This article was originally published by JAMA Medical News

When a patient presents to the emergency department for a heart attack, their medical team might not be thinking about underlying factors.

After treating the patient, the physician would likely input the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) code for a myocardial infarction, which, in the US, doubles as a billing code.

But behind the heart attack could be many hidden factors. One of them is chronic heat exposure.

For people who live or work in hotter areas, heat stress can exacerbate underlying ailments ranging from cardiovascular disease and diabetes to asthma and mental health conditions, according to the World Health Organization.

Tracking the various ways heat can harm or kill is perhaps more important now than ever. One climate modeling study published earlier this year predicted an increase in temperature-related deaths in hundreds of cities in Europe alone without aggressive mitigation.

If a physician doesn’t identify heat as a cause and note it in the patient’s chart, the data—and the true impact of heat on health—are lost.

“When a physician is aware that there’s a heat wave, they’re more likely to code for heat-related illness,” like acute heat stroke, said Ashley Ward, PhD, director of the Heat Policy Innovation Hub at Duke University. But, she noted, “it is not those short-term extreme heat events that actually result in the greatest health outcomes. It’s the everyday chronic exposure throughout heat season” that exacerbates other conditions.

If more physicians included ICD-10 codes for heat, including chronic exposure, experts say policymakers would have a clearer picture of its health effects, ideally leading to initiatives that protect patients from its harms. Coding for heat-related illness is just one part of a larger push to better document social determinants of health. But difficulty in determining the role of heat in a patient’s condition, a lack of physician familiarity with the codes, and balancing thoroughness and efficiency during clinical encounters all present challenges.

There are 2 ways to estimate heat-related illness and death, explained Gregory Wellenius, ScD, an epidemiologist and professor of environmental health at Boston University. The first model, which is statistical, excludes individualized patient data, something Wellenius says is essential. “If you want to get better at preventing those excess deaths, you need to understand what went wrong in each case.”

The other model does just that, Wellenius explained, taking into account death certificate or medical record diagnostic codes to determine how many deaths were coded as being attributable to heat after the fact. Although this model might not be imminently helpful for predicting waves of events that might burden hospitals in real time, it gives policymakers the ability to determine better strategies for addressing heat exposure moving forward.

But as researchers from the US Centers for Disease Control and Prevention (CDC) have noted, counts of heat-related deaths based on death records are known to fall short of reality.

Wellenius’ own research demonstrates this. He led a study in 2020 that related highly localized temperature estimates to mortality rates in nearly 300 US counties representing 70% of the country’s population. The analysis estimated that from 1997 to 2006, an average of about 5600 people died annually from heat-related causes in the US.

This was substantially higher than the heat-related deaths that were being recorded at the time. For instance, fewer than 700 annual deaths were coded as heat-related on death certificates between 1999 and 2009, according to a 2013 CDC report. And more recently, a study in JAMA of death certificate data identified fewer than 1000 annual heat-related deaths, on average, between 1999 and 2023.

In other words, people are dying from heat, but it appears that more often than not this isn’t being acknowledged on their death records.

“We can’t fix a problem until we see a problem,” said Jason Adler, MD, chair of the American College of Emergency Physicians (ACEP) Coding and Nomenclature Advisory Committee. “The intent of these codes is for surveillance.”

That’s where physicians can come in. As the authors of a 2023 study titled “Coding for Climate” put it, “Clinicians are already at the frontlines of assessing and treating climate sensitive health impacts, and therefore, regardless of their practice setting, are uniquely positioned to help quantify and provide regionality information on the impacts of climate change through tools like the ICD.”

Geoff Comp, DO, an emergency medicine physician at Valleywise Health Medical Center in Phoenix, developed protocols to treat patients with heat stroke and is “hyper aware” of heat-related issues. In practice, he said, physicians are not using billing codes for heat unless the disease is clearly associated, as with heat stroke.

Many physicians might be wary of including these codes because it’s difficult to determine causality in cases like cardiovascular events or asthma attacks, he said.

Wellenius agreed: “If two people came in with renal insufficiency on a hot day, how do you know which one came in because of the heat vs which one was unrelated?”

