PHILADELPHIA — Shrouds of smoke from wildfires in the Pacific Northwest again are congesting the skies over much of the nation — and they are forecast to persist well into Wednesday.

While smoky haze from distant wildfires isn’t all that unusual, said Tom Kines, senior meteorologist with AccuWeather Inc., “to have it this thick, it doesn’t happen very often.”

Air-quality alerts were in effect on both sides of the Delaware, with health officials advising those with heart or lung issues to eschew strenuous outdoor activity. However, the fact that the smoky veil will remain so high in the atmosphere should mitigate the health impacts, said Kines.