What smog is and why it matters

Smog is a thick haze of pollution that forms when sunlight reacts with emissions from cars, factories, and power plants. It is not fog — fog is water vapor. Smog is a mixture of ground-level ozone, nitrogen oxides, volatile organic compounds, and particulate matter that sits in the air you breathe. On bad days, you cannot see across a city block. On worse days, breathing it causes when ready chest pain and shortness of breath.

Smog forms most readily in warm, still air, which is why Los Angeles, Phoenix, and the Central Valley of California see it year-round, and why cities in the Northeast see it worst in summer. When air stagnates — when wind does not push pollution away — the chemical reactions that create smog accelerate. A city can wake up to clear skies and have dangerous air by noon.

The damage is not only what you see. Smog damages lung tissue, narrows airways, and makes existing asthma and heart disease worse. Children who grow up breathing smog have smaller lung capacity as adults. People over 65 and anyone with respiratory or cardiac disease face the highest risk, but smog harms everyone who breathes it.

Key Takeaways

  • Smog forms when sunlight triggers chemical reactions between car and factory emissions, creating ground-level ozone and particulate matter that sits in still air.
  • Warm, stagnant air in valleys and coastal cities traps smog for hours or days, making it worst in summer and in geographic areas that do not get wind.
  • Breathing smog causes when ready symptoms like chest tightness and coughing, and long-term damage to lung tissue and heart function.
  • Children, older adults, and people with asthma or heart disease face the highest health risk, but smog harms anyone exposed to it regularly.
  • Air quality forecasts and real-time air quality indexes tell you when smog is dangerous and what precautions to take.

How smog forms from vehicle and industrial emissions

Smog begins with emissions you cannot see. Cars, trucks, and buses release nitrogen oxides and unburned fuel into the air. Factories and power plants do the same. These chemicals are invisible when they leave the tailpipe or smokestack. They become visible only when sunlight hits them.

Ultraviolet light from the sun breaks apart nitrogen dioxide molecules. This releases oxygen atoms that recombine with oxygen gas to form ozone — the same molecule that protects Earth in the upper atmosphere, but dangerous at ground level where you breathe it. At the same time, volatile organic compounds (VOCs) from paint, gasoline, and solvents react with nitrogen oxides to form secondary organic aerosols — tiny particles that hang in the air and give smog its brown or gray color.

The reaction accelerates in heat. A 90-degree day produces far more smog from the same emissions as a 70-degree day. This is why smog is worst in summer, and why inland valleys that trap heat and still air see worse smog than coastal cities where ocean breezes push pollution away.

Why geography traps smog in certain cities

Smog does not spread evenly. Cities surrounded by mountains or located in valleys act as bowls that hold pollution in place. Los Angeles sits in a basin ringed by mountains. The San Francisco Bay Area has similar geography. Phoenix is in a valley. When air masses stagnate in these areas, smog accumulates hour after hour.

Coastal cities and cities on plains have an advantage: wind moves pollution away. But even these cities see smog when high-pressure systems park over them for days. A high-pressure dome creates calm, warm conditions that are perfect for smog formation and terrible for dispersal.

Elevation matters too. Smog is heavier than clean air and sinks into low-lying areas. If you live at the bottom of a valley, you breathe worse air than someone on a hillside above the smog layer. This is why some neighborhoods in the same city have measurably different air quality.

when ready and long-term health effects of breathing smog

When you breathe smog, ozone enters your lungs and damages the tissue lining your airways. This causes inflammation, which narrows the passages air travels through. You feel this as chest tightness, coughing, and difficulty breathing — symptoms that appear within minutes to hours of exposure. People with asthma feel it first and worst because their airways are already inflamed.

Repeated exposure causes permanent damage. Ozone scars lung tissue. Over years, this reduces how much oxygen your lungs can absorb. Children who grow up in high-smog areas have lung capacity 10 to 15 percent lower than children who grow up in clean air. This deficit persists into adulthood, even if they move away. Older adults with existing heart or lung disease face the highest risk of hospitalization or death on high-smog days.

Particulate matter in smog — the visible brown haze — carries toxic compounds deep into the lungs and even into the bloodstream. These particles trigger inflammation throughout the body, worsening heart disease and increasing the risk of heart attack and stroke. People with diabetes, obesity, or high blood pressure face compounded risk.