Comp gave the same example, noting that a patient with renal insufficiency could be more dehydrated on a hot day because they’re sweating more. “Maybe we should be adding heat to that diagnosis,” he said.

To help determine causality, Ward recommends taking emergency care a step further by asking the patient for a more thorough history and using best judgment to add supplementary codes if the physician determines heat likely played a part. Wellenius added that it’s easier to determine if heat is a contributing factor if the physician is familiar with the patient’s background, including where they work and live.

“I understand that the ask is a big ask, to train clinicians to be able to think differently about how they use those diagnostic codes,” Ward said. “I could see a physician saying, I can’t conclusively say it was heat, so therefore I don’t want to write it.”

Comp stressed the need for emergency department physicians, in particular, to code for heat-related illnesses and injuries when they can reasonably tie them to heat exposure.

“Frankly, it would be lost if we didn’t code for it, because if a person’s symptoms were directly worsened by heat, but we didn’t acknowledge that on the intake, then it probably wouldn’t end up as a final diagnosis code from the internal medicine team,” he explained.

But there’s a reality at play in the ED: the constant trade-off between asking more questions and trying to see the next patient. For busy emergency department physicians, being asked to evaluate for and code another item—especially when knowing the underlying factors doesn’t change the patient’s immediate care—simply may be too big an ask.

As for which ICD-10 codes to use, the 2023 study identified 46 unique codes that could be applied for climate-sensitive health conditions. A smaller number of codes are available for heat-related illnesses specifically, the most common being T and X codes, used for primary and secondary diagnoses, respectively.

Challenges in selecting one of the many codes might itself be a reason not to use them. A good one to know, therefore, is the X30 code for “exposure to excessive natural heat” and its variations, which can be used when chronic heat exposure is a secondary diagnosis, explained Jeffrey Linzer, MD, a representative to ACEP’s ICD-10-CM Coordination and Maintenance Committee.

“If I have somebody who has an exacerbation of their asthma because it’s hot out, those T codes wouldn’t apply,” he said. “It’s going to be that X30 code that says exposure to natural heat.”

To encourage physicians to input the codes, Ward suggested creating an automated supplement to electronic medical record (EMR) systems that prompts physicians to add on heat-related codes. For example, a patient admitted to the emergency department for an asthma exacerbation during a heat wave might trigger a pop-up asking a clinician to query the patient on heat exposure and to consider adding an ICD-10 code.

Some questions Ward suggested asking include: “Do you have access to air conditioning?” “Do you work outside?” And, “If you work inside, is there air conditioning?” She also recommended considering overnight temperatures, which commonly exacerbate heat-related illness and are often overlooked. When air temperatures remain high overnight, the body is not able to cool down as efficiently, she explained.

Comp, however, cautioned that the EMR alerts are a good idea in theory but may not be in practice because physicians might end up ignoring them. “Sometimes they help. Other times, it’s just seen as another box to have to click through,” he said.

Comp suggested a regional approach might help, where health care professionals in high-heat areas are trained selectively to keep an eye out for a history of chronic heat exposure. But, he added, “we’re also seeing increase in incidence in heat stroke in different places that we don’t classically think of as being hot.” Limiting the training regionally might ignore the realities of climate change and leave historically cooler areas unprepared.

“Providing proper documentation so that information can be captured is a key issue that goes back to medical schools,” Linzer said. Although the percentage of medical schools that teach climate change has increased in recent years, Linzer noted a focus on chronic heat exposure in the curriculum could help to address the problem.

Wellenius wants standardization in medical education: teaching medical students which codes to use, and when, instead of leaving it up to interpretation. “Having a succinct, standard way to document these things so that we, as epidemiologists, can do a better job of counting those events would be very helpful.”

(Epidemiologists outside the federal government like Wellenius may be increasingly important to this work. A statement at the top of the CDC’s Heat & Health Tracker page says the site “is no longer being maintained, and no new data will be added.” The last date for which data on emergency department visits associated with heat-related illness was available was May 21.)

Without detailed data from physicians who have direct patient access and can most accurately determine causality, Ward said it will be difficult to sway policy change that protects the public from the health effects of a changing climate. “If you have data that show the actual impact of heat, then it’s a lot easier to argue for more allocation of resources toward it and show a clear need,” she said.

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