A heatwave walloping the Southwest claimed seven more lives in the Phoenix, Arizona area within the last week as temperatures exceeded 110 F during the day. 

The Maricopa County Department of Public Health has confirmed at least 25 heat-associated deaths this year, with seven of those occurring between July 16 and July 22. Nearly 250 additional deaths are under investigation. 

The region's county and city governments, hospitals, schools and nonprofit groups that operate several hundred cooling and hydrating stations across the area are closely watching the confirmed death figures, along with daily forecasts from the National Weather Service.

The deaths come as Phoenix has recorded nearly a month straight of temperatures reaching above 110 F. There has not been a single overnight low under 90 F since July 9, which means people's bodies aren't adequately recovering after the sun goes down, making them susceptible to heat illnesses that can result in death.

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Both this year and last, about 80% of the people who died fell ill while outdoors; more than a third were homeless; more than half of all heat-associated deaths involved drug use.

All of those who died indoors both this year and last were in uncooled environments. Most of the people had an air conditioner that was nonworking, turned off or nonexistent. In three cases this year, there was no electricity to power a cooling system.

The Tucson sector of the U.S. Border Patrol reported that during the first three weeks of July it had received 151 calls for help and rescued more than 1,100 migrants in the sweltering desert near the U.S.-Mexico border. Agents have also encountered human remains.

The Associated Press contributed to this report.