When we think of pollution, images of cars, factories, and power plants spewing dark plumes typically come to mind. That's air pollution, essentially, and it's mostly made up of carbon dioxide and methane.
Most emissions of CO2 and methane come from burning fossil fuels like coal, oil, and natural gas. As these emissions of greenhouse gases add up, they trap more heat on the planet, rather than letting it dissipate into space — leading to global warming.
Burning fossil fuels also sends carcinogenic particles into the air.
Factories and gas-powered vehicles also produce other air pollutants like nitrogen oxide, sulfur dioxide, and hydrocarbons. These chemicals can react with sunlight to create smog.
Currently, 91% of the world’s population lives in places where the air quality does not meet hte standards for safety set by the World Health Organization.
In some parts of the world, like China and India, smog can get so thick that people can't see the sun and have to wear masks.
Sometimes those nitrogen oxide and sulfur dioxide chemicals can react with the moisture in the atmosphere and change into acids. That water-chemical mixture then returns to Earth as acid rain.
Acid rain can kill trees and devastate lakes and streams. The addition of too much acid rain to a body of water makes it too acidic for fish and other marine life to survive.
Another air pollutant is called chlorofluorocarbon (CFC), a type of chemical used in foam products, aerosol cans, and refrigerator coolants.
CFCs rip holes in the planet's ozone layer, a region of the upper atmosphere that absorbs harmful radiation from the sun and protects species on Earth. Without an ozone layer, people would be exposed to high levels of ultraviolet radiation and be more likely to develop skin cancer and eye diseases.
The Montreal Protocol, an environmental agreement that 197 nations have ratified, calls for the phasing out of ozone-depleting substances like CFCs. The US signed the accord in 1988.
But not all ozone is "good." While atmospheric ozone protects the Earth from radiation, ozone that forms at ground level is harmful to human health.
Ground-level ozone, or tropospheric ozone, is a result of chemical reactions when sunlight interacts with pollutants emitted by cars, power plants, industrial boilers, and refineries.
This type of ozone is can trigger a variety of breathing problems, particularly for children and the elderly, according to the US Environmental Protection Agency (EPA). Breathing in ground-level ozone can reduce a person's lung function and harm lung tissue, exacerbating conditions like emphysema and asthma.
It's one of the main ingredients in smog, too.
Exposure to polluted air in general can trigger asthma, change the way children's brains develop, and make older adults more likely to succumb to cognitive decline.
Studies conducted in China and Canada show that children who breathe poor air are more likely to have breathing difficulties and asthma. A study involving New York City schoolchildren also found that kids who breathe poor air are more likely to need academic intervention.
Research in the US shows that dementia and cognitive decline rates are higher in places with more air pollution as well.
Researchers recently calculated that air pollution contributes to an estimated 8.8 million extra deaths globally every year — nearly double previous estimates.
The new study, published in March, also showed that nearly 800,000 Europeans died from air pollution-derived issues in 2015 alone; 48% of those deaths were from cardiovascular disease and stroke.
In addition to greenhouse gases, human activity also creates light pollution.
If you look at the night sky in Times Square, you won't see any stars. That's because the area has an enormous amount of light pollution.
Light pollution is the excessive use of artificial light, including the brightening of the sky over inhabited areas (called skyglow). It's a category that also includes glare, the excessive brightness to the point of visual discomfort.
For many of us, light pollution means we're no longer experiencing truly dark nights. According to the International Dark Sky Association, artificial light at night increases our risk for obesity, depression, sleep disorders, diabetes, and breast cancer. It also suppresses melatonin production, a hormone that induces sleep and boosts the immune system.
Global light pollution is so bad that more than one-third of humanity can't see the Milky Way — ever.
According to a 2016 study, 80% of the world’s population lives under skyglow. In the United States and Europe, 99% of the public lives under light-polluted skies.
Another subtle form of pollution that you might not think much about is noise pollution.
Honking car horns, wailing sirens, and chugging trains all contribute to the levels of ambient noise that surround us.
The EPA defines noise pollution as an excess of a type of sound that's “unwanted or disturbing.” According to the agency, there are direct links between noise and poorer health. Problems related to prolonged exposure to excessive noise include stress-related illnesses, high blood pressure, hearing loss, sleep disruption, and loss of productivity.
A 2011 report found that at least 1 million healthy years of life are lost each year in Europe due to noise pollution.
The report, published by the World Health Organization, ranked traffic noise as one of the worse environmental threats to public health, second only to air pollution.
And then there's the solid waste that humans create.
A 2012 report from the World Bank’s Urban Development department estimated that by 2025, the amount of municipal solid waste generated worldwide would rise to 2.4 billion tons.
Most of that garbage comes from organic waste, like food. Nearly 60% of it goes into landfills.
Landfills leak harmful pollutants like methane into the air and leachate into nearby soil and groundwater.
Over time, the organic waste languishing in landfills breaks down and produces methane, a greenhouse gas that contributes to climate change.
What's more, some of the solid waste in landfills can release harmful toxins that leach into the soil. Fluorescent lightbulbs release mercury, for example, while electronics like computers and dead batteries can leak arsenic and lead.
Another major pollutant from landfills is called leachate — it's a liquid cocktail that forms when water seeps through landfill waste and collects toxins. Leachate can include various toxic pollutants, heavy metals, and ammonia-nitrogen compounds that are toxic for aquatic life, according to the Water Quality Association.
