Public health officials across several US states warned residents on Wednesday to limit outdoor exposure as wildfire smoke pushed air quality into unhealthy ranges, threatening people with asthma, heart disease and other conditions. The National Weather Service issued air quality alerts for parts of the West and Midwest.

Research from the University of California San Diego and other institutions shows that wildfire smoke is more toxic than ordinary air pollution and can linger for weeks. Fine particulate matter, known as PM2.5, penetrates deep into the lungs and bloodstream, raising risks of respiratory and cardiovascular harm.

The US Environmental Protection Agency's AirNow system reported elevated PM2.5 readings in affected regions, with several cities recording air quality index values in the "unhealthy" range. State health departments advised residents to keep windows closed, use HEPA filters and wear N95 respirators outdoors where necessary.

Physicians said children, older adults, pregnant women and people with chronic lung or heart conditions faced the greatest danger. The American Lung Association urged those groups to monitor local alerts and avoid strenuous outdoor activity during peak smoke periods.

The alerts coincided with severe heat and wildfire activity across the Northern Hemisphere, compounding pressure on emergency services. State and federal officials said conditions could persist as long as active fires continued to burn and weather patterns carried smoke across state and national borders.