What the Air Quality Index Means for Your Health—and How to Stay Safe

Earlier this month, the catastrophic wildfires in LA coincided with a 16-fold rise in hospital visits for fire-related injuries, including smoke exposure, according to the Los Angeles Times. In 2024, Hurricane Helene devastated the Southeast, including Georgia, North Carolina, Tennessee, and Virginia, triggering concerns about mold, dust, and toxic chemicals in the aftermath of record-breaking rainfall. In 2023, plumes of smoke from Canada’s most destructive wildfire season clouded the skies of the Great Lakes region, from Illinois to Ohio to New York and beyond.

Meanwhile, 2024 was a record-breaking year for global carbon emissions from fossil fuels—and the Environmental Protection Agency (EPA) reports that about 40 percent of people in the U.S. live in areas where air pollution is high enough to potentially affect their health.

Breathing in bad air could lead to immediate symptoms like a scratchy throat or a tight feeling in your chest, but scientists are unearthing a host of possible long-term health effects, too. That’s why paying attention to the air quality index (AQI) is so important, no matter where you live. Here’s how you can use it to protect your well-being.

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What each category in the air quality index means

The EPA relies on a number of tools—weather forecast models, satellites, air sampling data, and more—to come up with the air quality index (AQI), which lets people know what the outdoor air quality is like in their area. (FYI, you can find your local AQI on your phone’s weather app or AirNow.) The scale is split into six categories organized by color, each linked to a different level of public health concern.

“The darker the color on the index, the worse the concentration of pollutants in the air from a variety of sources,” says Loren Wold, PhD, a professor in the Division of Cardiac Surgery at The Ohio State University College of Medicine who has researched the effects of particulate matter for 20 years. Here’s the breakdown:

• Green (0–50) — Good: Air is satisfactory and poses little to no risk
• Yellow (51–100) — Moderate: Air quality is acceptable, though it might affect people who are highly sensitive to pollution (such as people with allergies or asthma)
• Orange (101–150) — Unhealthy for Sensitive Groups: Air quality may pose some health effects on infants and children, older adults, pregnant people, and people living with chronic conditions like heart disease, lung disease, and diabetes
• Red (151–200) — Unhealthy: Air quality will cause people in sensitive groups to experience more serious health effects, while all other groups will also experience respiratory effects (e.g., coughing, throat irritation, etc.)
• Purple (210–300) — Very Unhealthy: Air quality causes everyone to face some risk of health effects or respiratory symptoms
• Maroon (301+) — Hazardous: Air quality has turned into an emergency and public health is a top concern

What causes poor air quality?

The EPA is actively trying to regulate tons of hazardous air pollutants—188, to be exact. When reporting the AQI, however, the agency keeps close tabs on five major pollutants regulated by the Clean Air Act, a federal law that was first passed in1963 which requires the government to regulate air pollution:

Ground-level ozone

Not to be confused with the good kind of ozone, which naturally exists in Earth’s upper atmosphere, protecting us from the sun’s harmful ultraviolet rays. Bad ozone cooks up near the ground when pollutants emitted by, say, cars, power plants, and industrial boilers spur a chemical reaction in the sunlight, acting as a key player in the mix of smog. Ground-level ozone tends to spike in the summer when lots of sun and higher temps are the norm. It’s most common in cities but wind can also whisk it into rural areas.

Carbon monoxide

You can’t see or smell carbon monoxide, but it forms when the carbon in fuels doesn’t totally burn off. Car exhaust makes up about 75 percent of all outdoor carbon monoxide emissions in the U.S., and up to 95 percent in our cities specifically, per the EPA. Fuel combustion in industrial processes (like metal manufacturing or oil or gas extraction) as well as natural sources like wildfires can also spike levels. Carbon monoxide tends to be more of an issue in the winter since colder temps can mess with car systems.

It’s also worth noting that a bunch of things in your home—a leaky furnace or chimney, gas stove, portable generator, and more—can emit carbon monoxide and affect indoor air quality too. Breathing in high concentrations of carbon monoxide can cause carbon monoxide poisoning, which can be dangerous, and sometimes even deadly.

