Air quality is a measure of how clean or polluted the air is.

The two main culprits of poor air quality are particulate matter and ground-level ozone.

Particulate matter (PM), or airborne particles, includes dust, dirt, soot, and smoke.

These particles are so small that they can be inhaled, causing serious health problems.

The smallest are a fraction of the size of a human hair, even smaller than grains of sand.

Ground-level ozone, on the other hand, is a gas (and the main ingredient in smog).

It forms when certain pollutants chemically react in the presence of heat and sunlight.

Even low concentrations can trigger a variety of health problems.

It can cause respiratory issues, hospitalization for heart or lung diseases, and even death.

The EPA estimates that poor air quality causes more than 100,000 deaths in the U.S. annually.

Seniors, children, and those with compromised immune systems are especially at risk.

How can you protect yourself?

Outdoor activity and exercise of all kinds can help your heart stay healthy and resilient.

But stay inside when the air quality is poor.

If you must go out, limit your time outside.

The Air Quality Index (AQI) reports the daily air quality.

It tells you how clean or polluted your air is, and what associated health effects might be a concern for you.

See the current AQI for your area at airnow.gov.