Odors, infections, and exhaust fumes: Pollution linked to Alzheimer's disease
The sense of smell may weaken over the years before noticeable memory problems. Scientists are also testing whether infections and polluted air can affect the brain through the olfactory system. About how odors and memory are related, what was found in the brain tissues of residents of polluted cities and what the experiment with diffusers showed, is in the Izvestia article.
Why does Alzheimer's change the sense of smell?
To feel that something smells in the kitchen and to recognize the smell of coffee are different tasks. In the second case, the brain needs to match the signal with an already familiar smell. Therefore, scientists separately check how faint a smell a person is able to detect, whether he can distinguish one smell from another and whether he recognizes a familiar fragrance. It is the recognition of odors that, according to some reports, can deteriorate even before a person notices memory problems.
In 2025, researchers published an analysis of data from the American ARIC project, in which the health of participants has been monitored for many years. Scientists studied the results of odor recognition tests in 4,470 people. Their average age at the time of the examination was 75 years. The relative risk of dementia in participants with severe olfactory impairment, who were classified as anosmia according to the test results, was about 3.5 times higher than in people with a good sense of smell. The calculations took into account age, education, smoking and other factors. The association was especially noticeable in those whose sense of smell remained reduced or deteriorated significantly between examinations.
In another study published in 2022, researchers at the University of Chicago analyzed data from 515 elderly people. A rapid deterioration in the ability to recognize odors with preserved cognitive functions was associated with a lower volume of gray matter in areas involved in odor processing and memory. Among them are the amygdala and the entorhinal cortex associated with the hippocampus.
British mathematician Hannah Fry cites a 2014 paper in which scientists observed 3005 Americans aged 57 to 85 years. After taking into account concomitant diseases, including cancer, the statistical chances of dying within five years for participants with anosmia were 3.37 times higher than for people with a preserved sense of smell. The authors considered the sense of smell as a possible indicator of overall health. It can weaken with age, after respiratory infections, with inflammation of the sinuses and nasal polyps.
How changes in the brain affect the sense of smell
One of the signs of Alzheimer's disease is the accumulation of altered tau protein inside neurons. The other is plaques of beta—amyloid between the cells. In April 2026, the authors of a paper at Alzheimer's & Dementia compared the distribution of tau protein at different stages of the disease.
At earlier stages, accumulations of tau protein were detected in the layer of the anterior olfactory nucleus. The peripheral layers of the bulb were affected later. The test results were consistent with this. The recognition of familiar odors worsened even before the pronounced cognitive symptoms, the ability to distinguish odors later. Sensitivity to faint odors in this sample decreased at the stage of dementia. The authors considered that this sequence is consistent with the spread of changes from the central parts of the olfactory system to the peripheral ones.
Can an infection enter the brain through the nose
The blood-brain barrier regulates the transfer of substances from the blood into the brain tissue. The olfactory system has a direct anatomical connection to the brain. Neurons in the upper part of the nasal cavity react to odorous substances, and their fibers pass through small openings of the latticed plate to the olfactory bulb. This connection has interested researchers who study how infections can affect the nervous system.
In a paper published in Scientific Reports in 2022, scientists studied the bacterium Chlamydia pneumoniae in mice, which can cause respiratory tract infections. In the first three days after infection, the bacterium was found in the olfactory and trigeminal nerves, the olfactory bulb and the brain of animals. In the olfactory system, near the areas where the bacterium was present, the researchers found accumulations of beta-amyloid. The experiment showed that such a reaction is possible in animals. Its role in the development of Alzheimer's disease in humans remains a hypothesis.
The interest in this hypothesis is related to the possible role of beta-amyloid in the immune response. In an experimental study in 2016, scientists obtained evidence that this protein fragment can help the body resist infection. This raised the question of whether some amyloid deposits can form in response to the presence of microorganisms. To link this reaction to Alzheimer's disease, it is necessary to establish whether it precedes the characteristic changes in the human brain and persists after the end of the infection.
What was found in the brains of the inhabitants of Mexico City
Another hypothesis concerns air pollution. In a 2003 study, scientists compared the tissues of 40 dogs. Of these, 26 lived in the Mexico City metropolitan area, where pollution levels were high, and 14 lived in a less polluted area in the state of Tlaxcala. In dogs from Mexico City, signs of inflammation and deposits of beta-amyloid were more often found in brain tissues. The researchers also found nickel and vanadium. Concentrations of these metals were higher in the olfactory mucosa, lower in the olfactory bulb, and even lower in the frontal cortex. This distribution has become one of the arguments in favor of the possible penetration of particles through the nose.
The researchers then turned to human tissues. In 2004, they compared postmortem samples of residents of cities with different levels of pollution. In subsequent studies, nanoparticles were found in the brain tissues of young residents of Mexico City, which the authors associated with combustion and wear products. Among the possible sources, scientists named transport emissions and particles formed during braking.
In 2018, scientists studied the postmortem samples of 203 residents of the agglomeration of Mexico City. The average age in the sample was about 25 years old. Early changes in tau protein were also found in children, including an 11-month-old child. Early deposits of beta-amyloid were also found in adolescent samples. Such samples can be used to assess the condition of the tissues at the time of death. To assess how these changes affect memory and the risk of dementia in the future, observations of living people are needed.
In 2025, the authors of a paper in JAMA Neurology analyzed autopsy materials from 602 people. In people exposed to higher concentrations of fine PM2.5 particles, the tissue changes characteristic of Alzheimer's disease were more pronounced. The pollution level was assessed at the place of residence. This approach made it possible to compare the effects of polluted air with the state of the brain, and the specific path of exposure in work was not checked separately.
In the same year, a meta-analysis was published in The Lancet Planetary Health. The part devoted to PM2.5 particles up to 2.5 microns in diameter brought together 21 studies with more than 24 million participants. With prolonged exposure, every additional 5 micrograms/m was associated with an increase in the relative risk of dementia by an average of 8%. The estimates in individual papers varied markedly. Among the possible mechanisms, scientists are studying the effects of pollution on blood vessels and inflammatory processes throughout the body.
Can odors improve memory
In 2023, scientists at the University of California at Irvine tested whether regular exposure to odors affects the memory of older people. The study included 43 people aged 60 to 85 years without cognitive impairment. The participants used diffusers at night for six months. The odors were distinct in one group, barely noticeable in the control group.
A significant difference was found in the task where it was necessary to recall the words after the fifth presentation of the list. In the group with distinct flavors, the result improved in six out of 12 people, in the control group — in three out of 11. The 226% indicated by the authors relate to the difference in the dynamics of the result of one task between the groups. No statistically significant differences were found in other cognitive tasks.
The 2025 meta-analysis combined six randomized olfactory training trials with 302 participants. The authors found a slight improvement in individual memory indicators. There was no significant effect in the overall assessment of cognitive functions. To find out whether olfactory training can affect the risk of dementia, the researchers will have to observe participants for longer and compare how often they develop cognitive impairments years later.
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