Humans have been dumping an unprecedented amount of plastic into the ocean, too. On average, 8.8 million tons of plastic enters the ocean every year.
That's just the average amount, though — the maximum could be closer to 14 million tons.
These plastics accumulate in polluted patches of the ocean like the Great Pacific Garbage Patch.
Plastic in the ocean threatens marine life — animals sometimes confuse the items for food and consume them. This can cause them to change their behavior, suffer strangulation, and die.
Some plastics take hundreds of years to break down. Even then, they just splinter into very small pieces called microplastics, which will likely never biodegrade.
These microplastics are everywhere — including the food we eat and beverages we drink. They even show up in our poop, according to the Smithsonian Institute.
That's because the fish and shellfish we eat ingest these microplastics. According to a 2013 study, marine animals can accumulate potentially hazardous chemicals from eating those plastics, and can get tumors and liver problems because of it.
Those chemicals then move up the food chain.
Oil spills from tankers and off-shore drilling also pollute the ocean.
The Exxon Valdez tanker famously hemorrhaged 11 million gallons of oil into the Gulf of Alaska in 1989, while the Deepwater Horizon disaster dumped some 200 million gallons of oil into the Gulf of Mexico over the course of 87 days in 2010.
Only 25% of the leaked oil from Deepwater Horizon was recovered, leaving more than 154 million gallons in the ocean. It harmed or killed an estimated 82,000 birds, 6,100 sea turtles, and 25,900 marine mammals, according to the Center for Biological Diversity.
While oil spills are unintended forms of pollution, humans have intentionally dumped toxic chemicals like methylmercury into the water, too.
Between 1932 to 1968, a Japanese fertilizer company called the Chisso Corporation dumped industrial wastewater that was contaminated with an estimated 30 tons of poisonous methylmercury into Minamata Bay.
The toxic chemical accumulated in local fish and shellfish, which Japanese citizens consumed. This led them to suffer from a condition called Minamata disease, which wreaks havoc on the brain and nervous system and causes physical deformities.
Other toxic pollutants come in the form of fertilizers, pesticides, and insecticides that are used for ranching or farming and can accumulate in soil.
A report by the European Commission noted that some soils are contaminated by asbestos, a chemical linked to lung cancer.
Sometimes, those chemicals can move from the soil to a major water source via run-off.
Run-off is when water — and the contaminants it contains — moves across and through the soil, eventually joining local waterways that can flow into the ocean.
Run-off can accumulate motor oil, solid waste, pesticides, and fertilizers, and bring those toxic chemicals along for the ride.
According to NOAA, 80% of pollution to the marine environment comes from run-off from the surrounding land. Septic tanks, cars, trucks, boats, farms, and ranches all contribute to polluted run-off.
When humans log an area, the absence of trees leads to even more run-off.
Deforestation contributes to soil erosion, which increases the amount of polluted run-off that flows into rivers and streams. That worsens flooding.
Every year, more than 18 million acres of forest disappear worldwide. That's about 27 soccer fields' worth every minute, according to the World Wildlife Fund.
Pollutants from farms, septic systems, and industrial sites can also leach into the ground water we drink.
Pesticides, fertilizers, herbicides and animal waste are agricultural sources of groundwater contamination, while wastewater disposal systems can also leak human waste into the groundwater and nearby drinking water wells.
Ground water supplies drinking water for 51% of the total US population and 99% of the country's rural population.
Industrial manufacturing and mining operations are a primary source of groundwater pollution. Often, industrial waste contaminates the water with arsenic, lead, mercury, and chromium 6 — all known carcinogens.
Chemical companies release the most contaminants, the Center of Public Integrity reported. Plastics and rubber manufacturers, mining companies, and producers of petroleum and coal take the rest of the top polluting spots.
These types of heavy metals can cause gastrointestinal, respiratory, reproductive, and developmental problems in humans.
Industrial activities also release dioxins — known carcinogens that have been linked to heart disease and diabetes, according to the National Institutes of Health.
Radioactive waste is yet another form of pollution that has devastating health effects on humans and wildlife.
Radioactive pollution occurs when radioactive substances are released into the environment. This can be caused by nuclear weapons testing, production, and decommissioning; nuclear explosions; and the handling and disposal of radioactive waste.
Accidents at nuclear power plants like Chernobyl and Fukushima have also contributed to radioactive pollution in the surrounding air and water.
The Chernobyl disaster, for example, contaminated vast areas of Europe and Eurasia, and the wildlife living in the area.
Chernobyl released 400 times more radioactivity into the atmosphere than the bomb dropped on Hiroshima in 1945, according to the International Atomic Energy Agency.
The 2011 Fukushima disaster, by comparison, released only one-tenth as much radioactive material, the Conversation reported. Still, after Fukushima, fish off the Japanese coast had elevated levels of radiation, which prompted the Japanese government to ban or limit their sale.
Animals at both sites now have smaller and less diverse populations. Birds and other mammals at Chernobyl have cataracts in their eyes and smaller brains, according to a 2016 study. Some birds have malformed sperm or are sterile.
Exposure to high amounts of radioactive material like cesium is detrimental to humans. Cesium can cause radiation sickness, which includes nausea and vomiting in the short-term and sometimes death in the following days or weeks.
According to the Centers for Disease Control and Prevention, radiation exposure can also result in long-term health effects like cancer and cardiovascular disease.