Girl holding phone lays on bed in a pink room filled with trash and smog.
Have you ever taken a relaxing sunset cruise or gone kayaking from Pier 96 in the Hudson River around New York City? If so, have you ever spotted a fish there? Neither have I. Due to factors such as climate change and pollutants, aquatic life in the Hudson has seen a great decline over the past couple of decades and is at risk of collapsing entirely .
Climate change and pollution are putting our lives and our planet in grave danger. Besides large-scale consequences, these factors might also be affecting us in covert, subtle ways that have gone largely unnoticed until now . In recent years, scientists have devoted their time and attention to learning more about two very important pollutants—carbon and plastic . Since then, numerous regulations and movements have targeted greenhouse gas emissions like carbon dioxide and plastic products such as straws . But what if the presence and effects of these pollutants go far beyond what is typically discussed in the media? It’s no secret that in an estimated 30 years, we will see the catastrophic long-term effects of climate change culminate here on Earth, affecting our atmosphere and various ecosystems [2, 4]. But in the meantime, pollutants and other environmental toxins have slowly been infiltrating our bodies . One of their targets? Our brains.
Background: Climate Change
The current environmental climate is filled with chaos and uncertainty, and it can often be difficult to discern the facts. Since the early 1990s, the United States Environmental Protection Agency (EPA) has produced an annual report, “Inventory of U.S. Greenhouse Gas Emissions and Sinks,” outlining environmental damage from greenhouse gas emissions, which contribute to the warming of our planet . According to these reports, carbon dioxide (CO2) accounts for 80 percent of these pollutants. In 2019 alone, the US produced 6,558 million metric tons of CO2 equivalents . That is equivalent in weight to nearly 18,000 Empire State buildings [7, 8]. Due to these increasingly high greenhouse gas emissions, scientists estimate that by 2050, the earth will have warmed by 2˚C . Though this temperature increase may seem slight, it can lead to dire long-term consequences for the environment, including rising sea levels, increasing ocean acidity, and decreases in biodiversity . Reducing the amount of CO2 in our atmosphere can mitigate this global temperature increase to only 1.5˚C [4, 9]. Limiting this rise by just 0.5˚C would protect over ten and a half million people from these environmental effects [4, 11].
Climate change will not only threaten us with drastic environmental conditions; it will also infiltrate our society, welfare, and bodies. These effects include but are not limited to increasing the prevalence of vector-borne diseases and inequities across cultures and habitats that rely more heavily on natural resources [4, 11, 12]. Moreover, as our bodies are reflective of our surrounding environment, an imbalance in the atmosphere creates an imbalance in our bodies and our brains.
Air Pollution and Carbon
Pollution is defined as the introduction of harmful substances into our environment . It is the driving factor of climate change and takes many forms . Air pollutants are classified as any type of airborne matter, mostly due to anthropogenic (human-caused) activities, that are found in concentrations high enough to threaten the wellbeing of humans, animals, and vegetation or adequately "toxify" an ecosystem . In addition to threatening cardiovascular and respiratory health, air pollutants have been shown to affect our brains and nervous systems as well [15-17].
Carbon dioxide, a greenhouse gas emitted into the air by our daily activities, is one particularly impactful air pollutant . It is important to remember that carbon is not inherently harmful; in fact, this element makes up all organic matter, including our bodies . However, when carbon is present in high concentrations and in certain forms such as CO2 and particulate matter, it can disrupt Earth’s balance and have detrimental effects . In a controlled experiment that modeled an office space, 24 individuals were exposed to varying levels of CO2 during six full work days . Participants were tested via computer simulation in which they completed complex real-world tasks, such as being the mayor of a town. Results indicated that as CO2 levels rose, strategy, information use, and crisis response among participants decreased . Currently, indoor CO2 levels have increased so significantly that our ability to do complex cognitive tasks may be reduced by half by the year 2100 . While the specific mechanism by which CO2 affects cognition is unclear, some suggest that inhaling large amounts of it alters our cerebral metabolic rate of oxygen consumption, meaning it prevents our brains from utilizing oxygen efficiently .
Keep in mind that all of these studies used CO2 measures of 1,000 to 2,500 parts per million, a level of CO2 believed to already be present in the average indoor space such as schools and office buildings today . In fact, it seems that these levels were already present ten and 20 years ago, and they have only been rising [22, 23].
