The effects of pollution on the brain are felt throughout the lifespan. Prenatal exposure to pollution is associated with an increased risk of negative fetal outcomes, including central nervous system damage and developmental delays. Exposure to pollution can lead to neurodevelopmental disorders involving brain development, cognitive function, intellectual development, and emotional and behavioral regulation, often becoming apparent during childhood or adolescence (e.g. ADHD, autism). Pollution is also linked to mental health outcomes that are commonly diagnosed in adolescence or adulthood such as schizophrenia, neuroticism, depression, anxiety, and suicide. Pollution is associated with neurological conditions such as migraines and stroke, cognitive impairment, and neurodegenerative disorders such as multiple sclerosis, Alzheimer's disease, Parkinson's disease, and dementia.
The criteria air pollutants particulate matter, nitrogen dioxide, sulfur dioxide, carbon monoxide, ozone, and lead have been identified as harmful to human health by the World Health Organization and the U.S. Environmental Protection Agency (EPA). Many of them are linked to the burning of fossil fuels, through vehicle use, heating and cooking. Indoor and outdoor air pollution can contain particulate matter, dioxins, heavy metals, and endocrine-disrupting chemicals and microplastics that have negative effects on the central nervous system (CNS). Increased exposures to pollutants such as particulate matter and black carbon are associated with increases in cognitive impairment. Pollutants can interfere with neuroplasticity (the brain's ability to grow new neurons, adapt, and reorganize itself) and lead to long-term structural changes. Neurotoxicity is mediated by processes such as oxidative stress, mitochondrial dysfunction, and neuroinflammation. Pollutants such as microplastics may affect the neurophysiology of individuals after the structure of the CNS has mostly stabilized.
In terms of long-term social and economic impacts, air pollution exposure is linked to poorer cognitive development, academic performance and learning capacity in children. In adults, the effects of air pollution are linked to lower incomes, and decreased personal and workplace productivity. Economically, lowered IQ levels (as a measure of cognitive ability) are also related to increased healthcare costs and greater burden on the health system.
As a contributor to disease burden, environmental pollution is potentially preventable. Steps can be taken to minimize exposures to pollutants such as avoiding smoking, minimizing dust, using glass instead of plastics, carefully selecting cleaning products and cosmetics, and exercising away from heavy traffic routes. Control strategies can also be implemented to improve industrial safety and public health.
Pollutants
Pollutants can be inhaled, eaten, drunk, or absorbed through skin, with inhalation of air pollution being a dominant exposure route. Air pollution can result both from natural causes such as sea spray or desert dust and from human activities such as first and second-hand tobacco and cannabis smoke, household air pollution (HAP), and automobile exhaust fumes from fossil fuel combustion. Air pollution is a complex mixture that includes solids, liquids and gases. Air pollution also contains combinations of tiny particles of solid or liquid matter suspended in air, called particulate matter.
Air pollution is consistently associated with neurostructural and neurofunctional changes such as increased inflammation and oxidative stress and altered activity of neurotransmitters, neuromodulators and their metabolites. Exposure to air pollution is also associated with changes to gray matter volume and white matter structure within some brain regions.
Particulate matter
Particulate matter is made up of tiny particles of solid or liquid matter suspended in air, which are referred to in terms of the size of the particles measured (e.g. PM, PM, PM, ultrafine particles). PM contains particulates with a diameter of 2.5 micrometers or less. Small particulates are extremely harmful as they can penetrate deep into the lungs and travel through the bloodstream to the brain. Particulate matter can contain many toxic components, including carbon and heavy metals, with its chemical composition influencing its effects. As of 2022, the World Health Organization reported that 99% of the world’s population lives in places with unhealthy levels of PM2.5 pollution. Increased exposures to pollutants such as particulate matter (PM, PM, PM) and black carbon are associated with increases in cognitive impairment and neurodegenerative diseases such as dementia, Alzheimer's disease and Parkinson's disease. Airborne particulate matter is a Group 1 carcinogen. There is no safe level of particulate exposure.
Particulate matter can be both manufactured and naturally occurring. Hot volcanic lava, ocean spray, and wildfire smoke are common natural sources of particulates. Aerosols can be intentionally fabricated to serve a vast range of applications in both medicine and technology. Other particulates are produced as byproducts of specific processes or combustion reactions, such as printer toner and automobile exhaust. Anthropogenic sources of particulate matter include combustion of gas, coal, and other hydrocarbons (fossil fuels); burning of biomass fuels (firewood, charcoal, agricultural crop residues, and animal dung) in kitchens, domestic wood stoves, and outdoors; cigarette and cannabis smoking; human-caused forest fires and agricultural burns; waste disposal; industrial emissions; vehicular traffic and wear to roads, tires, and brakes; air traffic; seaport and maritime transportation; construction, demolition, restoration and concrete processing.
