What air quality means and why smog matters

Air quality is a measure of how much pollution is in the air you breathe. Smog is a visible form of air pollution — usually a brownish or grayish haze — created when sunlight reacts with emissions from cars, factories, and power plants. The term "smog" originally combined "smoke" and "fog," and it describes real chemical reactions happening in the atmosphere, not just reduced visibility.

Governments measure air quality using an index that tracks five main pollutants: ground-level ozone, particulate matter (PM2.5 and PM10), nitrogen dioxide, sulfur dioxide, and carbon monoxide. Each pollutant has a standard — a legal limit set by environmental agencies — and when air exceeds those limits, the air quality is considered unhealthy for certain groups or for the general public.

The reason this matters to you is practical: on high-pollution days, breathing that air can trigger asthma attacks, worsen heart disease, reduce lung function, and cause other health effects. Children, older adults, and people with respiratory or heart conditions face the greatest risk, but anyone can be affected by severe pollution.

Key Takeaways

  • Smog forms when sunlight reacts with vehicle and industrial emissions, and it indicates the presence of ground-level ozone and other harmful pollutants.
  • Air quality is measured on an index that tracks five main pollutants, each with legal limits set by environmental agencies.
  • High air quality days pose health risks to children, older adults, and people with heart or lung conditions, but severe pollution can affect anyone.
  • You can check your local air quality daily through government websites and apps that show whether the air is good, moderate, unhealthy for sensitive groups, or unhealthy for everyone.
  • When air quality is poor, reducing outdoor activity and using air filters indoors are the main ways to protect yourself.

How the Air Quality Index works

The Air Quality Index (AQI) is a standardized scale used in the United States and many other countries to communicate how polluted the air is on a given day. The EPA (Environmental Protection Agency) calculates the AQI for five pollutants and reports the highest one — meaning if ozone is bad but particulate matter is moderate, the AQI reflects the ozone reading.

The index runs from 0 to 500, divided into color-coded categories. A reading of 0–50 is green and labeled "Good." 51–100 is yellow and "Moderate." 101–150 is orange and "Unhealthy for Sensitive Groups." 151–200 is red and "Unhealthy." 201–300 is purple and "Very Unhealthy." Above 300 is maroon and "Hazardous." Each category comes with guidance about who should limit outdoor activity and how.

The AQI is reported daily for most U.S. cities and regions. You can find your local reading on AirNow.gov, which is run by the EPA, or through weather apps and local news websites that pull data from the same source. Many cities also send alerts when air quality reaches unhealthy levels.

Where smog comes from and why it forms

Smog is not a single pollutant but a mixture created by a chemical process. The main ingredient is ground-level ozone, which forms when nitrogen oxides (NOx) from car exhaust and industrial emissions react with volatile organic compounds (VOCs) in the presence of sunlight. This is why smog is often worst in the afternoon and early evening, when the sun is strongest, and why it tends to build up on hot, still days when air does not disperse.

The second major component of smog is particulate matter — tiny particles of dust, soot, smoke, and chemical compounds suspended in the air. These particles come directly from tailpipes, smokestacks, and construction sites, and they can remain in the air for days, traveling hundreds of miles from their source.

Geography and weather play a large role in smog formation. Cities in valleys or near coasts, where air tends to stagnate, experience worse smog than cities in open areas. High temperatures, low wind speeds, and atmospheric conditions that trap air near the ground all make smog worse. This is why Los Angeles, the San Francisco Bay Area, and cities in the Southwest and Midwest often report unhealthy air quality during summer and fall.

Health effects of smog and poor air quality

Breathing smog and polluted air can cause when ready and long-term health problems. Short-term exposure — even a few hours on a bad air day — can trigger coughing, wheezing, shortness of breath, and chest pain. People with asthma are especially vulnerable; a high-pollution day can bring on an asthma attack even in people whose asthma is usually well-controlled.

People with heart disease face a different risk: air pollution can cause inflammation in the lungs and bloodstream, raising blood pressure and increasing the risk of heart attack or stroke. Older adults and children have less developed or declining lung function, making them more susceptible to damage. Pregnant women exposed to high pollution may face risks to fetal development, though research is ongoing.