Sulfur dioxide

Sulfur dioxide is a colorless gas that forms when sulfur-containing fuels like coal and oil are burned. If you live near big industrial facilities like power plants, you might have more of this stuff in your air. Locomotives, ships, and other vehicles and heavy equipment also burn fuel that packs a lot of sulfur.

Nitrogen dioxide

This reddish-brown gas develops when fossil fuels like coal, oil, methane, or diesel are burned at high temps. You might breathe more nitrogen dioxide in if you live near a factory or highway.

Particulate matter

Also known as soot, particle pollution is made up of extremely tiny solid particles and liquid droplets, making them especially risky because they can nestle deep into your lungs. The finest particles measure 2.5 micrometers in diameter, or about the size of a red blood cell, says Dr. Wold. Major sources of these microscopic troublemakers include natural disasters like wildfires and hurricanes, wood or agricultural burning, cars, dust, and a slew of industrial processes. Coarser particles up to 10 micrometers in diameter (smaller than the width of a human hair) are also included in the AQI.

So, how does air quality affect your health?

When you inhale air pollutants, your body sees those teeny-tiny particles as a threat and tries to expel them ASAP—say, by causing you to hack them up, and by setting off an inflammatory response, Dr. Wold explains. When the AQI creeps toward 100 (orange on the AQI chart) and up, you might notice respiratory symptoms like wheezing, shortness of breath, and coughing, he adds.

Dr. Wold also stresses that studies have shown those super-fine particles can pass through your lungs and infiltrate your bloodstream, setting off sudden heart problems like an arrhythmia (an irregular heartbeat) or a heart attack, especially in people with an increased risk of cardiovascular events, such as older adults and people with underlying heart conditions.

As for long-term exposure to lots of air pollution? Research has linked living or working in areas with poor air quality to all sorts of chronic health conditions, including heart disease, respiratory and lung diseases, cancer, reproductive concerns, neurological conditions, and autoimmune disorders.

A lot of this goes back to inflammation, Dr. Wold says. When your body is persistently trying to protect you with a cascade of inflammatory chemicals, research shows it can start to go rogue and attack healthy tissues, damage DNA, and cause oxidative stress (an imbalance of molecules that harms cells)—all well-established precursors to chronic illness.

Let’s not overlook the possible psychological effects: Studies² have also linked long-term exposure to air pollution (especially fine particulate matter) with an increased risk of mental health conditions like anxiety, depression, and schizophrenia.

Keep in mind that not everyone is affected equally. While sensitive groups are physically more susceptible to these risks, the American Lung Association emphasizes that social determinants of health—such as your race and ethnicity, income level, and access to healthcare—can affect your susceptibility to experiencing pollution-related health issues. While some of these risk factors are non-modifiable (meaning, you can't change them), there are some things you can do to help lower your risk of breathing in air on a poor AQI day.

When it’s a good idea to stay inside (and how to stay safe if you can’t)

“If you’re generally healthy, as long as the AQI is not in the unhealthy (red) range, it’s okay to be outside,” Dr. Wold says. “But anyone in sensitive groups really should limit their exposure outside when the AQI is above moderate (orange).”

When the air is particularly bad outdoors, Dr. Wold says you should consider taking a few steps to keep the air cleaner inside your home. Keep your windows and doors shut, run an air purifier with a HEPA filter, and limit cooking that releases particles into the air (like frying). If you have an HVAC system, make sure you regularly swap those filters, too.

If you absolutely need to go outside when the AQI is high, avoid doing strenuous exercise or hanging out for too long. “It’s not the best time to be going for a walk, training for a marathon, or working in your yard all day,” Dr. Wold says. Wearing any type of facial covering over your nose and mouth also helps, but a high-quality N95 mask is the most effective at filtering out those risky ultra-fine particles, he adds.

The bottom line: It’s nearly impossible to control something as vast as the air you breathe but being aware of how it can affect your body—and the little changes you can make to stay a little safer—can go a long way in protecting your health.