Dirty air can harm your brain and stress the body | Science News for Students

Dirty air can harm your brain and stress the body

Studies show pollution can alter brain function in students and disrupt hormones
Apr 26, 2018 — 6:45 am EST
Mexico City

Mexico City is one major urban area frequently plagued by heavy air pollution. Studies conducted there and elsewhere show now link such pollution with lesions in the brains of adults and even children.


The year was 1952. The setting: London, England. “On December 8, cool air from across the English Channel settled over the Thames River valley and did not move. London’s 8 million residents did what they had been doing for centuries: They huddled indoors and warmed themselves by their coal stoves. Smoke ran like tap water from a million chimneys. In the motionless air, the hot vapors chilled and, instead of rising, settled back to the ground. The smoke became so thick that visibility dropped to near zero.”   

 — Devra Davis, When Smoke Ran Like Water, Basic Books 2002

In December 1952, an event known as the Great Smog blanketed London, England in a toxic haze. In just five days, 4,000 people died from breathing-related problems.
NT Stobbs/Wikimedia Commons (CC BY-SA 2.0)

That super-thick fog of pollution, 65 years ago, turned the air into a toxic soup that lasted five long days. Local reporters would call it the Great Smog. Inhaling the blackened air sent 150,000 people to the hospital with breathing problems. In all, some 4,000 would die. This disaster provided some of the strongest, early evidence that urban air could prove deadly.

It evolved owing to an unfortunate combination of bad weather and especially heavy pollution. Yet even today, air pollution sickens and kills people. Lots of people. A 2016 study reported that breathing dirty air is now the fourth-leading cause of deaths worldwide.

Air pollution tends to pose the biggest risk to the very old, to the very young and to people already suffering from some chronic ailments. What types? Asthma (a breathing disorder) and heart disease are two major conditions that put people at risk.

But scientists are learning that air pollution can pose serious risks to anyone. You don’t have to be old or sick. You don’t have to suck in horrible fumes or air so full of pollutants that you can see and taste them.

Indeed, emerging data show that even pollutants too small to see with the naked eye can harm healthy children and teens. That pollution can alter how their brains function. It can make it hard for kids to concentrate. It also can throw out of whack hormones — chemical messengers that direct the body’s activities. In short, it can seriously damage young minds and bodies.  

Polluting the brain

Lilian Calderón-Garcidueñas works at the University of Montana in Missoula. As a pathologist, she’s a doctor who looks at the body’s tissues to diagnose disease. She looks for disease caused by pollutant particles that are small — often way too tiny to see. Scientists call these tiny specks particulates. They come from many sources. Power plants, factories, homes and cars — all spew particulates. So can forest fires.

Once inhaled, these particles can move deep into the lungs. The oxygen you breathe passes from the lungs through a thin membrane. From there it enters the blood. Many particulates are small enough to cross that membrane into the blood, too. They’re known as nanoparticulates. They can trigger inflammation wherever they travel. And the blood can carry them everywhere, even to the brain.

Some particulates may enter the brain more directly. If breathed in through the nose, they can contact nerves that bring scent signals to the brain through a structure known as the olfactory bulb. Just as the pollutants’ small size allows them to slip into blood through lung membranes, nanoparticles’ size lets them enter that bulb’s nerve cells. And from there, those inflammatory pollutants can climb into the brain.

Inflammation can be a good thing. The body uses it to kill off damaged cells and harmful germs. But inflammation in the brain is dangerous. It can destroy sensitive cells, causing memory problems.

Digging into the brain

Mexico City, where Calderón-Garcidueñas does her research, is home to nearly 9 million people. Every day, they get around using some 3.5 million cars. And the cars’ exhaust pollutes the air. “High-traffic roads are a very important source of particulate [pollution],” notes Calderón-Garcidueñas.

traffic jam
Automobile exhaust is a major source of air pollution. It’s been linked to health problems in adults and kids.

Research has shown that older adults who live in areas with lots of traffic-related air pollution are more likely to suffer Alzheimer’s disease than are people at cleaner sites. Alzheimer’s causes a type of brain damage that results in memory loss and other problems with thinking, language and behavior.

