Half of the U.S. population was urged on Friday to take precautions when they are outdoors, either due to a relentless heat wave or poor air quality caused by Canadian wildfires ahead of the long Fourth of July weekend.

Some 80 million people living in California, the Deep South and the lower Mississippi Valley were under excessive heat warnings and heat advisories through the day and into the weekend, before Tuesday's holiday, the National Weather Service said.

Heat indexes - a measure that combines temperature and humidity - were expected to soar well past 100 degrees Fahrenheit in many parts of California and the two regions on Friday. Forecasters recommended that people should not participate in strenuous activities outdoors, especially in the middle of the day, and drink plenty of water.

Much of New York State will continue to be under air quality health advisory on Saturday, according to a release on Friday by the New York State Department of Environmental Conservation.

The whole state has been under the advisory from Wednesday to Friday due to impacts of haze and smoke from Canadian wildfires.

The air quality was ranked as unhealthy or unhealthy for sensitive groups on Friday with air quality index (AQI) value topping 160 in New York City.

"Individuals sensitive to poor air quality, including the elderly and the young, should limit strenuous activities and time outdoors," said a post by National Weather Service New York.

Detroit, Washington, D.C. and New York City ranked the top six major cities in the world with the worst air quality on Friday morning, according to IQ Air's live ranking of major world cities.

(With input from agencies)