We have known about the harmful effects of CO2 since 1957, yet its pollution levels are at an all-time high. In 1957, Woodbury et al. showed that in addition to disrupting cognition, CO2 affects neuronal excitability or stimulation . Hyperexcitability in the brain can lead to seizures, which have been implicated in neurological disorders such as epilepsy . Besides inducing seizures or anesthesia, a temporary loss of sensation and often awareness, abnormal changes in cortical excitability have been associated with neurodevelopmental issues, movement disorders, and strokes, which may even contribute to migraine and dementia [16, 25, 26]. Additionally, the study pointed to the ability of CO2 to alter electrolyte distributions which can cause acidification of blood and body tissues, known as cellular acidosis [16, 27]. This can disrupt not only our nervous system but any system in our body .
Pink trash scattered on a desaturated beach.
Carbon pollution does not just come in the form of CO2, however. Particulate matter (PM), also known as particle pollution, is an airborne mixture of solid and liquid particles, many of which are made up of organic material . While some PM, such as dust, soot, and smoke, can be seen with the naked eye, others, which can be as small as 2.5 microns, require an electron microscope to be detected. For reference, a single human hair has a diameter of 50-70 microns. These particles are emitted from power plants, factories, automobiles, and even fires .
Though the EPA has already regulated such pollutants via air quality standards, we are still seeing a significant effect of PM on cognition . One study examined 19,000 older women across the United States and found greater cognitive decline in women exposed to higher levels of PM over a month than their nonexposed counterparts . Airborne exposure was estimated using geographic information system-based spatiotemporal smoothing models. Additionally, long-term exposure (7 to 14 years) seemed to exacerbate these effects . Black carbon, or soot, a type of PM usually produced via diesel exhaust or traffic-related pollution, has been associated with such decline in older men as well .
Not only does exposure to PM impair cognition in old age, but it also can affect learning and memory in young people. A longitudinal study following 200 children from birth to ten years old found that residential exposure to black carbon led to lower scores on memory tests and both verbal and nonverbal intelligence tests . Another longitudinal study in New York, which observed children from the fetal stage to six or seven years old, found that fetal exposure to high levels of polycyclic aromatic hydrocarbons, a type of urban air pollutant released by the burning of fossil fuels, was associated with attention problems as well as mood disorders like anxiety and depression . Similar effects to the two studies above have been shown in mouse models as well. In a study by Fonken et al., mice were exposed to either PM or normal filtered air for five days a week over ten months . The former condition mimicked the PM levels that a human commuter living in the suburbs and working in the city is exposed to, scaled down for a mouse subject. As expected, toxin-exposed mice showed impaired performance and learning in a maze task compared to those exposed to filtered air. These mice also exhibited depressive-like symptoms indicated by decreased effort to stay afloat in a forced swim test and decreased time spent drinking normally-desirable sugar water .
The damage done by PM goes beyond just cognitive and psychological deficits and can even cause changes in brain physiology. In the study by Fonken and colleagues described above, mice exposed to PM had much higher levels of proinflammatory cytokines throughout the brain . Cytokines are signaling molecules that help govern our inflammatory response . However, their malfunctioning can lead to various forms of cancer [35-37]. The same study showed fewer dendritic spines in the hippocampus—an important brain region for spatial memory—of PM-exposed animals [33, 38]. The effect of PM on dendritic spines can be detrimental as well. Dendrites are the inputs of a neuron, gathering information from other neurons . Dendritic spines help form connections between these neurons, which transmit electrical signals throughout the brain and body . As such, low dendritic spine count in the hippocampus is associated with poor memory . This may have contributed to the learning results from the maze tasks . Additionally, low dendritic spine count and growth are associated with various neurodegenerative diseases such as Alzheimer’s Disease, a condition associated with severe memory loss in humans .
The increased inflammation described above was also seen in dogs in Mexico City, one of the most PM-polluted cities in the world at the time the study was conducted . One study dissected and compared subjects’ brains with those from a less polluted city. The high-pollution group showed increased inflammation with a buildup of amyloid plaques and neurofibrillary tangles, two molecular markers indicative of neuropathologies such as Alzheimer’s disease. When viewing this group’s brains, researchers also found traces of metals such as nickel and vanadium, which are types of PM typically resulting from oil-combustion . Similar inflammatory effects were seen in humans when studying adolescents in Mexico City between the ages of eight and 13 . Neuroinflammation can disrupt our blood-brain barrier, a selective network of blood vessels and tissues that regulate the contents allowed to pass to and from the brain . Subjects also showed damage to the prefrontal cortex (PFC) through MRI scans, specifically white matter lesions . The PFC is a key area of the brain for our complex, high-order processes such as learning and cognition . This prefrontal cortex damage can have profound implications for many central nervous system disorders, such as stroke, multiple sclerosis, and even schizophrenia [40, 42]. Not only can too much carbon in the form of CO2 and PM be detrimental to our physiology and our lives, but plastic can as well.
