Heat wave death counts? Don't trust them. The true toll is higher.

Summer 2024 is barely 3 weeks old and it's already been a record-smashing season for heat extremes, with scores of record highs set from the deserts of California (124 degrees) to the mountains of Maine (98 degrees) over the past few weeks. And forecasters say still more extreme heat is on the way in upcoming weeks.

But it's not just about a number on a thermometer: The USA's ongoing heat wave has also been deadly. At least 30 fatalities across the West this month are suspected to be related to extreme heat, according to state officials and media reports.

The death toll will increase as the summer continues, but experts say the numbers won't ever be right. That's in part because heat kills in subtle and complex ways.

Extreme heat stresses the human body, which means its true toll can't be measured simply by counting obviously heat-related deaths (such as heat stroke or hot car deaths). Heat may not show up on a death certificate even if high temperatures played a role in causing a person's death.

Extreme heat kills and maims: Here are some of its victims from across the US.

“We know the number of workplace and traffic-related accidents increases during heat waves,” Bharat Venkat, director of the UCLA Heat Lab, told USA TODAY on Thursday. “If you fall off a ladder at work because it was a particularly hot night and you didn’t have air conditioning and you didn’t sleep well, that wouldn’t be counted.”

Researchers have previously tied heat to increases in overall deaths. If the entire U.S. experienced extreme heat for just one day, an estimated 154 adults would die, according to a USA TODAY analysis of data from a 2022 study.

Making things even more complicated: There’s no national standard or data collection for heat deaths, and many counties collect the information differently.

The end result?

“Heat deaths are absolutely undercounted,” Venkat said.

Who tracks weather and heat-related deaths?

Reporting weather-related deaths is far from an exact science and up-to-the-minute reports are gathered from a variety of sources. The National Weather Service tracks deaths conveyed by its weather forecast offices, where information is gathered from local officials and news media reports. The information is timely and publicly available, but not complete.

The Centers for Disease Control and Prevention is the official source of cause of death reporting, and is based on death certificate data, so its information is more complete but often delayed by years after the weather event. (But even the CDC’s data is not precise nor exact because the process of filling out death certificates isn’t standardized across the nation.)

Even so, the CDC reported heat-related deaths averaged nearly 900 per year for the five years between 2016-2021, double the five-year average from 15 years earlier.

Meanwhile, initial media reports of heat deaths often look to local authorities as a source. But local governments have different procedures for disclosing deaths – and they aren't consistent in what is considered a heat-related death.

Among the more than 3,000 counties completing death certificates in the U.S., there’s reason to believe the forms are handled differently in every county, said Ashley Ward, director of the Heat Policy Innovation Hub at Duke University’s Nicholas Institute for Energy, Environment and Sustainability.

Death certificates have multiple slots for adding information, with many potential codes available from a standardized set of codes for reporting primary and contributing causes of death.

When it’s time to complete the form, if heat exposure or other environmental factors contributed to a stroke or heart attack for example, Ward said information about the patient’s activities in the days leading up to their hospitalization or death might only be found deep in the notes from an emergency room doctor.

Heat triggers other health problems. So what counts as a heat death?

Most deaths that occur due to extreme heat are likely exacerbations of preexisting medical conditions, said Sameed Khatana, assistant professor of medicine at the University of Pennsylvania and a cardiologist at the Philadelphia VA Medical Center. “Can you say confidently that this particular heart attack happened because of this heat wave and it wouldn't have happened if that heat wave didn't occur? That's a big challenge.”

When heat is a direct cause of death the question is simpler, said Khatana. “When it’s a contributing factor, that’s where the challenge lies.”

There’s also no way to definitively know if heat killed someone unless the signs are very clear.

“Autopsies often don’t show heat exposure,” Venkat said. “It’s hard to read heat off the body. Were they working in a hot warehouse, were they exposed to a long period of high temperature? An autopsy won’t necessarily show that.”

Heat has been historic

This heat wave has been historic and record-setting: More than 50 cities in California and Nevada broke all-time heat records in the first five days of July, according to AccuWeather. Some areas experienced temperatures up to 20 degrees over historical averages.

Las Vegas soared to its all-time record high of 120 degrees last week, breaking the previous record by the wide margin of 3 degrees. Palm Springs, California, also ballooned to its all-time record of 124 degrees last week.

The National Weather Service warns that more temperature records are possible in the West this weekend and also that the heat is headed across the country. "This long-duration heat wave remains extremely dangerous and deadly if not taken seriously," the weather service said.

"Dozens of daily record-high temperatures are forecast over much of the West through Saturday. Hazardous heat will expand in coverage over portions of the central and eastern U.S. late this weekend."

Next week looks brutal too: "Confidence is increasing in extremely dangerous heat, particularly for urban areas in the Southeast and East Coast beginning Monday."

The heat wave will also bring danger at night.

Higher nighttime temperatures are especially dangerous for those without access to air conditioning, which is especially a problem if there are power outages during heat waves.

“The stresses on your organs to keep you alive when it’s really hot are intense. When temperatures drop at night, your body has a chance to recover,” Venkat said. “When you don’t get that, it’s pretty serious. There’s a certain threshold beyond which your body just can’t do it anymore.”

“If you’re not able to get sleep, and let your heart and lungs and kidneys and liver take a break, it’s more likely one of these systems will fail the next day,” said Venkat. “It’s like any sort of mechanical system, if you run it long enough it will break down.”

Contributing: Chris Cann, USA TODAY

This article originally appeared on USA TODAY: Heat wave death count keeps growing, but true toll is higher