Long-term exposure — living in a polluted area for years — is linked to reduced lung development in children, accelerated lung decline in adults, increased risk of asthma, and higher rates of heart disease and premature death. The effects are dose-dependent: the more pollution you breathe over time, the greater the risk.

How to check air quality in your area

AirNow.gov is the official U.S. government source for air quality information. You can enter your zip code and see the current AQI, a forecast for the next few days, and health recommendations for each category. The site also shows which pollutant is driving the unhealthy reading, so you know whether the problem is ozone, particulate matter, or something else.

Many weather apps — including Apple Weather, Weather.com, and others — now include AQI data. Local news stations and city health departments often post air quality alerts on their websites or send them via email or text. If you live in a state with frequent air quality issues, your state environmental agency may run its own alert system.

For real-time, hyperlocal data, some cities and regions have installed air quality monitors that report readings every hour. If you have a smartphone, apps like BreezoMeter and IQAir show pollution levels block by block in some areas, though these rely on a mix of government monitors and private sensors.

What to do when air quality is poor

When the AQI reaches "Unhealthy for Sensitive Groups" (101–150), people in those groups — children, older adults, and those with heart or lung conditions — should reduce prolonged outdoor activity. This does not mean staying indoors entirely, but limiting strenuous exercise outdoors and taking breaks.

When the AQI reaches "Unhealthy" (151–200) or higher, the general public should also limit outdoor activity. Strenuous exercise like running or playing sports outdoors should be postponed or moved indoors. If you must go outside, wearing an N95 or P100 mask can reduce the amount of polluted air you inhale, though masks are not perfect and do not filter ozone.

Indoors, you can reduce exposure by keeping windows closed on high-pollution days, using air conditioning with a clean filter, and running a HEPA air filter in rooms where you spend the most time. Avoid activities that generate indoor pollution — cooking with gas, burning candles, or smoking — on days when outdoor air is already poor.

Air quality standards and who sets them

In the United States, the Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) for six pollutants: ozone, particulate matter, nitrogen dioxide, sulfur dioxide, carbon monoxide, and lead. These standards are based on health research and represent the concentration levels below which the EPA believes the public is protected from harm.

States are required to meet these federal standards. If a region fails to meet them, it is designated a "nonattainment area," and the state must develop a plan to reduce pollution. This can mean stricter emissions rules for cars and factories, limits on outdoor burning, or requirements for businesses to reduce their pollution output.

Other countries have their own standards. The European Union, China, India, and Australia all measure air quality and set limits, though the specific numbers and pollutants tracked vary. Some countries have stricter standards than the EPA; others have looser ones.

Frequently Asked Questions

Is smog the same as air pollution?

No. Smog is a visible form of air pollution — the brownish haze you can see — but air pollution includes many invisible pollutants like carbon monoxide and ozone. You can have unhealthy air quality without visible smog, and you can see smog on days when the AQI is only moderate.

Can an air filter in my home protect me from smog?

A HEPA filter can reduce particulate matter indoors, but it does not filter ozone, which passes through most home filters. Keeping windows closed and using air conditioning with a clean filter is more effective than relying on a portable air purifier alone.

Why is air quality worse in summer than winter?

Smog forms when sunlight reacts with emissions, so it is worst when days are long and hot. Winter has shorter days, lower temperatures, and more wind, all of which reduce smog formation. However, winter can bring other air quality problems, like smoke from wood burning and temperature inversions that trap pollution.

Do masks protect me from smog?

N95 and P100 masks filter particulate matter but not ozone or gases. They reduce but do not eliminate exposure to smog. Masks work best when fitted properly and worn consistently, and they are most useful for people doing outdoor exercise on high-pollution days.

What is a "code red" air quality day?

A code red day typically refers to an AQI reading in the "Unhealthy" range (151–200 or higher), though the exact definition varies by region. On these days, the general public is advised to limit outdoor activity, not just sensitive groups.