By Brendan O'Brien
CHICAGO (Reuters) - 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 (38 Celsius) in many parts of California and the two regions on Friday. Forecasters recommended that people not participate in strenuous activities outdoors, especially in the middle of the day, and drink plenty of water.
To the north, across the Midwest and East, another 100 million Americans faced another day of smoky skies and poor air quality alerts due to raging wildfires in Canada.
The weather service said individuals, especially those who are young, elderly and suffer from respiratory problems, should consider limiting strenuous outdoor activities in those areas.
New York and Washington had the second and third worst air quality respectively of any major cities around the globe, according to IQAir.com, which tracks global air pollution.
Some people in eastern Illinois and western Indiana faced a hot and humid day of compromised air quality on Friday without electricity after fierce storms on Thursday evening knocked down power lines and trees.
About 280,000 homes and businesses in the area were without power as of Friday morning, according to Poweroutage.us, which tracks outages across the U.S.