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 gases and tiny particles that hang in the atmosphere near the ground. The two main types are photochemical smog, which forms on sunny days when car exhaust and factory emissions meet ultraviolet light, and London smog, which forms in cold, still air when coal smoke and fog trap pollution close to the earth.
The pollution that creates smog comes from specific sources. Cars, trucks, and buses release nitrogen oxides and volatile organic compounds. Power plants and factories emit sulfur dioxide and particulates. On hot, sunny days with little wind, these pollutants accumulate and react with sunlight to form ground-level ozone — the main ingredient in photochemical smog. In winter or in areas surrounded by mountains, cold air can trap pollution near the ground, creating a thick, visible layer.
Smog is different from dust or haze because it is chemically active. The ozone in smog is not the protective ozone high in the atmosphere — it is a harmful gas that forms right where people breathe. This is why smog days feel different from clear days with poor visibility: smog actively damages lung tissue, while regular haze is mostly a visibility problem.
Key Takeaways
- Smog forms when sunlight reacts with vehicle exhaust and industrial emissions, creating a visible haze that contains harmful ground-level ozone.
- Photochemical smog is worst on hot, sunny, windless days, while London-type smog forms in cold, still conditions that trap pollution near the ground.
- Breathing smog can irritate your lungs and airways, and repeated exposure may worsen asthma, heart disease, and other respiratory conditions.
- Air quality indexes and smog alerts tell you when pollution levels are high enough to limit outdoor activity, especially for children and older adults.
How smog affects your lungs and heart
Ground-level ozone in smog irritates the lining of your airways. When you breathe it in, it can cause coughing, throat irritation, and chest tightness — even in people without lung disease. The irritation happens because ozone is a reactive chemical that damages cells on contact. Repeated exposure over hours or days can make these symptoms worse and can reduce how much air your lungs can hold.
People with asthma, chronic bronchitis, or emphysema are at higher risk because their airways are already inflamed or narrowed. For them, a smog day can trigger an attack or make breathing noticeably harder. Children are also more vulnerable because their lungs are still developing and because they tend to be more active outdoors, so they breathe in more polluted air per minute.
Smog also affects your heart. Ozone and fine particles can enter the bloodstream through the lungs and trigger inflammation throughout your body. People with heart disease, high blood pressure, or a history of heart attack face a higher risk of heart problems on high-smog days. Older adults are at greater risk overall because they are more likely to have existing heart or lung conditions.
Reading air quality reports and smog alerts
Most cities and regions publish an Air Quality Index, or AQI, which rates pollution on a scale. The scale usually runs from 0 to 500, with color codes: green (good), yellow (moderate), orange (unhealthy for sensitive groups), red (unhealthy), and purple (very unhealthy). The AQI is based on measurements of ozone, particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide — the main pollutants that make up smog.
You can find your local AQI on your city or county's air quality website, on the EPA's AirNow website, or through weather apps that include air quality data. Most services update the forecast hourly or several times a day. When the AQI reaches orange or higher, local health departments often issue smog alerts recommending that sensitive people limit outdoor activity.
The AQI number tells you not just whether smog is present, but how much it matters for your health. A yellow day means the air is acceptable for most people but may bother those with asthma or heart disease. An orange day means even healthy people may feel effects if they spend hours outside. A red day means most people should stay indoors or limit time outside.
Who is most at risk on high-smog days
Children face higher risk because they spend more time outdoors, their lungs are developing, and they breathe faster than adults — so they inhale more polluted air per minute. Older adults face higher risk because they are more likely to have asthma, heart disease, or other conditions that smog can worsen. People with diagnosed asthma, chronic obstructive pulmonary disease (COPD), or heart disease should take smog alerts seriously and adjust their outdoor time accordingly.
Pregnant people may also want to limit exposure to high smog levels, though research on this is still developing. People who work outdoors — construction workers, landscapers, mail carriers, police officers — face higher exposure straightforward because they spend more hours breathing the air. Athletes and people who exercise outdoors should check the AQI before intense activity, since heavy breathing during exercise means more pollutant intake.
Even healthy people can feel effects on very high-smog days. Symptoms might include a scratchy throat, coughing, or mild chest discomfort. These usually go away once you move to cleaner air, but they are a sign that pollution levels are high enough to affect your body.
Steps to take when smog levels are high
On orange or red AQI days, limit outdoor activity, especially strenuous activity. This means postponing outdoor exercise, yard work, or sports practice if possible. If you must be outside, try to stay outside during early morning or evening hours, when ozone levels are typically lower than midday. Ozone forms throughout the day as sunlight builds, so the worst hours are usually late afternoon.
Keep windows closed on high-smog days, especially if you have air conditioning. Air conditioning filters out some particles and ozone, while open windows let polluted air flow directly into your home. If you do not have air conditioning, closing windows and staying in the coolest room of your house is still better than opening them to outside air.
If you have asthma or another respiratory condition, keep your rescue inhaler with you and follow your doctor's instructions about when to use it. Some people need to take their regular maintenance medications more often on high-smog days. If you notice symptoms that do not improve with your usual treatment, contact your doctor or seek urgent care.
Long-term health effects of repeated smog exposure
Living in an area with frequent smog can affect your health over months and years, not just on individual high-pollution days. Studies show that people who live in high-smog areas have higher rates of asthma, reduced lung function, and increased risk of heart disease compared to people in cleaner areas. Children who grow up in smoggy regions may develop smaller lung capacity than they would have otherwise, which can persist into adulthood.
Repeated ozone exposure can cause permanent changes to the airways, making them more reactive and sensitive to other irritants. This means someone who has lived with chronic smog exposure may develop asthma symptoms even after moving to a cleaner area. The risk is higher for people who already have genetic factors that predispose them to lung disease.
Older adults with existing heart or lung disease face the most serious long-term risk. Studies have linked chronic smog exposure to earlier death in people with heart disease or COPD. This does not mean one smog day is dangerous, but it means that if you live in a chronically polluted area and have a heart or lung condition, reducing your exposure over time matters for your long-term health.
Frequently Asked Questions
Is smog the same as air pollution?
Smog is a type of air pollution — specifically, a visible mixture of gases and particles that forms when sunlight reacts with certain pollutants. Air pollution is a broader term that includes all harmful substances in the air, including pollution you cannot see. You can have poor air quality without visible smog.
Can I get sick from smog if I stay indoors?
Smog outdoors does not directly reach you indoors if your windows and doors are closed. However, some ozone and particles can seep in through cracks and ventilation systems. If you have air conditioning with a good filter, your indoor air will be cleaner than outside. If you do not have air conditioning, your indoor air quality will be closer to outdoor levels.
Does a mask protect me from smog?
A regular cloth or surgical mask does not filter out ozone gas, which is a major component of smog. An N95 or P100 respirator can filter out some particles, but it does not filter ozone either. The most effective protection is limiting time outdoors and staying in filtered indoor air when smog levels are high.
Why is smog worse in some cities than others?
Smog depends on three things: the amount of pollution sources (cars, factories), the weather (sunlight, temperature, wind), and geography (mountains that trap air). Los Angeles and other cities surrounded by mountains tend to have worse smog because air gets trapped. Cities with more cars and less wind also see more smog. Cold, still weather in winter can trap pollution near the ground.
Does smog go away on its own?
Smog clears when wind disperses the pollutants or when weather changes. A strong wind can push smog out of an area in hours. Rain can wash some particles out of the air. At night, when sunlight stops, photochemical smog stops forming, though it may not disappear completely. The only permanent solution is reducing the pollution sources that create smog in the first place.