Alzheimer’s symptoms usually show up in old age. Yet researchers believe the disease may start years — even decades — earlier. Calderon-Garcidueñas wants to know just how early. The answer, she says, might one day help researchers prevent some cases of this memory-robbing disease.

Some elderly dogs can develop brain abnormalities seen in people with Alzheimer’s disease. Waxy clumps of protein, called plaques (PLAKS), may start to litter their brains. But 15 years ago, Calderon-Garcidueñas reported finding these same plaques in the brains of 11-month-old pups! The dogs had been living outdoors, exposed to Mexico City’s heavy air pollution.

At the time, she told Science News, this is “definitely worrisome.” Even more concerning was her finding of the same type of plaques in the brains of seemingly healthy Mexico City children. The plaques hadn’t caused symptoms. She was only able to find those plaques because of autopsies done on kids who had died in car crashes or other accidents.

But not all children had them. Those living in distant suburbs, breathing cleaner air, had no brain plaques. Air pollution seemed to explain these brain lesions.

Later, Calderon-Garcidueñas showed that teenagers in Mexico City had more problems with memory and attention than did teens in less-polluted cities. We were picking up these problems with attention and thought-processing “that appeared to be linked to high pollution areas,” she said. Her next step was to look at how that might happen. She wondered whether there could be clues in a person’s genetic code.  

When pollution conspires with our genes

Today, Calderón-Garcidueñas studies how the genes that some people were born with might combine with air pollution to raise their risk of memory problems, including Alzheimer’s disease. Her work has focused on a gene called APOE. That’s short for apolipoprotein E (AY-poh-lih-poh-PRO-teen E). This gene gives the body instructions for making proteins that help it process the fats in food.

Alzheimer’s plaques
Large circular blobs near the center and top of this image are clumps of protein, called plaques, in the brain of a patient with Alzheimer’s disease. This disease causes problems with memory, thinking, language and behavior.
KGH/Wikimedia Commons (CC BY 3.0)

People can be born with any of a few slightly different versions of the gene. Those with the type called APOE 4 have a higher risk of developing Alzheimer’s disease. Calderon-Garcidueñas recently studied children and teens with this version of the gene. Some lived in Mexico City; others in cleaner towns. Of the two groups, those in Mexico City were more likely to have developed the telltale brain plaques. Her team described those findings in the October 2016 Environmental Research.

In a more recent update, Calderon-Garcidueñas’ team found further support for pollution’s link to Alzheimer’s disease. And their latest data are the most disturbing. They viewed the brains of 203 people who had died in Mexico City — from babies (11 months old) to middle-age adults (40 years old). Abnormal proteins that serve as “hallmarks” of Alzheimer’s disease showed up in 99.5 percent of those brains. The researchers conclude that Alzheimer’s disease actually can start in early childhood. And the disease had progressed at a faster pace in those people with the APOE 4 gene.

Details of that work appear in the July 2018 Environmental Research.

“Our findings show that kids need to be protected against air pollution,” says Calderon-Garcidueñas. Most kids can’t control where they live. It might be hard for them to avoid air pollution if they live on a busy road or in a city with lots of cars. That means it’s all the more important to stay away from other substances that can hurt the brain, Calderon-Garcidueñas says. Such things include alcohol, drugs and tobacco. “It’s important to lower the exposures they can control,” she says of kids in polluted communities.

Air pollution is linked to poorer attention

You might not be able to see it or feel it, but air pollution can vary a lot from one day to the next. Such fluctuations could lead to similar day-to-day changes in how a child’s brain functions, says Jordi Sunyer. That’s what his work shows. As an epidemiologist, Sunyer studies the link between pollutants and disease. He coordinates a child health program at the Barcelona Institute for Global Health, ISGlobal. It’s in Barcelona, Spain.

“Attention is critical for school success,” Sunyer points out. It’s the first step in the learning process. Students who are easily distracted will have a harder time focusing on information and remembering it.

school kids
Researchers found that air pollution can reduce kids’ attention, which is important for learning. What’s more, some of the most dangerously small pollutants won’t show up as haze. They can be present even on clear, sunny days.

