What smog is and where it comes from

Smog is a visible haze that forms when sunlight reacts with pollution already in the air. It is not a single substance — it is a mixture of ground-level ozone, nitrogen oxides, volatile organic compounds, and particulate matter that combine under certain weather conditions. The word itself comes from "smoke" and "fog," which describes what it looks like.

Smog forms in two main ways. Photochemical smog happens when sunlight hits emissions from cars, factories, and power plants on hot days, creating ozone at ground level — the kind that harms your lungs. Sulfurous smog, also called London-type smog, forms when coal or oil burning releases sulfur dioxide into cold, stagnant air, trapping pollution close to the ground. Most smog you see today in cities is photochemical, worse in summer and early fall.

Weather patterns trap smog in place. When warm air sits above cooler air near the ground — a condition called a temperature inversion — pollution cannot rise and disperse. Valleys and basins surrounded by hills or mountains hold smog longer because wind cannot push it away. This is why cities like Los Angeles, Denver, and Salt Lake City see persistent smog, while coastal cities with steady breezes often do not.

Key Takeaways

  • Smog forms when sunlight reacts with vehicle and industrial emissions, and temperature inversions trap it near the ground where you breathe it.
  • Ground-level ozone in smog damages your lungs and is worse on hot, sunny days when air is still.
  • Reducing personal vehicle use, supporting cleaner energy sources, and checking air quality forecasts are the most direct ways individuals can reduce smog.
  • Government regulations on emissions from cars and power plants have cut smog in most U.S. cities over the past 30 years, though some areas still exceed safe levels.
  • Smog is not the same as wildfire smoke, though both reduce air quality and both require similar protective steps during high pollution events.

How smog damages your health

Breathing smog irritates your airways and reduces how much oxygen your lungs can absorb. Ground-level ozone — the main ingredient in photochemical smog — inflames the lining of your respiratory tract, causing coughing, wheezing, and shortness of breath. Even healthy people experience these symptoms during high smog days, and the effects are worse if you exercise outdoors.

Repeated exposure to smog over weeks or months can cause lasting damage. Children whose lungs are still developing, older adults, and people with asthma, emphysema, or heart disease face the highest risk. Smog also carries fine particles that bypass your nose and throat, entering deep into your lungs where they can trigger inflammation and reduce oxygen flow to your bloodstream.

The Air Quality Index (AQI), reported daily by the EPA and local air agencies, measures smog and particle pollution on a scale from 0 to 500. An AQI below 50 is "good," 51 to 100 is "moderate," 101 to 150 is "unhealthy for sensitive groups," and above 150 is unhealthy for everyone. You can check your local AQI at AirNow.gov or through most weather apps.

Steps you can take to reduce smog

The most direct action is to reduce emissions from vehicles. Drive less by combining trips, using public transit, carpooling, or biking when possible. If you are buying a vehicle, choosing one with lower emissions or an electric model removes a major source of smog-forming pollution. Even keeping your current car well-maintained — regular oil changes, tire pressure checks, and emissions inspections — prevents leaks that add to air pollution.

At home, reduce energy use from power plants that burn fossil fuels. Switch to LED bulbs, seal air leaks around windows and doors, use a programmable thermostat, and run major appliances during off-peak hours if your utility offers time-of-use rates. These steps lower your electricity demand, which means fewer emissions from the grid.

Avoid activities that release volatile organic compounds on high smog days. Refuel your car in the evening when it is cooler, postpone lawn mowing and painting, and limit charcoal grilling. These everyday sources add up, especially during temperature inversions when the air is already stagnant.

Support policies that reduce emissions. This can mean voting for candidates who back clean energy standards, attending local air quality hearings, or joining community groups focused on air quality. Many cities and states have set targets to phase out coal power plants and require cleaner vehicle standards — public support helps these policies move forward.

