What smog is and where it comes from

Smog is a visible haze made of ground-level ozone and fine particles suspended in air. It forms when sunlight reacts with nitrogen oxides and volatile organic compounds released by cars, factories, power plants, and other sources. Unlike the fog that rolls in from water, smog is pollution — it does not clear on its own and gets worse on hot, still days when air cannot disperse.

Ground-level ozone, the main ingredient in smog, is different from the ozone layer high in the atmosphere. At ground level, ozone is a harmful gas created when sunlight cooks emissions from vehicles and industry. Fine particles — called PM2.5 because they are 2.5 micrometers or smaller — come directly from exhaust, smoke, and dust, or form when gases react in the air. Both ozone and particles can lodge deep in your lungs.

Smog concentrations vary by region and season. Areas downwind of major highways, ports, and industrial zones see higher levels. Summer and early fall typically bring the worst smog because heat and sunlight drive the chemical reactions that create it. Winter smog can occur too, especially in valleys where cold air traps pollution close to the ground.

Key Takeaways

  • Smog is ground-level ozone and fine particles that form when sunlight reacts with vehicle and industrial emissions, and it worsens on hot, still days.
  • Breathing smog can trigger asthma attacks, reduce lung function, and cause lasting damage with repeated exposure, especially in children and older adults.
  • The Air Quality Index (AQI) tells you how polluted the air is in your area on any given day, with readings above 150 considered unhealthy for the general public.
  • You can reduce your exposure by checking the AQI before outdoor activity, exercising indoors on high-pollution days, and using air filters in your home.
  • Long-term smog exposure increases the risk of heart disease, stroke, and early death, making it a serious public health issue even when you cannot see it.

How smog damages your lungs and heart

Ozone irritates the airways and causes inflammation. When you breathe smog, ozone can trigger coughing, wheezing, and chest pain within hours. People with asthma are especially vulnerable — smog exposure can provoke attacks even in those whose asthma is usually controlled. Children are at higher risk because their lungs are still developing and they spend more time outdoors and playing.

Repeated exposure to smog reduces how much air your lungs can hold and how efficiently they work. This decline in lung function can be permanent, even after smog levels improve. Older adults and people with chronic lung disease face the same risk. Fine particles bypass the upper airway and embed themselves deep in the lungs, where they trigger inflammation and can enter the bloodstream.

Smog also harms the heart. Particles and ozone can cause inflammation in blood vessels, increase blood pressure, and make blood clots more likely. Studies show that people exposed to high smog levels have higher rates of heart attack and stroke. The risk is greatest for people who already have heart disease, diabetes, or obesity, but it affects the general population too.

Reading the Air Quality Index for your area

The Air Quality Index, or AQI, is a daily number that tells you how polluted the air is where you live. It ranges from 0 to 500, with higher numbers meaning worse air. The Environmental Protection Agency (EPA) publishes AQI readings for thousands of locations across the United States, updated hourly or several times a day depending on the monitoring station.

An AQI of 0 to 50 is green and means air quality is good. An AQI of 51 to 100 is yellow and means air quality is moderate — sensitive groups like children, older adults, and people with lung disease should limit prolonged outdoor exertion. An AQI of 101 to 150 is orange and means air quality is unhealthy for sensitive groups; they should avoid outdoor activity. An AQI of 151 to 200 is red and means air quality is unhealthy for everyone; the general public should reduce outdoor activity. Above 200, air quality is very unhealthy or hazardous, and everyone should stay indoors.

You can check the AQI for your zip code through AirNow.gov, which is run by the EPA and the National Oceanic and Atmospheric Administration (NOAA). Many weather apps and local news stations also display the AQI. Some cities and counties send alerts when the AQI reaches unhealthy levels, so check whether your area offers that service through your local health department or air quality agency.

Protecting yourself on high-smog days

On days when the AQI is orange or higher, limit time outdoors, especially during peak hours — usually late morning through early evening when the sun is strongest and ozone levels peak. If you must go outside, avoid strenuous activity like running or heavy yard work, which forces you to breathe more deeply and take more pollution into your lungs. Walking and light activity are lower-risk alternatives.

Wear an N95 or P100 mask if you go outside on high-AQI days. These masks filter out fine particles, though they do not filter ozone gas. Make sure the mask fits snugly around your nose and mouth — a loose mask offers little protection. Surgical masks and cloth masks do not filter particles effectively enough to be useful for smog protection.