“We know there are many factors throughout the day that can cause attention to fluctuate,” Sunyer says. Feeling sleepy can make it harder to stay focused on a task. So can being too hot or too cold. Sunyer had a hunch that air pollution could do this, too.

So his group studied 2,687 children. All were 7 to 10 years old. Each attended one of 39 different Barcelona schools.

Every participant took a computer test four times during the school year. It contained a number of tasks that required paying attention. For instance, one asked kids to watch fish swim in a stream. They were told to click left or right depending on which direction the fish swam. “It’s a simple task, but when kids get bored and lose attention, they start to click the wrong answer,” Sunyer notes.

The researchers also measured levels of air pollution on days that the kids had been tested. Then they compared the kids’ scores when pollution levels in Barcelona had been relatively high or low. And they showed that the kids scored slightly worse on days with higher levels of certain air pollutants. Which types? Nitrogen dioxide and soot (elemental carbon). Both can be spewed by traffic and industrial smokestacks. The researchers shared their results in the March 2017 issue of Epidemiology.

Sunyer now is focused on helping schools in Barcelona identify ways to lower levels of the pollution to which kids are exposed.

For instance, schools could create more zones around them where cars and buses aren’t allowed to idle, meaning run their engines while parked. Another idea: Plant more trees and greenery around schools. Plants can help remove pollutants from the air, Sunyer points out. They do this by absorbing gases and small particles through their leaves and roots.  

Air pollution can boost stress hormones

Another reason to control air pollution: It can stress out the body.

Imagine you’re trapped in a room that’s filling with smoke. Your heart is racing. You jump into action. In just seconds, you open the window and climb out the burning building to reach safety. The body’s quick reaction to such an emergency is called the fight-or-flight response.

Scary events, such as a fire, aren’t the only things that can activate that fight-or-flight response. Air pollution can too, new data show. It does so through hormones.

Hormones are the body’s chemical messengers. They control many important activities. Stress hormones help people react quickly to life-threatening situations. When we’re faced with a fearful situation, the body ramps up production of certain hormones, such as adrenaline (Ah-DREN-uh-lin). Breathing polluted air can trigger a similar response, researchers in China now report.

Huichu Li works at Fudan University, in Shanghai. He and his colleagues studied 55 students attending college in that city. Air pollution there is some of the worst in the world. The researchers gave all of these students air purifiers to use in their dormitory rooms. Half received working air purifiers. These helped remove particulates from the air. The other half received real air purifiers that appeared to work — but didn’t. The researchers had removed a filter. Now the devices could no longer remove pollution particles from the air.

The students used the air purifiers for nine days. Afterward, the researchers collected blood and urine from each student. Those with nonworking air purifiers had higher levels of stress hormones in their blood.

In an emergency, stress hormones and the fight-or-flight response can be helpful. But when the body ups their production too often or for too long, those hormones can harm the heart and blood vessels. Fortunately, the students in this test were healthy. None were sickened by their 9-day exposure.

Still, their bodies were “clearly having harmful reactions to the . . . air pollution,” notes Robert Brook. He’s a doctor of internal medicine at the University of Michigan in Ann Arbor. He was not involved in the study. But Brook can understand its impacts. He has been focusing on how air pollution affects the heart.

Brook worries that repeated short-term exposures to air pollution may add up. “Over many years of exposure, these small health effects might lead to more long-term serious health effects”. After years of exposure, he says, “That could place healthy young people at risk of developing high blood pressure, diabetes or even heart diseases or strokes.”

The Fudan University team published its results last August in the American Heart Association journal Circulation. Future studies, its authors say, should look at whether using air purifiers in homes and buildings might, over time, help protect health.

New studies like the one published by the Fudan team show how far we’ve come in understanding the health effects of air pollution since London’s Great Smog. Yet as in many areas of science, “there’s still a lot to learn,” says Brook. He says one thing is becoming more and more clear: Everyone — even healthy young people — can be harmed by air pollution.

Epidemiologist Jordi Sunyer talks about the short-term effects of air pollution on school children.

