All eyes and temperatures are on Phoenix as the temperatures rise, after temperatures in the city topped 110 degrees Fahrenheit for a record 55 days last summer.

The 2023 Phoenix heatwave broke records, drew national attention and led to a record number of heatstroke deaths, with 645 people killed. The Maricopa County coroner is tasked with investigating each of those deaths and determining whether heat was a significant factor.

Chief Medical Examiner Dr. Geoff Johnston is preparing to conduct more research in the hopes that the data can help public health officials prevent heatstroke deaths.

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Jeff Johnston: We had two summers where the number of deaths spiked, especially in July. We've learned a lot of lessons about where our processes are bottlenecked and we need to do something different to better address that high mortality. So we're really applying the lessons of the last two years and ramping up our training at this stage to prepare for the coming weeks.

LAUREN GILGER: So have you ramped up or are you able to ramp up staffing or equipment, and are you basically anticipating the morgues being full?

Johnston: So we're always planning for surges like that, and we're also thinking about storage capacity. And a lot of it is really about in- and out-flows — how many patients we have that need testing for hospitalizations, deaths, and it obviously takes time to go through those processes. So it's about how do we make that as quick as possible without compromising the quality of our work.

And then there's the release aspect, which depends on the funeral home, the family's decision and all those kinds of factors. So there are a lot of variables.

GILGER: So let's talk about when we determine if someone has died from heat stroke or heat exhaustion. What does that mean specifically? This is the first time we've had a heat stroke death this year, and that was before temperatures hit triple digits, right?

Johnston: Yes, what we do is the same as what we do with every other death we investigate here at the Coroner's Office. Approximately 17% of all deaths that occur in the county each year are those that are required by law to have a professional medical death investigation.

So we're going to dig a little deeper so we can be more certain about what the cause is and look at other factors associated with that death so people can intervene and reduce deaths in their communities.

In other words, death due to heatstroke refers to a case where a person is exposed to an extremely hot environment and the heat is thought to have had a significant impact on death.

GILGER: So what does “significant” mean? You're looking at all the things that caused the deaths, and does that include heat? Does that count as a heat-related death?

Johnston: Yes. So we want to make sure that we're covering all the things that we think he played a significant role in. So we're not just going to count people who died because they happened in the summer or because they were outdoors.

We're really looking at what kind of exposure they had, what other risks they might have, what evidence is there that this person couldn't get out of that heat and into a cooler environment, things like that.

GILGER: I think a lot of this has to do with rising temperatures in the Valley, but we're also seeing an increase in homelessness. Talk to us about other factors that are at play here and other people who are really at risk.

Johnston: There's a significant number of people over the age of 40. Indeed, as you say, the percentage of heat stroke deaths has increased in recent years. Are any of you experiencing homelessness? This is not surprising. If you don't have access to cooler environments, you are at greater risk for longer periods of time.

Indeed, people suffering from substance use disorders, especially methamphetamines, can weaken our bodies' ability to adapt to heat, helping to mitigate some of the effects of heat on our bodies.

GILGER: We've been hearing a lot about the rise of fentanyl and how deadly that drug is in particular. Are you seeing a connection with heatstroke deaths?

Johnston: Fentanyl and methamphetamine are the two leading causes of drug-related deaths, including cases where heat is also a contributing factor. We tend to see more methamphetamine than fentanyl in homeless people, and I think part of that is the heat connection, because heat counteracts a lot of the mechanisms in your body that are trying to cool you down.

GILGER: So, in that case, how do these things get counted, and when you count the numbers — and the numbers are becoming more significant every year, right? If there were something like 640 heat stroke-related deaths last year, if someone overdoses on fentanyl or methamphetamine and they're outside, would that be considered a heat stroke-related death?

Johnston: Yes. So we qualify it as fever-related. And it depends on the individual details in each individual case. And then our forensic pathologists look at all the data and determine whether it's primarily drug-related and the fever is just contributing to that, or the other way around, where the fever is actually something new and the fentanyl or methamphetamine intoxication is contributing.

GILGER: What do you want people to know about this? Do you think enough people take this into consideration when, like, going out in the summer or making life decisions? It seems like the conversation has really changed.

Johnston: Yeah, I'm noticing that too. I'm glad there's more awareness out there. I think it's really hard. We all know it's hot here in the Sonoran Desert in the summer. It's not surprising.

The real question is, when is it too hot, and when is it too hot? People who have cardiovascular disease and other medical risk factors are probably older. Their tolerance to heat is probably lower than someone who is younger and healthier. But that doesn't mean there aren't some hot situations where even young, healthy people shouldn't go hiking. It's just too dangerous.

The National Weather Service has introduced a new tool, which has been around for a while in the US, but has opened it up to other regions that are really looking at heat risk. It has three color-coded levels: moderate, severe, and extreme. With this information, you can get a sense of what the conditions will be like for the day.

There is a serious risk over the next three days. It is dangerous for many people. It is wise to try to avoid the heat and danger of the day. Check on people who may be at risk. Check on neighbors more often if they are elderly or at risk. These things can save lives in our community.

A transcript of KJZZ's The Show will be produced by deadline. This text may not be in its final form. The official record of KJZZ's show is the audio recording.