PHOENIX — It's a hot morning in Phoenix and Paul Yager is getting his vital signs checked at a mobile clinic providing care to homeless patients. He's 64, he's HIV positive and on most nights he sleeps in a park nearby. He credits this team with keeping him alive.

"I've got a lot of life to live, and with God's help, maybe I can live another 10 years," Yager said.

But surviving summers in Phoenix without shelter is hard. In July, when temperatures here stayed above 110 for over a week, Yager said he collapsed and couldn't get up for hours.

"I'm not good anyhow, so it's just not good — it's not healthy for me to be out in this kind of weather," Yager said.

No major U.S. city gets more triple-digit days than Phoenix. But that famous desert heat is harming more and more Arizonans each year. The Phoenix metro area averaged 78 heat-associated deaths per year from 2005 to 2015, according to county records. But the death toll has reached a record-breaking high every summer since 2016. Last year, the region saw an unprecedented 339 heat deaths. This year is on track to be the deadliest yet. Advocates say the real concern is not that Arizona has too much hot weather, but that it doesn't have enough homes.

"This is a really bad summer for us," Dr. Kevin Foster, director of the Arizona Burn Center, told reporters in July.

Pavements can heat up to more than 150 degrees in the Phoenix sun. Every summer, Foster treats patients who fall, can't get up and develop severe burns.

The Arizona Burn Center has treated a high volume of patients this year. And Foster said patient demographics are changing. In the past, patients have typically been older adults who struggle with balance. Recently, Foster's patients have been younger. He said that now they are more often homeless and that more of their falls are related to substance abuse.

"They go down and they stay down for a long time. They end up not only getting really bad burns, but they suffer heat prostration and heatstroke. Oftentimes, their temperatures coming in are 108 or 109 degrees Fahrenheit."

County records show similar demographic shifts. Heat deaths are increasingly occurring outdoors among homeless people. About 60% of cases involve substance use.

"Each and every one of these deaths can be prevented," said David Hondula, director of Phoenix's newly launched Office of Heat Response and Mitigation. "My interpretation is the increase [in heat fatalities] is much more related to what's happening with social services than it is related to climate."

Hondula is concerned that the region's already-hot temperatures are rising. The National Weather Service projects Phoenix will average more than 120 days per year with triple-digit heat by the end of this decade.

But Hondula is more troubled by another trend. The unsheltered homeless population of Maricopa County, where Phoenix is located, has tripled since 2016.

A construction shortage dating back to the 2008 Great Recession, paired with explosive population growth, has sent housing prices skyrocketing. That's contributing to a growing population of Arizonans without homes. Hondula said that's turning heat into a more critical public health threat.

"Our unsheltered neighbors are absolutely at the highest risk of heat-associated death," Hondula said. "Our best estimate is that the unsheltered community is at about 200 to 300 times higher risk than the rest of the population."

It's not just the long hours spent outdoors. Hondula said people without shelter also have limited access to medical care, increased likelihood of chronic health problems and high rates of addiction, all of which can raise risk.

Dehydration and exhaustion also can be disastrous for mental health, said psychiatric nurse practitioner Nina Gomez, at the mobile medical clinic run by the nonprofit Circle the City.

"The stress from the heat really exacerbates psychosis, and then it becomes so much harder to get people in to engage in any services," Gomez said.

The city of Phoenix is making large investments to address the housing crisis, announcing in June that it was allocating ​$70.5 million for affordable housing and homelessness programs. But these issues can't be solved overnight. So for now, organizations like Circle the City try to deliver short-term solutions.

"We're trying to intervene early, so get people hydrated, get them some food, see if they need anything before it gets to a full crisis," Gomez said.

And as the summer drags on, Yager and other unsheltered people at the clinic say they'll drink water, keep a hat on and just try to stay cool.