Power Words

(more about Power Words)

activate     (in biology) To turn on, as with a gene or chemical reaction.

adrenaline     A hormone produced by glands (adrenal) when someone is stressed by fear, anger or anxiety. It can make the heart beat faster and allow muscles to perform better than normal. Adrenaline is part of the body’s “fight or flight” response to stress. It can briefly help someone run faster or temporarily boost the performance of muscles (as for lifting weights).

Alzheimer’s disease     An incurable brain disease that can cause confusion, mood changes and problems with memory, language, behavior and problem solving. No cause or cure is known.

asthma     A disease affecting the body’s airways, which are the tubes through which animals breathe. Asthma obstructs these airways through swelling, the production of too much mucus or a tightening of the tubes. As a result, the body can expand to breathe in air, but loses the ability to exhale appropriately. The most common cause of asthma is an allergy. Asthma is a leading cause of hospitalization and the top chronic disease responsible for kids missing school.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

blood pressure     The force exerted against vessel walls by blood moving through the body. Usually this pressure refers to blood moving specifically through the body’s arteries. That pressure allows blood to circulate to our heads and keeps the fluid moving so that it can deliver oxygen to all tissues. Blood pressure can vary based on physical activity and the body’s position. High blood pressure can put someone at risk for heart attacks or stroke. Low blood pressure may leave people dizzy, or faint, as the pressure becomes too low to supply enough blood to the brain.

blood vessel     A tubular structure that carries blood through the tissues and organs.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chronic     A condition, such as an illness (or its symptoms, including pain), that lasts for a long time.

circulation     (adj. circulatory) A term that refers to the pumping of some fluid repeatedly throughout a system of vessels. (in medicine) The pumping of blood through the arteries and smaller types of vessels (and from there into other organs and tissues).

colleague     Someone who works with another; a co-worker or team member.

diabetes     A disease where the body either makes too little of the hormone insulin (known as type 1 disease) or ignores the presence of too much insulin when it is present (known as type 2 diabetes).

diagnose     To analyze clues or symptoms in the search for their cause. The conclusion usually results in a diagnosis — identification of the causal problem or disease.

disorder     (in medicine) A condition where the body does not work appropriately, leading to what might be viewed as an illness. This term can sometimes be used interchangeably with disease.

epidemiologist     Like health detectives, these researchers figure out what causes a particular illness and how to limit its spread.

exhaust     (in engineering) The gases and fine particles emitted — often at high speed and/or pressure — by combustion (burning) or by the heating of air. Exhaust gases are usually a form of waste.

factor     Something that plays a role in a particular condition or event; a contributor.

fat     A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.

fight-or-flight response     The body’s response to a threat, either real or imagined. During the fight-or-flight response, digestion shuts down as the body prepares to deal with the threat (fight) or to run away from it (flight).

filter     (in chemistry and environmental science) A device or system that allows some materials to pass through but not others, based on their size or some other feature.

fluctuate     (n. fluctuation) To vary at irregular intervals and often by amounts that are hard to predict.

gene     (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic     Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

germ     Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

high blood pressure     The common term for a medical condition known as hypertension. It puts a strain on blood vessels and the heart.

hormone     (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body.

inflammation     (adj. inflammatory) The body’s response to cellular injury and obesity; it often involves swelling, redness, heat and pain. It also is an underlying feature responsible for the development and aggravation of many diseases, especially heart disease and diabetes.

journal     (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject.

lesion     A tissue or part of the body that shows damage from injury or disease. Lesions come in all shapes and sizes, both inside the body and on its outside. A pus-filled wound on the skin is one example. Cells with holes in them or missing parts due to disease represent a totally different class of lesions.

membrane     A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

nitrogen dioxide     A gas — often abbreviated as NO2 — that has become an important pollutant in urban air. Most of this gas in the Earth’s lower atmosphere has been released by human activities. Gasoline- and diesel-fueled vehicles are among the most common sources of NO2 in cities. Coal- and natural-gas-burning power plants also can emit regionally high amounts. This pollutant can cause breathing problems and aggravate lung-related diseases such as asthma. Nitrogen dioxide is among the pollutants that contribute to the formation of smog and acid rain.

olfactory bulb     A region in the front of the brain that receives information from smell-receptor nerves in the nose (and nasal cavity).

