Thousands of Americans in parts of California have been advised to stay inside due to "unhealthy" air conditions.

Air quality is currently in the "unhealthy" Air Quality Index (AQI) category around the Lassen National Forest in Lassen County as of 3.40 a.m. ET, according to The Environmental Protection Agency's (EPA) AirNow map . The area includes the towns of Greenville and Quincy.

The warning means that individuals in sensitive groups - such as those with respiratory illnesses - may experience "more serious health effects," AirNow said.

What To Know

The AQI, used by AirNow and developed by the EPA, is a nationwide system that communicates air pollution levels. It ranges from 0 to 500 and uses color-coded categories to reflect the intensity of air quality concerns. It rates air pol

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