Health warnings have been issued in multiple US states on Tuesday, warning millions of Americans to limit outdoor activity as dangerous air pollution spreads across the country.

The National Weather Service (NWS) has issued air quality alerts in New York, New Jersey, Connecticut, Massachusetts, Rhode Island, Maine, New Hampshire, Pennsylvania, Delaware, Maryland and California, from 11am to 11pm.

The alerts across the Northeast and Mid-Atlantic were largely tied to dangerous levels of ground-level ozone, a pollutant created when heat and sunlight react with emissions from vehicles, industrial facilities and other sources. 

Officials warned the gas can irritate the lungs, trigger asthma attacks and cause breathing difficulties, particularly for children, older adults and people with heart or respiratory conditions.

Southern California was placed under separate warnings for wildfire smoke and fine particulate pollution known as PM2.5, microscopic particles small enough to penetrate deep into the lungs and bloodstream. 

'Particles in wildfire smoke can get deep into the lungs and cause serious health problems such as heart attacks, strokes, asthma attacks and difficulty breathing,' the NWS shared in the advisory.

'Everyone can be affected, but people with lung or heart disease, older adults, people who are pregnant, children, and those who spend a lot of time outdoors are at greater risk.'

Residents in affected regions have been urged to remain indoors when possible, avoid strenuous outdoor exercise and use air purifiers or N95 masks if they need to go outside.

In New York, health advisories covered New York City, the Bronx, Brooklyn, Queens, Staten Island, Long Island, Westchester and Rockland County, where officials warned the Air Quality Index could rise above 100 due to ozone pollution. 

Residents were urged to avoid strenuous outdoor activity, particularly children, older adults and people with asthma or heart disease.

New Jersey issued multiple Code Orange Air Quality Alerts spanning Bergen, Hudson, Essex, Union, Middlesex, Mercer, Monmouth, Ocean, Camden and several other counties, warning that ozone concentrations could become unhealthy for sensitive groups.

'A code orange air quality alert means that air pollution concentrations within the region may become unhealthy for sensitive groups,' according to the NWS.

'Sensitive groups include children, people suffering from asthma, heart disease or other lung diseases and the elderly. The effects of air pollution can be minimized by avoiding strenuous activity or exercise outdoors.'  

Massachusetts placed Boston, Cambridge, Quincy, Brockton and surrounding communities under alerts for elevated ozone levels, while Rhode Island issued similar warnings for Providence, Newport and other coastal regions.

Connecticut advisories stretched across Fairfield, New Haven, Middlesex and New London counties, with officials cautioning that air quality could deteriorate throughout the day as temperatures climbed.

In New Hampshire and Maine, environmental agencies warned that ozone levels were expected to rise into the ‘Unhealthy for Sensitive Groups’ category, especially along coastal regions including Portsmouth, Portland and Bar Harbor.

Pennsylvania, Delaware and Maryland also activated Code Orange alerts across the Philadelphia metro area.

The Baltimore region and multiple coastal counties are warning that children, seniors and people with lung or heart conditions face increased health risks if exposed outdoors for extended periods.

Meanwhile, Southern California faced a separate crisis tied to wildfire smoke and dangerous fine particulate pollution known as PM2.5.

The alerts covered most of Los Angeles County and all of Orange County, including downtown Los Angeles, Beverly Hills, Santa Monica, Pasadena, Malibu, Long Beach, Anaheim, Irvine and Huntington Beach.