Millions of Americans sweltered in stifling heat and humidity Thursday with dangerous temperatures expected to hit swaths of densely populated areas through the Fourth of July holiday weekend.

Heat alerts span from Illinois to Florida and across the Northeast and New England.

As millions plan to celebrate the Fourth of July outdoors, meteorologists warn temperatures could soar to record-breaking, life-threatening highs.

Sweltering conditions are expected to last through the weekend

Sweltering conditions are expected to last through the weekend