Like carbon, plastic contributes to air pollution and thus climate change. Unlike carbon, plastic is foreign and detrimental to our bodies, even in small amounts . Plastic is a common synthetic material used in many products today, and not only is it airborne, but it is its own category of pollutant as well. We often ingest plastic without even knowing it . One may question why plastic is dangerous and should be abolished—besides saving the turtles, of course. Unfortunately, plastic is non-biodegradable, meaning it is incapable of being naturally degraded by microorganisms . The energy it takes to industrially break it down increases emissions and traps heat in the atmosphere . Furthermore, plastic degrades into microscopic pieces tiny enough to enter and harm animal and human bodies through ingestion, absorption, or inhalation .
One type of plastic pollutant that is especially concerning for parents and health officials is bisphenol A (BPA). BPA is the chemical that makes up polycarbonate plastics, a type of durable or tough plastic found in items such as cars, office supplies, baby bottles, and even digital equipment . BPA migration is a term for the frequent leakage of BPA from these plastics into the food and beverages we consume . One study took urine samples from a large, diverse population of adults across ages, geographic regions, and other demographics to study such leakage . Researchers detected BPA in over 95 percent of samples, suggesting that even moving to the countryside will not protect you from this pollutant . Currently, over a million pounds of BPA are being produced and released into the environment annually .
Even low doses of BPA can have deleterious effects. A study conducted by Yale University and Ontario Veterinary College exposed monkeys to levels of BPA corresponding to the EPA’s safe daily limit to mimic the slow but continuous way in which humans are exposed to this toxin . Researchers saw a significant reduction in connections between brain cells in the hippocampus and PFC. These brain areas and specifically the remodeling of their connections, or spine synapses, have been implicated in learning and memory, mood, and overall cognition. Not only did BPA reduce synaptic connection formation in these areas, even at low levels of exposure, but it abolished the heightened synapse formation response normally seen to estradiol, a type of estrogen hormone . BPA is a synthetic xenoestrogen, or “foreign estrogen,” meaning its molecular structure mimics that of estrogen so well that estrogen receptors mistake it for the real molecule . Thus, the presence of BPA in the body blocks the correct binding of estrogen and promotes other hazardous effects . While the role of estradiol in reproductive and sexual development is well known, it has also been implicated in controlling synaptic formation between brain cells . Monkeys’ endogenous estrogen levels, as discussed in the study above, were controlled by removing the ovaries and administering exogenous estradiol . Another group, however, was also given BPA. As expected, the estradiol group’s synaptic connections increased tremendously (as much as 198.1 percent in the hippocampus and 159.9 percent in the PFC). In the group given estradiol along with BPA, this effect completely dissipated in both of these areas . While this study was done in female monkeys, previous studies by the same lab have shown similar results in gonadectomized female and male rats [51, 52].
BPA’s effects on hormone balance and regulation can even start as early as in utero. BPA has been shown to disrupt neurogenesis, the development of new brain cells, as early as embryonic development [53, 54]. This can impede the synthesis of neuronal networks or connections throughout the brain. Not only does BPA reduce the amount of neural connections and network formations during development, but a study conducted in mice showed that many of the networks that do develop in its presence seem to be malformed or not positioned correctly within the brain . For instance, using cell labeling techniques, it was shown that BPA-treated mice developed more randomized, dispersed networks without convergence whereas control mice exhibited clear networks in specific layers of the cortex with their axonal or projection endings converging by the 12th-week mark of development. Convergence of neural networks represents a neural network learning to respond correctly to a condition within some margin of error. Thus, the BPA-treated mice developed malformed networks as they were unable to converge . A variety of studies have shown similar effects of BPA due to prenatal and perinatal exposure . This can have long-lasting effects on everything from learning to memory to behavior .
Though "BPA-free" reusable water bottles are a great start towards cultivating mindful, eco-friendly lives, other chemicals and toxins in plasticware can still be harmful! Phthalates are chemicals used to make plastic softer, more flexible, and more durable . They’re found in everything from personal care products to baby toys to pharmaceuticals to vinyl flooring, and they exhibit toxic biological effects . Since they are not chemically bound to the plastics they soften, they can easily migrate or leak out into our food, skin, or saliva, causing a host of toxic biological effects .
