Photochemical smog forms when sunlight reacts with car and industrial emissions

Los Angeles once suffered some of the worst smog on Earth

From the 1940s through the 1960s, Los Angeles was blanketed by dense, eye-burning smog so severe that drivers sometimes couldn't see traffic lights. The issue led to the nation's first local air pollution controls and inspired the passage of the 1970 Clean Air Act.

Pineapple research helped scientists solve Los Angeles' smog mystery

In the 1950s, chemist Arie Haagen-Smit applied methods he'd used studying pineapple plant chemistry to smog-filled air—discovering that sunlight reacting with car exhaust produced photochemical smog from hydrocarbons and nitrogen oxides, not factory smoke.

Rainfall and lush vegetation make the Great Smoky Mountains smoky

The national park is a rainforest that receives significant rainfall. When the vegetation on the mountains gets wet, it releases a gas that is smoky, and sometimes even appears blue, a phenomenon that gave the mountains (and the park) its name.

Chemtrails are formed from condensing water vapor and soot from jet fuel

Conspiracy theories about the phenomenon—also known as contrails—began circulating in the 1990s, but have gained significant traction on social media platforms. Chemtrails are more common in high-humidity conditions, where they persist and form cirrus clouds.

Tornadoes can cause significant air pollution and diminish air quality

Asbestos, a hazardous material that can cause lung cancer, is commonly found in buildings constructed before 1980. Tornadoes that damage such buildings can release asbestos into the air, along with other toxic pollutants and dust as materials are lifted. Individuals, particularly those with asthma or other chronic lung diseases, are encouraged to wear masks after a tornado.

Ozone protects against the sun's UV radiation and enhances the greenhouse effect

In the stratosphere, trioxygen molecules can absorb harmful ultraviolet radiation from the sun, preventing it from reaching Earth's surface, where it can harm organisms and raise surface temperatures. In the lower atmosphere, ozone acts as a greenhouse gas, trapping thermal radiation.

Particles found in Martian air pose significant risks to human lungs

Ultrafine dust, just 3 micrometers wide, is small enough to bypass lung defenses and enter the bloodstream, where its highly reactive chemicals can cause thyroid disease. Silica in the dust can also cause silicosis, a chronic and incurable lung disease.

Atmospheric composition and temperature impact the color of lightning

Just as fire ranges from red to blue with increasing temperature, lightning's blueish-white comes from the superheating of air. The emission of light from the predominantly nitrogen- and oxygen-rich atmosphere produces a purple tint, with additional color caused by light scattering from humidity, hail, dust, and ash.

Review the health effects of living near oil refineries, including increased respiratory risks

An analysis of 54 studies published between 1977 and 2024 in 15 international regions found evidence that proximity to refineries increased the risks of leukemia, asthma, bronchitis, and respiratory symptoms. Several studies also identified worse cardiovascular function in children and adults living near refineries, alongside other acute and chronic health impacts.

The Office of Chemical Safety and Pollution Prevention regulates toxic substances

This office evaluates and manages risks from industrial chemicals and pesticides. It reviews new chemicals before they enter the market, restricts hazardous substances, and promotes safer alternatives to protect human health and the environment.

City structures cause localized heat zones hotter than nearby rural areas

Urban materials like asphalt and concrete trap heat, while cities lack cooling features like trees and water. Due to hot runoff, urban streams can heat up by as much as 18°F more than forested streams.

The food industry's hard-to-solve pollution

One-quarter of the world's greenhouse gas emissions result from food and agriculture. That whopping number includes production and post-farm processes like distribution. See a visualization of the data along with a breakdown explaining how each process contributes to emissions, and why it’s so difficult to improve the agriculture industry.

What is freezing fog?

Freezing fog is a weather condition in which fog containing supercooled water freezes on contact with objects, forming a layer of ice and possibly dangerous driving conditions. Get the breakdown with this explainer.

During emergencies, backup diesel generators in data centers emit substantial pollution

During power outages or when the power grid is at insufficient capacity, these generators emit 200 to 600 times more nitrogen oxides—a greenhouse gas—than natural gas plants, as well as fine particulate matter linked to respiratory disease.

Synthetic textiles and tires are the top sources of secondary ocean microplastics

These sources contribute more than 60% of the primary microplastics polluting the oceans, followed by city dust and road markings. City dust is made of pieces of building coatings, synthetic footwear, and other human-made objects that have broken off due to weathering and abrasion.

Beyond burns, fireworks can harm respiratory, auditory, and visual health

While mishandling fireworks can cause severe burns and loss of appendages, there are also risks for viewers of displays. Released particulate matter can cause the surrounding air to resemble wildfire smoke, and the explosions can cause hearing loss and damage to the corneas.

Carboxylic acids are one reason teens are smelly

Scientists compared the smells of babies and teenagers and identified some of the compounds that make teens smelly. Carboxylic acids present in teens, they wrote, seemed to give off an "earthy, musty, or cheesy" scent. This 2024 research could help inform future odor-control products.

Will-o'-the-wisps may be the result of microlightning between bubbles

Methane and air bubbles moving through water can generate small sparks between them, which can ignite gas. The above-ground phenomenon may have historically been the result of passing travelers igniting swamp gas with their lanterns. (Some readers may experience a paywall.)

