Smog Plus measures ground-level ozone and particle pollution together

Smog Plus is an air quality index that combines two of the most common outdoor pollutants: ground-level ozone and fine particulate matter (PM2.5). Unlike a standard air quality report that might list these separately, Smog Plus shows you a single number that reflects how both pollutants together affect the air you breathe on a given day.

Ground-level ozone forms when sunlight reacts with emissions from cars, factories, and power plants. Fine particles come from vehicle exhaust, wildfires, dust, and industrial sources. Both can irritate your lungs and make breathing harder, especially during physical activity or for people with asthma. Smog Plus exists because these two pollutants often spike at the same time and in the same places, so tracking them together gives you a more realistic picture of outdoor air safety.

The index typically uses a color scale — green, yellow, orange, red, and sometimes purple — with higher numbers meaning worse air quality. Each color corresponds to health guidance: what activities are safe, who should stay indoors, and when to use a mask or avoid strenuous exercise.

Key Takeaways

  • Smog Plus combines ground-level ozone and fine particle pollution into one number so you can see both threats at once.
  • The color scale (green to purple) tells you whether outdoor activity is safe for your age and health status.
  • Ozone peaks on hot, sunny days; particles peak during wildfires or heavy traffic, so Smog Plus readings change with season and weather.
  • People with asthma, children, older adults, and people doing outdoor work should check Smog Plus before spending time outside.

How the Smog Plus scale works

Smog Plus assigns a number from 0 to 500+, and each range gets a color and a health message. A reading of 0–50 (green) means air quality is good and outdoor activity is safe for everyone. A reading of 51–100 (yellow) means moderate air quality; people with respiratory conditions should limit strenuous outdoor activity. Orange (101–150) means unhealthy for sensitive groups — children, older adults, and people with lung or heart disease should avoid prolonged outdoor exertion.

Red (151–200) means unhealthy for the general population; everyone should reduce outdoor activity. Purple or maroon (201+) means very unhealthy or hazardous; most people should stay indoors, and outdoor workers should wear respiratory protection. The exact thresholds and color names can vary slightly by region or the organization publishing the index, so check your local air quality agency's definitions.

The index updates hourly or several times a day, depending on your location. Some areas have real-time monitors; others update once daily. Knowing whether your area uses hourly or daily updates helps you decide whether to check in the morning for the day ahead or closer to when you plan to go outside.

When Smog Plus readings are highest

Ground-level ozone peaks in summer and early fall, especially on hot, sunny, windless days. Heat and sunlight speed up the chemical reaction that creates ozone, so afternoon readings are usually worse than morning readings. In winter, ozone drops because there is less sunlight and cooler temperatures slow the reaction.

Fine particles spike during wildfires (which can happen any season but are most common in late summer and fall in many regions), during heavy traffic periods, and when weather patterns trap air close to the ground. A temperature inversion — a layer of warm air sitting above cooler air — can trap pollutants and send Smog Plus readings into the orange or red range even if emissions are normal.

If you have asthma, work outdoors, or exercise regularly, tracking Smog Plus over a few weeks teaches you when your area's worst days typically occur. That knowledge helps you plan indoor activities or reschedule outdoor work to safer times.

Where to find your local Smog Plus reading

Your state or regional environmental or air quality agency publishes Smog Plus data. In the United States, the Environmental Protection Agency (EPA) maintains AirNow.gov, which shows real-time air quality for your zip code using the standard Air Quality Index (AQI). Some states and cities also publish their own Smog Plus or similar combined-pollutant indices on their environmental agency websites.

Search "[your state] air quality index" or "[your city] air quality" to find the official source for your area. Many weather apps and services also display air quality data, though they may use the standard AQI rather than Smog Plus specifically. If you want the Smog Plus index in particular, contact your state's environmental agency or air quality division to confirm whether they publish it and where.

Some areas offer email or text alerts when air quality reaches unhealthy levels. Signing up for alerts means you do not have to check manually every day; you only get notified when conditions change enough to affect your plans.

What to do on high Smog Plus days

On yellow days, people with asthma or heart disease should reduce strenuous outdoor activity — that means limiting intense exercise, heavy yard work, or outdoor sports. Children and older adults should also be cautious. On orange days, everyone should limit outdoor exertion; this is a good day to move workouts indoors or reschedule outdoor tasks.

On red or purple days, most people should stay indoors. If you must go outside, wear an N95 or P100 mask, which filters out fine particles. A cloth mask or surgical mask does not filter particles effectively. If you work outdoors and cannot avoid it on a high-pollution day, wear a respirator and take breaks indoors to let your lungs recover.

Keep windows closed on high-pollution days, and run your air conditioner on recirculate mode if you have one. If you do not have air conditioning, stay in the coolest, most sheltered room and use a portable air purifier with a HEPA filter if you have access to one. Avoid activities that generate additional pollution indoors, like cooking with gas, burning candles, or smoking.

Smog Plus and your health over time

A single day of orange or red air quality is uncomfortable but usually not dangerous for healthy people. Repeated exposure over weeks or months, though, can worsen asthma, increase heart attack risk, and reduce lung function — even in people without diagnosed lung disease. Children whose lungs are still developing and older adults whose lungs have aged are at higher risk from long-term exposure.

If you live in an area where Smog Plus regularly reaches orange or higher during certain seasons, talk to your doctor about whether you should take extra precautions, use preventive inhalers, or consider temporary relocation during peak pollution months. Keeping a log of your symptoms and Smog Plus readings can help your doctor see patterns and adjust your care.

On a community level, high Smog Plus readings are a sign that local air quality regulations may need strengthening or that emissions sources (traffic, industry, power plants) need to be addressed. If your area has consistently poor air quality, local environmental or public health groups often work on policy changes; joining or supporting those efforts is one way to address the problem at its source.

Frequently Asked Questions

Is Smog Plus the same as the Air Quality Index?

Smog Plus and the standard Air Quality Index (AQI) both measure air quality, but Smog Plus specifically combines ozone and fine particles, while the AQI can include other pollutants like nitrogen dioxide or sulfur dioxide depending on your region. Some areas use Smog Plus; others use the standard AQI. Check your local air quality agency to see which one they publish.

Can I exercise outdoors on a yellow Smog Plus day?

Light exercise is usually safe on yellow days for people without asthma or heart disease. If you have asthma, heart disease, or are over 65, avoid strenuous activity like running or intense sports. Walking at a normal pace is generally lower-risk than activities that make you breathe hard.

Do air purifiers help on high Smog Plus days?

A HEPA air purifier removes fine particles from indoor air and can help, especially in one room where you spend time. It does not remove ozone, though, so it is less effective if ozone is the main pollutant. Keeping windows closed and using your air conditioner on recirculate is usually the most effective indoor strategy.

Why does Smog Plus change so much from day to day?

Ozone and particle levels depend on weather, traffic patterns, industrial activity, and whether wildfires are burning upwind of your area. A hot, sunny, windless day traps pollution and raises ozone. A rainy day clears particles from the air. Wind can blow pollution in or out of your region. These changes happen naturally and are hard to predict more than a few days ahead.

Should I wear a mask indoors on a high Smog Plus day?

No. A mask protects you from outdoor pollution when you go outside. Indoors with windows closed, air quality is much better. Wearing a mask indoors all day is uncomfortable and unnecessary. Wear a mask only when you are outdoors or moving between indoors and outdoors on high-pollution days.