollution." This study was conducted on mice, with specific models (ApoE-deficient) and exposure conditions.ScienceDirect

However, the value of having more perspectives is significant.

• The possibility that not only the lungs and heart but also "metabolic organs" throughout the body, including the digestive tract and liver, may be involved

• The recognition that "air issues" can be as much a background of metabolic stress as diet and lifestyle

• In the future,research may progress in evaluating exposure effects using gut microbiota and metabolic markers as clues.

In fact, UCLA has been working on this theme for some time. In 2017, research was introduced suggesting that exposure to ultrafine particles could alter the gut microbiota, potentially leading to changes in blood cholesterol and atherosclerosis.UCLA Health

The 2025 study can be seen as an update, adding "concrete metabolic footholds" such as liver stress and SCFAs to the previous findings.ScienceDirect

Reactions on Social Media (Summary of Post Trends)

※The following is a summary of "points of discussion" seen on social media regarding the same theme as this article (air pollution × gut microbiota × systemic effects), compiled by the editorial team from public posts and comments (avoiding definitive quotes from specific individuals).

1) Surprise at the fact that "air pollution ≠ just lungs"

On LinkedIn, there are posts introducing the angle that "air pollution can disrupt the microbiome (gut bacteria)," perceived as surprising and as a "health pathway often overlooked."LinkedIn

2) Realization of "everyday exposure" (commuting, traffic, urban areas)

The topic of air pollution is often discussed not only in terms of extreme smog but also as a concern for being "in places with heavy traffic every day." Comments mentioning traffic and daily exposure are indeed observed.LinkedIn

3) Voices calling for both "individual measures" and "policy/structural" changes

Opinions emphasizing a two-pronged approach are prominent, stating, "We do what we can, but ultimately we need to reduce emissions at the societal level." Comments typically discuss individual actions alongside "larger systemic changes."LinkedIn

4) Cautious views on "applying mouse research to humans"

While acknowledging the interest of the research, cautious reception is common in health-related topics on social media, with calls for "verification in humans" and "not misjudging the distance of causality." For this case, the next step required is likely "reproducibility in humans."ScienceDirect

What should we do now? (A realistic conclusion)

The research indicates that "the impact of air pollution is broader than it appears." Countermeasures involve both individual ingenuity and societal rule-making.

• Individual Level: Check air quality information from municipalities or apps, consider outdoor activity adjustments, ventilation timing, and indoor environment management

• Societal Level: In addition to PM2.5, how to establish frameworks for monitoring and evaluating ultrafine particles (there is a movement for expanded monitoring in Europe)European Environment Agency

The new perspective that "the gut might be an entry point" helps reframe air pollution from a "distant environmental issue" to a "risk factor for metabolism and circulation." The next report will likely focus on how much this route can be reproduced in human studies.ScienceDirect