Di(2-ethylhexyl) phthalate, also known as DEHP, is one of the most common types of phthalates . It is found in common products such as upholstery, perfumes, insulation, shower curtains, mattresses, clothing, and umbrellas. Phthalates like DEHP are even present in very high concentrations in medical tubing and blood bags . Thus, there are many sources of DEHP exposure, including ingestion, inhalation, absorption, and intravenous exposure . DEHP is known to be toxic towards animal and human reproductive systems and development . For instance, like BPA, phthalates are known hormone disruptors, and studies have shown defects in genitalia and pubertal development to be correlated to exposure to DEHP early in life [59, 60]. Other reproductive effects include suppressed estrogen synthesis, inhibited testosterone and sperm production, and reduced serum testosterone levels even at low exposure all of which are necessary for masculinization or development of male brains [61-63].
Close up of a girl's mouth and nose inhaling small brown pollutants.
There is also evidence that phthalates alter neuroplasticity, especially in the hippocampus . Neuroplasticity is a natural mechanism by which we form new neural pathways, strengthen existing ones, and weaken or eliminate unnecessary or rarely used ones. It is also thought to be the basis of learning and memory. Phthalate exposure, however, can lead to burst firing—overaction of certain neuroreceptors. This can result in convulsions, uncontrollable muscle contractions, and neurotoxicity instead of plasticity. Furthermore, phthalates can reduce the length of dendritic spines, thereby reducing or disrupting neural connections, as seen with PM. Perinatal exposure to phthalates had the most severe impact on the brain, as a great amount of neuroplasticity occurs during that time, and again demonstrates the negative impacts of these chemicals on brain development. Furthermore, early exposure during critical periods, important time windows of development, was shown to have long-lasting effects, specifically on spatial and other types of learning, memory, and social development. It is important to note, however, that plasticity occurs throughout adulthood. Therefore, even adults are vulnerable to the harmful effects of phthalates .
Another way phthalates exhibit neurotoxic effects is through ion and enzyme regulation, similar to the effects of CO2 on the brain . For instance, Dhanya et al. studied exposure to DEHP in rats and found reduced Na+/K+-ATPase activity in neural membranes. Na+/K+-ATPase is a critical protein for ion transport and regulation as well as overall cell function, as this protein pump produces ATP, the body’s unit of energy used to carry out critical cellular and metabolic processes . Thus, this reduction in activity coincided with neuronal degradation, as when too little ATP is produced, such processes can’t be carried out, the cell loses its ability to regulate itself, and it can die [64-66]. This decrease in activity has also been implicated in a variety of neurodegenerative and psychiatric disorders, stroke, Syndrome X, and tumors . Furthermore, DEHP puts our homeostasis and essentially every system in our body at risk of disruption .
So… What can we do?
While we know the adverse effects of pollution on the brain, perhaps what is more frightening is that we know very little about the mechanisms by which these effects occur. In fact, there’s even more waiting to be discovered, especially as we continue to be surrounded by an environment with rising temperatures and levels of pollutants.
In the meantime, what can we do to reduce such harm? Besides government-wide mitigation efforts on climate change and regulations on air quality standards, there are many things you can do in your own home and local community to protect yourself, your loved ones, and future generations from harm.
In regards to carbon pollution, spend more time outside, as indoor CO2 concentrations are elevated and only rising . Next, if you have the resources, divest your funds away from carbon-pumping, polluting corporations such as Exxon, Chevron, Shell, and BP that are currently putting all of us in harm's way . Those of us with the financial stability to do so should also try reducing our personal carbon footprint, or the amount of CO2 emissions due to our daily production and consumption habits . Some concrete ways to do this include walking or biking instead of driving for some of your shorter trips, eating more food made or grown locally, and eating less red meat .
Girl wearing glasses with earphones plugged in while writing in a notebook. She is surrounded by round and course particulate matter as well as grey smog.
To protect yourself against the dangers of plastics or plastic-associated particles, try to buy kitchen utensils and baby toys made out of other materials, especially recycled material or metals. The plastic versions of such items are common mechanisms of ingestion of plastic particles, especially BPA . Furthermore, if you cannot afford to buy organic or locally-grown produce, make sure to wash your fruits and vegetables! Many pesticides on fruits and vegetables contain phthalates, and this can be another source of ingestion. There are also organic personal care products and makeup brands that you can buy to avoid absorption of phthalates .
Lastly, vote! Vote for politicians that are aware of the issues at hand, will raise funding for research efforts towards identifying particular pathways and molecules of these neuronal effects, and are committed to making your local community and the world a healthier, less-polluted place.
Not many people think about these minute effects when discussing climate change or pollution. However, it’s important to raise awareness and dispel the notion that they are problems of the future. We are already seeing the effects of climate change in real time. Our planet is not the only reflection of the consequences of our actions on our environment: our bodies and brains are too.
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