Use of power tools and hand tools for demolition, renovation and construction can produce large amounts of dust particulates. High energy tools (cut-off saws, grinders, wall chasers and grit blasters) can produce lots of dust in a short time. Using power tools to cut, grind, drill or prepare a surface, and sanding taped plaster board joints, can produce high dust levels. The more enclosed a work area, the more dust will collect there. The longer one works, the more dust there will be. Work methods such as dry sweeping raise more dust than vacuuming or wet brushing. Knowing that dust is generated during construction and can cause serious health hazards, some manufacturers market power tools equipped with dust collection systems or integrated water delivery systems. However, there are few industry standards for production of dust from tools. As of Q1 2024, California banned the sale of new gas-powered landscaping and outdoor equipment <25 horsepower.
Pollutants such as particulate matter are theorized to affect the brain via multiple pathways. The small particles found in PM2.5 can reach the brain through different entry routes via the olfactory system, lungs, circulatory system, and digestive system. PM2.5 can enter the nervous system through the nose and olfactory bulb. Inhaled PM2.5 can lead to respiratory inflammation and trigger systematic inflammation. This produces proinflammatory cytokines that can disrupt the blood–brain barrier, enabling pollutants to enter the brain. Once PM2.55 enters the circulatory system, it can be transported to the brain and across the blood–brain barrier. Within the brain, PM2.5 interferes with cellular and molecular mechanisms and can injure neurons. Neuroinflammation involves the activation of microglial cells and the production and release of inflammatory cascade mediators, including reactive oxygen species (ROS), proinflammatory cytokines, and nitric oxide. Elevated levels of ROS can interfere with the body’s ability to detoxify cells and to repair cellular damage, resulting in oxidative stress. This can damage cellular components such as DNA, proteins, and lipids. Responses to oxidative stress can lead to changes in mitochondria or other organelles within the cell, like the endoplasmic reticulum (ER), and trigger cell death. A buildup of misfolded proteins in the brain can lead to cognitive decline and neurodegenerative diseases.
Nitrogen oxides (NOx) Nitrogen and oxygen combine during combustion to produce nitrogen oxides (NOx) such as nitric oxide (NO) and nitrogen dioxide (NO2). NO2 is generated by fuel burning in vehicles, power plants, and industrial facilities, and is often used as an indicator of the presence of NOx generally and of traffic-related air pollution (TRAP). NO2 concentration tends to be highest in urban areas but nitrogen oxides can also be produced through agriculture. NO2 reacts with other pollutants and can dissipate quickly, varying hour by hour and from one neighborhood to another. When sunlight reacts with NOx and volatile organic compounds (VOCs) this can produce the pollutant ground level ozone (O3).
NO2 can increase oxidative stress and inflammation in the brain, resulting in neuronal damage. Meta-analysis suggests that NO2 is also related to depression, possibly due to an increase in reactive oxygen species, oxidative stress and inflammation. Prolonged exposure can interfere with the generation of new neurons and the brain's ability to adapt, and lead to structural changes. Long-term exposure to NO2 is associated with lower gray and white matter volume and enlargement of ventricles (fluid-filled spaces) in the brain. This has been linked to accelerated biological aging, reduced cognitive abilities, cognitive decline, increased susceptibility to depression, and higher risk of dementia. NO2 may also be linked to Alzheimer's Disease and Parkinson's Disease.
Sulfur dioxide (SO2) SO2 is a widespread air pollutant, emitted from natural sources such as volcanoes, hot springs, geysers, wildfires, and marine processes, and through human activities such as fossil fuel combustion, manufacturing, and wine preservation. Both acute and chronic exposure to SO are known to cause respiratory and cardiovascular illnesses and fatalities. Chronic exposure to SO also appears to cause neurological damage, through mechanisms such as oxidative stress, neuroinflammation, and vascular compromise. SO2 exposure can damage synapses and affect the expression of synaptic structural and functional proteins.
Meta-analysis and reviews indicate that exposure to SO is associated with decreases in global cognitive function, and with increased risk of cognitive decline and cognitive impairment. Cognitive function includes a broad range of skills such as attention, perception, memory, learning, and executive function. Cognitive impairment can include difficulty in thinking, learning, remembering, judgement and decision-making. SO2 is linked to memory impairment in aging. Synaptic alterations are considered an early marker for cognitive decline and are correlated with neurodegenerative processes in Alzheimer's and other neurodegenerative diseases. SO2 has a potential role in the onset of epileptic seizures and in relapses from schizophrenia.
Carbon monoxide (CO) Carbon monoxide (CO) is most commonly associated with acute carbon monoxide poisoning, in which hemoglobin in the blood bonds to CO instead of oxygen, preventing the body from getting enough oxygen to function. Exposure to CO most commonly occurs as a result of incomplete combustion of carbon-based fuels, from engine exhaust, fires, or gasoline-powered tools.
CO exposure is also associated with permanent brain damage which can occur days and weeks after exposure, and involve long-term neurological and cognitive deficits, PTSD, and depression. Mechanisms involved are not well understood, but may involve reactive oxygen species (ROS), oxidative damage, inflammation, and changes to myelin basic protein (MBP) that trigger immune responses and lead to brain damage. Meta-analysis also suggests that exposure to CO is associated with Parkinson's disease.