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

particle     A minute amount of something.

particulate     A tiny bit of something. A term used by pollution scientists to refer to extremely tiny solid particles and liquid droplets in air that can be inhaled into the lungs. So-called coarse particulates are those with a diameter that is 10 micrometers or smaller. Fine particulates have a diameter no bigger than 2.5 micrometers (or 2,500 nanometers). Ultra-fine particulates tend to have a diameter of 0.1 micrometer (100 nanometers) or less. The smaller the particulate, the more easily it can be inhaled deeply into the lungs. Ultra-fine particulates may be small enough to pass through cell walls and into the blood, where they can then move throughout the body.

pathologist     Someone who studies disease and how it affects people or other infected organisms.

plaque     An accumulation of materials in the body from the fluids that move through an area or bathe it. They can be minerals, proteins or other substances that collect as deposits. (in heart disease) Fatty deposits that accumulate in vessels as a result of a disease known as atherosclerosis. This plaque is made up of fat, cholesterol and other substances carried by the blood. Eventually these deposits will harden and narrow the internal openings of the arteries. This reduces the flow of oxygen and blood to organs throughout body. 

pollutant     A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

power plant     An industrial facility for generating electricity.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

resident     Some member of a community of organisms that lives in a particular place. (Antonym: visitor)

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

smog     A kind of pollution that develops when chemicals react in the air. The word comes from a blend of “smoke” and “fog,” and was coined to describe pollution from burning fossil fuels on cold, damp days. Another kind of smog, which usually looks brown, develops when pollutants from cars react with sunlight in the atmosphere on hot days.

smoke     Plumes of microscopic particles that float in the air. They can be comprised of anything very small. But the best known types are pollutants created by the incomplete burning of oil, wood and other carbon-based materials.

soot     Also known as elemental carbon, it's the residue of incompletely burned materials, from plastics, leaves and wood to coal, oil and other fossil fuels. Soot particles can be quite small — nanometers in diameter. If inhaled, they can end up deep within the lung.

stress     (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional or behavioral reaction to an event or circumstance (stressor) that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative. (in physics) Pressure or tension exerted on a material object.

stroke     (in biology and medicine) A condition where blood stops flowing to part of the brain or leaks in the brain.

tissue     Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

urban     Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs. The development or buildup of urban areas is a phenomenon known as urbanization.

vapors     Fumes released when a liquid transforms to a gas, usually as a result of heating.


Journal:​​L. Calderón-Garcidueñas et al. Hallmarks of Alzheimer disease are evolving relentlessly in Metropolitan Mexico City infants, children and young adults. APOE4 carriers have higher suicide risk and higher odds of reaching NFT stage V at ≤ 40 years of age. Environmental Research.Vol. 164, July 2018, p. 475. doi: 10.1016/j.envres.2018.03.023.

Journal:​​L. Calderón-Garcidueñas et al. Interactive and additive influences of gender, BMI and Apolipoprotein 4 on cognition in children chronically exposed to high concentrations of PM2.5 and ozone. APOE 4 females are at highest risk in Mexico City. Environmental Research. Vol. 150, October 2016, p. 411. doi: 10.1016/j.envres.2016.06.026.

Journal:J. Sunyer et al. Traffic-related air pollution and attention in primary school children. Epidemiology. Vol. 28, March 2017, p. 181. doi: 10.1097/EDE.0000000000000603.

Journal:H. Li et al. Particulate matter exposure and stress hormone levels: A randomized, double-blind crossover trial of air purification. Circulation. Vol. 136, August 15, 2017, p. 618. doi: 10.1161/CIRCULATIONAHA.116.026796.

Journal: R. Brook and S Rajagopalan. “Stressed” about air pollution: Time for personal action.” Circulation. Vol. 136, August 15, 2017, p. 628. doi: 10.1161/CIRCULATION.117.029688