PHOENIX (AP) — Postal worker Eugene Gates Jr. was delivering mail in the suffocating Dallas heat this summer when he collapsed in a homeowner’s yard and was taken to a hospital, where he died.

Carla Gates said she’s sure heat was a factor in her 66-year-old husband’s death, even though she’s still waiting for the autopsy report. When Eugene Gates died on June 20, the temperature was 98 degrees Fahrenheit (36.6 Celsius) and the heat index, which also considers humidity, had soared over 110 degrees Fahrenheit (43.3 Celsius).

“I will believe this until the day I die, that it was heat-related,” Carla Gates said.

Even when it seems obvious that extreme heat was a factor, death certificates don’t always reflect the role it played. Experts say a mishmash of ways more than 3,000 counties calculate heat deaths means we don’t really know how many people die in the U.S. each year because of high temperatures in an ever warming world.

That imprecision harms efforts to better protect people from extreme heat because officials who set policies and fund programs can’t get the financial and other support needed to make a difference.

“Essentially, all heat related deaths are preventable. People don’t need to die from the heat,” said epidemiologist Kristie L. Ebi, who focuses on global warming’s impact on human health as a professor at the University of Washington.

With a better count, she said, “you can start developing much better heat wave early warning systems and target people who are at higher risk and make sure that they’re aware of these risks.”

Currently, about the only consistency in counting heat deaths in the U.S. is that officials and climate specialists acknowledge fatalities are grossly undercounted.

“Deaths are investigated in vastly different ways based on where a person died,” said Dr. Greg Hess, the medical examiner for Pima County, Arizona’s second most populous county and home to Tucson. “It should be no surprise that we don’t have good nationwide data on heat-related deaths.”

Many experts say a standard decades-old method known as counting excess deaths could better show how extreme heat harms people.

“You want to look at the number of people who would not have died during that time period and get a true sense of the magnitude of the impact,” Ebi said, including people who would not have suffered a fatal heart attack or renal failure without the heat.

The excess deaths calculation is often used to estimate the death toll in natural disasters, with researchers tallying fatalities that exceeded those that occurred at the same time the previous year when circumstances were average.

Counting excess deaths was used to calculate the human impact of a heat wave in Chicago that killed more than 700 people in July 1995, many elderly Black people who lived alone. Researchers also counted excess deaths during the COVID-19 pandemic to provide more complete information about deaths directly and indirectly related to the coronavirus.

But as things stand now, the Centers for Disease Control and Prevention reports just 600 to 700 heat deaths annually in the United States. A study published last month in the journal Nature Medicine estimated more than 61,000 heat-related deaths last summer across Europe, which has roughly double the U.S. population but more than 100 times as many heat deaths.

Dr. Sameed Khatana, a staff cardiologist at the Philadelphia VA Medical Center and assistant professor at the University of Pennsylvania’s Perelman School of Medicine, has said deaths in which heat contributed significantly to fatalities from causes like heart failure should also be considered.

Khatana participated in research published last year that counted excess deaths in all U.S. counties. The findings suggested that from 2008 to 2017 between 3,000 to 20,000 adult deaths from all causes listed on death certificates were linked to extreme heat. Heart disease was listed as the cause of about half of the deaths.

After the Pacific Northwest heat wave in summer of 2021, the Canadian province of British Columbia reported more than 600 deaths due to heat exposure while Oregon and Washington each initially reported a little more than 100 such fatalities.

“It’s frustrating that for 90 years public health officials in the United States have not had a good picture of heat-related mortality because we have such a bad data system,” said Dr. David Jones, a Harvard Medical School professor who also teaches in the epidemiology department at the Harvard T.H. Chan School of Public Health.

There is no uniformity among who does the counting across U.S. jurisdictions. Death investigations in some places might be carried out by a medical examiner, typically a physician trained in forensic pathology. In other locales, the coroner could be an elected sheriff, such as the one in Orange County, California. In some small counties in Texas, a justice of peace might determine cause of death.

Utah and Massachusetts are among states that do not track heat-related deaths where exposure to extreme heat was a secondary factor.

The CDC, which is often several years behind in reporting, draws information on heat deaths from death certificate information included in local, state, tribal and territorial databases.

The CDC said in a statement that coroners and others who fill out death certificates “are encouraged to report all causes of death,” but they may not always associate those contributing causes to an extreme heat exposure death and include the diagnostic codes for heat illnesses.

Hess, the Arizona coroner, said determining environmental heat was a factor in someone’s death is difficult and can take weeks or even months of investigation including toxicological tests.

“If someone was shot in the head, it’s pretty obvious what happened there,” Hess said. “But when you find a body in a hot apartment 48 hours after they died, there is a lot of ambiguity.”

Hess noted that Pima County this year began including heat-related deaths in its tally of environmental heat fatalities. Maricopa County, home to Phoenix, America’s hottest big city, for years has included heat-related deaths. Clark County, Nevada, home to Las Vegas, now also considers deaths in which heat was a contributing factor.

Maricopa’s Public Health Department counted 425 “heat associated” deaths last year, including those where heat was a secondary factor, such as a heart attack provoked by high temperatures.

It reports there were 59 heat-associated deaths confirmed this year through Aug. 5, with another 345 under investigation. The latest count follows the hottest month in Phoenix on record, and a record 31 consecutive days that hit 110 degrees Fahrenheit (43.3 Celsius) or higher.

Dallas, which regularly sees summer highs over 100 degrees Fahrenheit (37.7 Celsius), sweltered through an excessive heat warning this month and also grapples with oppressive humidity.

Carla Gates, whose mail carrier husband died, noted cities worldwide now must learn to deal with extreme weather. She said her spouse, with 36 years on the job, tried to protect himself by taking a chest filled with ice and several bottles of cold water on his rounds.

“Our climate has changed,” she said. “And I don’t think it’s going back to how it was 20 years ago. So we’re going to have to get used to it and we’re going to have to make some adjustments.”

Now she wants to honor her husband by pushing legislation to ensure people working outside are better protected from the heat. Gates noted that the day her husband died he was in an old mail truck without working air conditioning.

“I don’t wish this on anyone, anyone to get a phone call that their loved one died working, doing something that they love in the heat,” she said.

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LaFleur reported form Dallas. AP writers from around the U.S. also contributed.