Protecting yourself when smog is high

On days when the AQI is above 100, limit outdoor activity, especially strenuous exercise. If you must go outside, wear an N95 or P100 respirator mask, which filters out fine particles and ozone. Cloth masks and surgical masks do not block ozone or the smallest particles, so they offer little protection during smog events.

Keep windows and doors closed on high smog days to prevent outdoor air from entering your home. Run your air conditioner on recirculate mode so it filters indoor air without pulling in outside pollution. If you do not have air conditioning, stay in air-conditioned public spaces like libraries, malls, or community centers during peak smog hours, usually mid-afternoon through early evening when sunlight is strongest.

People with asthma or heart disease should have a plan ready before smog season arrives. Talk to your doctor about whether you need to adjust medications or limit activities on high AQI days. Keep rescue inhalers accessible and know the symptoms that mean you need medical attention — chest pain, severe shortness of breath, or confusion warrant a call to 911.

How regulations have reduced smog over time

The Clean Air Act, passed in 1970 and updated in 1990, set national standards for air quality and required states to reduce pollution. The EPA established emissions limits for cars, power plants, and factories, and these rules have cut smog-forming pollution significantly. Most U.S. cities have seen smog decline over the past 30 years even as population and vehicle miles driven have increased.

California leads the nation in vehicle emissions standards, requiring cars sold there to produce fewer nitrogen oxides and hydrocarbons than federal rules demand. Other states have adopted California's standards, and the federal government has tightened its own limits several times. Power plants now use scrubbers to remove sulfur dioxide and selective catalytic reduction to cut nitrogen oxides before emissions leave the smokestack.

Despite progress, some areas still exceed the EPA's safe ozone standard. The South Coast Air Quality Management District in Southern California, the Four Corners region in the Southwest, and parts of the Ohio Valley still see unhealthy smog days most summers. These areas continue to tighten regulations and invest in cleaner transportation and energy.

Smog versus wildfire smoke and other air pollution

Smog and wildfire smoke are different pollutants that require the same protective response. Smog is created by chemical reactions in the air and is worst on hot, sunny, still days. Wildfire smoke is ash and particles from burning vegetation, worst during fire season and often visible as a brown or orange haze. Both reduce visibility and harm your lungs, so checking the AQI and wearing a mask during high pollution days protects you from either.

Industrial pollution and dust from construction or unpaved roads also degrade air quality but are tracked separately from smog. The AQI includes all these sources, so a high reading means some combination of ozone, particles, nitrogen dioxide, sulfur dioxide, and carbon monoxide — you do not need to know which one to know you should limit outdoor time.

Frequently Asked Questions

Is smog the same as haze?

Not exactly. Haze is any reduced visibility caused by particles or moisture in the air, while smog specifically refers to pollution created by chemical reactions with sunlight. You can have haze from dust or humidity without smog, but smog always creates haze. If the haze has a brown or orange tint and smells like exhaust, it is likely smog.

Can smog cause permanent lung damage?

Repeated exposure over years can reduce lung function, especially in children and people with existing respiratory disease. Studies show that people who grow up in high-smog areas have lower lung capacity as adults. However, moving to a cleaner area or reducing smog exposure can slow further decline and prevent new damage.

Why is smog worse in summer?

Heat and sunlight drive the chemical reactions that create ground-level ozone. Summer also brings temperature inversions more often — warm air aloft traps cooler, polluted air near the ground. Winter smog is usually sulfurous smog from heating and power plants, which is less common now due to cleaner fuels.

Do air purifiers remove smog?

HEPA filters in air purifiers remove fine particles but not ozone gas. If you want to filter both, look for a purifier with both a HEPA filter and an activated carbon filter. Running an air purifier indoors helps, but the most effective step is keeping windows closed and using your air conditioner on recirculate mode during high smog days.

What is the difference between an N95 mask and a P100 mask?

Both filter particles, but a P100 also blocks ozone gas, making it better for smog. N95 masks filter 95 percent of particles but not gases. For smog protection, a P100 or a mask labeled for ozone is more effective, though both are better than no mask during high AQI days.