Reading air quality forecasts and real-time pollution data

The Air Quality Index (AQI) is a number from 0 to 500 that tells you how polluted the air is right now. The EPA publishes it for every major city in the United States. You can find it on AirNow.gov, which updates every hour. Most weather apps and local news websites also display it.

An AQI below 50 is good. An AQI of 51 to 100 is moderate — acceptable for most people, but people with asthma or heart disease should limit outdoor exercise. An AQI of 101 to 150 is unhealthy for sensitive groups — children, older adults, and anyone with respiratory or cardiac disease should stay indoors. An AQI above 151 is unhealthy for everyone. An AQI above 200 is very unhealthy. Above 300 is hazardous.

The AQI measures multiple pollutants — ozone, particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide — and reports whichever one is worst that day. On a smoggy summer day, ozone is usually the culprit. In winter or near ports and highways, particulate matter often dominates.

Steps to protect yourself on high-smog days

When the AQI reaches 101 or higher, limit time outdoors. If you must go outside, wear an N95 or P100 mask — a cloth mask or surgical mask does not filter ozone or fine particles. Keep the mask sealed to your face; gaps around the edges let unfiltered air in.

Avoid strenuous outdoor exercise when the AQI is high. Exercise increases the amount of air you breathe, pulling more pollution into your lungs. If you run, bike, or play sports outdoors, check the AQI before you go. On high-AQI days, move your workout indoors or postpone it.

Keep windows and doors closed on high-smog days. Use air conditioning with the recirculation setting on, or use a HEPA filter in your home. If you do not have air conditioning, close windows during the hottest part of the day when smog is worst — usually 2 p.m. to 6 p.m.

People with asthma should keep rescue inhalers on hand and follow their doctor's action plan. People with heart disease should monitor symptoms closely and contact their doctor if they feel chest pain, shortness of breath, or unusual fatigue. Older adults should check on neighbors and relatives who live alone.

What regulations and policies aim to reduce smog

The Clean Air Act, passed in 1970 and updated in 1990, set national standards for ground-level ozone and particulate matter. States must meet these standards or face penalties. The EPA sets the standards; states decide how to meet them.

Most states have vehicle emissions standards that limit how much nitrogen oxide and VOCs cars can release. California has the strictest standards in the nation and has required zero-emission vehicles for a growing share of new car sales. Other states follow California's rules or the federal EPA standards, which are less strict.

Power plants and large industrial facilities must install pollution control equipment or pay for emissions credits. Refineries, chemical plants, and cement kilns are the largest stationary sources of smog precursors. Regulations on these facilities have reduced smog in many cities, though progress has slowed in recent years.

Despite these regulations, smog remains a problem in dozens of U.S. cities. Climate change is making it worse: higher temperatures accelerate smog formation, and drought reduces the wind patterns that disperse it. Many cities that met EPA standards 20 years ago now exceed them again.

Frequently Asked Questions

Is smog the same as air pollution?

No. Air pollution is a broad category that includes all harmful substances in the air — dust, pollen, industrial chemicals, and more. Smog is a specific type of air pollution made of ozone and particulate matter that forms when sunlight reacts with vehicle and factory emissions. You can have air pollution without smog, but smog is always a form of air pollution.

Can smog cause permanent lung damage?

Yes. Repeated exposure to ozone scars lung tissue and reduces lung capacity permanently. Children are at highest risk because their lungs are still developing. Adults who breathe smog regularly also show reduced lung function over time. The damage does not reverse if you move to a clean-air area.

Does an air purifier protect me from smog?

A HEPA filter or activated carbon filter in your home can reduce indoor smog exposure, but only if your windows and doors stay closed. The filter cannot protect you outdoors. On high-smog days, an N95 or P100 mask is the only portable protection, and it works only if it fits tightly to your face.

Why is smog worse in summer?

Smog forms faster in heat because sunlight triggers the chemical reactions that create ozone. Summer also brings stagnant air masses that trap pollution in place. Winter cold slows these reactions, and winter wind patterns often disperse pollution more effectively, though some cities see winter smog from other sources.

Can I see the AQI for my specific neighborhood?

AirNow.gov reports AQI by city and sometimes by neighborhood, depending on how many monitoring stations are in your area. Large cities may have multiple stations; rural areas may have only one for the entire county. The AQI you see is measured at the nearest station, which may not reflect your exact location if you live in a valley or near a major highway.