Smoke from Canadian wildfires continues to trigger air quality alerts in U.S. states, with health officials warning people, especially children, the elderly or those with respiratory conditions, to limit their time outdoors.

The Federal Aviation Administration issued a ground stop Thursday on all flights bound for New York City’s LaGuardia Airport due to poor visibility. The Air Quality Index spiked to “hazardous” levels in Philadelphia, where everyone was urged to stay inside. And employees at the National Mall in Washington, D.C., were encouraged to scale back all outdoor activities.

Updated forecast maps showed the lingering smoke slowly drifting south. And while conditions are expected to improve in the Northeast and Atlantic Coast on Thursday, they are likely to worsen in Pittsburgh, Cleveland and Detroit.

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