Keep windows and doors closed on high-smog days to prevent outdoor air from entering your home. Use an air conditioner set to recirculate indoor air rather than pulling in outside air. If you do not have air conditioning, stay in the coolest room you can find. Run a portable air purifier with a HEPA filter in the rooms where you spend the most time — bedroom and living room are priorities. Change the filter according to the manufacturer's instructions, as a clogged filter loses effectiveness.

Who faces the highest risk from smog

Children are more vulnerable to smog than adults because their lungs are still developing, they spend more time outdoors, and they breathe more air per pound of body weight. Smog exposure during childhood can stunt lung growth and lead to reduced lung function throughout life. Older adults face high risk because lung function naturally declines with age, and many have underlying heart or lung disease that smog can worsen.

People with asthma, chronic obstructive pulmonary disease (COPD), emphysema, or bronchitis are at elevated risk because their airways are already inflamed or damaged. Smog exposure can trigger attacks or worsen symptoms. People with heart disease, high blood pressure, or diabetes are also at higher risk because smog increases inflammation and blood clotting, which can lead to heart attack or stroke.

Outdoor workers — construction workers, landscapers, delivery drivers, and others who spend hours in the open air — face cumulative exposure that indoor workers do not. Pregnant people should also take smog seriously, as some research suggests that exposure during pregnancy may affect fetal development and birth outcomes, though more study is needed.

Long-term health effects of repeated smog exposure

Living in an area with chronic smog pollution increases your risk of premature death. Large studies following people over decades show that those exposed to high smog levels have higher rates of death from lung disease and heart disease compared to those in cleaner areas. The effect is dose-dependent — the more smog you breathe over time, the greater the risk.

Repeated ozone exposure can cause permanent scarring of the airways and loss of lung function that does not fully recover even after you move to a cleaner area. Children who grow up in high-smog regions have lower peak lung function as adults, which means they start with less breathing capacity and decline faster as they age. This can lead to disability and dependence on oxygen later in life.

Chronic smog exposure also accelerates atherosclerosis — the buildup of plaque in arteries — and increases inflammation throughout the body. This raises the risk of heart attack, stroke, and heart failure. The effects are not limited to people with existing disease; healthy people who breathe high smog levels for years show measurable changes in heart function and blood vessel health.

Steps communities and regulators take to reduce smog

The EPA sets National Ambient Air Quality Standards (NAAQS) for ozone and fine particles. States and regions that do not meet these standards must develop plans to reduce emissions from cars, factories, and power plants. These plans can include stricter vehicle emission standards, limits on industrial pollution, and requirements for cleaner fuels.

Many states have adopted California's vehicle emission standards, which are stricter than federal standards. Some cities restrict traffic on high-smog days or offer incentives for electric vehicles and public transit use. Power plants have been required to install pollution control equipment or retire older, dirtier units. Refineries and chemical plants face limits on volatile organic compound emissions.

Despite these efforts, many areas still exceed the ozone standard on summer days. Climate change is making the problem harder to solve because higher temperatures increase the rate at which ozone forms. Some regions that had improving air quality in the 1990s and 2000s have seen smog levels plateau or worsen in recent years.

Frequently Asked Questions

Can I see smog in the air, or is it invisible?

Smog can be visible as a brown or gray haze, especially on very polluted days, but it is often invisible. You cannot see ozone gas, and fine particles are too small to see individually. Just because the air looks clear does not mean the AQI is low — check the index rather than relying on what you see.

Does smog only happen in summer?

Ozone smog peaks in summer and early fall because heat and sunlight drive its formation. Winter smog can occur too, especially in valleys and basins where cold air traps pollution. Winter smog is usually made of fine particles and nitrogen dioxide rather than ozone, but it is still harmful to breathe.

Will an indoor air purifier remove all smog?

A HEPA filter removes fine particles but not ozone gas. To reduce ozone indoors, keep windows closed and use air conditioning set to recirculate. Some air purifiers have activated carbon filters that can absorb some ozone, but they are less effective than HEPA filters at removing particles. No home filter removes all pollution.

Is smog worse near highways and factories?

Yes. Areas within a few miles downwind of major highways, ports, and industrial facilities have higher smog levels than areas farther away. If you live or work near these sources, your exposure is higher even on days when the regional AQI is moderate. Check the AQI for your specific location rather than relying on a distant monitoring station.

Can smog cause permanent lung damage in healthy people?

Yes, though the risk is lower in healthy people than in those with existing lung disease. Repeated exposure over years can reduce lung function permanently, especially in children whose lungs are still developing. Even healthy adults show measurable declines in lung function after years of high smog exposure.