Increases in atmospheric carbon dioxide lead plants to produce more pollen

Pollen issues are emerging earlier in the year and lasting longer as rising levels of carbon dioxide in the atmosphere boost plants’ pollen production and also simultaneously extend warmer, pollen-producing seasons.

The composition of comets earned them the nickname 'dirty snowballs'

The icy relics of the solar system are composed of rock, gravel, dust, and frozen molecules, such as carbon dioxide, carbon monoxide, methane, ammonia, and water. Once close to the sun, these compounds sublimate into gases, creating a fuzzy cloud that earned the comets another nickname in history: hairy stars.

Dirty lightning is a thunderstorm created from friction between rising and falling flows of ash and smoke during volcanic eruptions.

Autonomous vehicles have been confirmed to reduce greenhouse gas emissions

Research indicates that self-driving cars can reduce harmful pollution through lower traffic, decreased parking demand, and optimized driving behavior. These benefits can be enhanced further in smart cities with responsive traffic signals.

Louisiana's 'Cancer Alley' is infamous for having some of the highest industrial pollution, toxic air emissions, and cancer risks in the US.

Cigarette smoke exposure boosts MS risk by up to 55%, though smokeless tobacco doesn't

The components of the smoke itself are associated with these significantly increased risks, so it's something beyond nicotine itself that drives that risk. What's more, smoking also increases the risk of developing antibodies to MS-treating drugs.

The legacy of Agent Orange: a photo gallery

An herbicide sprayed over the Vietnamese jungles during the war had a far greater impact than its supposed role as a defoliant, a chemical used to remove greenery. Agent Orange’s deleterious nature quickly impacted exposed people – and their children. This photo gallery, which includes graphic images, follows a photographer as they report on those affected by the chemical.

Internal-combustion engines convert fuel into motion using controlled explosions

Gasoline engines mix fuel and air, compressing it and igniting it to create thousands of tiny explosions that push pistons up and down. This movement spins the crankshaft, which moves a vehicle's wheels.

Allergies occur when our immune system mistakenly considers a substance a threat

Immunoglobulin E antibodies, or IgE antibodies, may bind to an allergen, effectively labeling it as a threat. That triggers a cascade of actions in the body, including the release of histamine, which causes the characteristic itching and hives of allergies. Sometimes a cooked version of an allergen—like a cooked carrot—may look different enough from its raw form to the immune system that antibodies ignore it, and no allergic response takes place.

An investigation suggests a Scottish island became awash in forever chemicals via seafoam and seaspray—raising questions about what that means for the safety of other coastal areas.

Stars appear to twinkle because turbulence in the motion of molecules in Earth's atmosphere causes light to bend and wiggle rapidly on its way to the surface—a phenomenon called scintillation.

The use of fungi to detoxify land and water—a process known as mycoremediation—has been used to clean up natural resources affected by the 2025 California wildfires.

See how fireworks are made and learn how metals like magnesium and copper are responsible for different colors.

The hormonal disorder PCOS was renamed PMOS in 2026, a move designed to improve care and research by shifting the focus to the hormonal causes of this condition that affects 1 in 8 women.

This poison ivy knowledge quiz—accompanied by photos of the plant's varied forms—may help you identify this common rash-inducing growth.

Before writing 'Walden,' transcendentalist Henry David Thoreau nearly burned down his hometown with a campfire that grew out of control.

Fear or disgust related to clusters of holes and cracks, called trypophobia, may stem from evolutionary processes to help us avoid pathogens.

Volcanic activity may have triggered the Black Death by spewing gases that led to crop failures in Europe, thus forcing the import of grain ships with plague-infested fleas.

Scientists can determine Earth's past climate conditions by analyzing air trapped in microscopic bubbles within ice cores—columns of compressed snow layers that build up over hundreds of thousands of years.

The price of gas is a composite made up of several different factors, with the federal gas tax making up less than 20% of what drivers pay at the pump.

The word 'jazz' evolved from 'jasm,' 19th-century American slang for 'energy'

When a Paris department store hesitated to sell Estée Lauder's perfume, she 'accidentally' broke a bottle, causing the smell to spread throughout the store and shoppers to start asking to buy the scent.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.

The name 'Silicon Valley' arose from the founding of Shockley Semiconductor Laboratory in Fairchild Semiconductor in 1956-57, founded near Stanford University in Palo Alto

Some Pokémon have been named after real and fictional organisms, while some real-world species have been named after Pokémon, such as Chilicola charizard and Binburrum zapdos.

Explore the known asteroids, comets, and spacecraft involved in asteroid studies within our solar system with 'Eyes on Asteroids,' a 3D visualization from NASA.

Graffiti dates back to at least ancient Pompeii, where citizens scrawled messages to one another on public walls

Mid-century modern graphic designers, known for their bright colors and bold lines in advertising, also found success with children's books.

In 1984, Operation Smash Hit intentionally crashed a 239-ton train into a nuclear waste canister at 100 miles per hour to prove the canister's safety, which remained completely intact and lost just 0.29 of its 100 pounds of pressure.

Gerrymandering works through two techniques: cracking opposing voters across districts to dilute their power, and packing them into a few to waste their votes in lopsided majorities.

Electrical grids worldwide must double their power delivery capacity by 2050 to meet surging demand from AI and electric vehicles.

A 2019 Facebook event called on attendees to "Storm Area 51, They Can't Stop All of Us," resulting in countless memes, the creation of a music festival, and eventually the arrests of those who did try to approach the area.