Coding for climate: sourcing better climate-health data from medical billing

The effects of climate change on human health are complex, extensive, and increasingly recognized as one of the defining public health threats of our time [1, 2]. The World Health Organization (WHO) identifies environmental factors that influence human health—including physical, chemical, and biological factors external to a person—as key drivers of adverse health outcomes [3]. However, despite this broadly accepted classification, the WHO and similar agencies charged with safeguarding public health have fallen short in quantifying the true health toll of climate change [4]. A 2021 WHO report estimated that climate change could cause approximately 250 000 additional global deaths per year between 2030 and 2050, yet this projection has been criticized as an underestimate. It also seems inconsistent with the Lancet's contemporary characterization of climate change as the greatest global health threat of the 21st century [1, 2, 5]. The discrepancy may be secondary to differences in how health exposure pathways are accounted for and due to a lack of data on the true burden of disease posed by climate driven hazards [4]. Health impacts linked to climate change including heat related illness, changing patterns of vector borne disease transmission, wildfire-related harms, direct and indirect harms of extreme weather events (EWEs), compromised sanitation, and food and water insecurity are expected to worsen with higher levels of warming. Therefore, with the world currently projected to warm to 2 · 4 °C–3 · 5 °C by 2100 compared to the pre-industrial era, stronger consensus on the magnitude of health impacts sensitive to climate variability is essential [6, 7].

While evidence points to climate change adversely impacting health and wellbeing, particularly for historically marginalized, under resourced, and overburdened communities, there remains a great need for more authoritative and actionable data that better describes the full magnitude and scope of the growing crisis [8]. Accurate and timely measurement is the cornerstone of any quality improvement initiative, including efforts to adapt to the health effects of climate change. 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 [9].

The ICD system is a global collaborative led by the WHO and is currently in its tenth revision (ICD-10). The codes are an alphanumeric system used to classify and track morbidity and mortality in a way that facilitates comparisons across nations [10, 11]. Since 2015 in the United States, the Centers for Medicare and Medicaid Services (CMS) ensures compliance with ICD-10 codes as a method of 'improving patient care and public health surveillance' [12]. ICD-10 codes are used in a patient encounter to systematically document the health status of the patient over the course of their interaction with the health care system and also subsequently used for medical billing purposes. On a population level, ICD-10 codes can be used to track prevalence and incidence of human disease and health outcomes [13]. ICD-10 codes are also used by national, regional, and state public health agencies to track incidence and outcomes of disease in populations of interest [14]. For example, the U.S. Centers for Disease Control and Prevention (CDC) has developed a publicly-accessible health surveillance platform, the Heat and Health Tracker, that provides daily rates of heat-related morbidity (emergency department visits) across the country based on uniform national application of a set of heat-specific ICD codes [15, 16]. Integration of ICD codes with publicly available healthcare cost and utilization data also enables researchers to estimate the health-related financial toll of specific conditions [11–13].

There is a need for expanded and more consistent use of coding for climate-sensitive health outcomes at the time of a patient encounter to better quantify the connection between longer, more intense, and more frequent EWEs resulting from climate change and specific health sequela [17–20]. Such improved documentation would allow for better documentation of health trends and understanding of the extent to which climate-sensitive exposures result in adverse human health outcomes—a particularly pressing problem for historically under-resourced populations [21, 22]. For instance, as EWEs become more common, accurate reporting is essential to inform effective evidence-based responses and to guide local, national, and global climate change mitigation and adaptation efforts centered on improving health and equity [23, 24]. There is evidence to suggest that the current application of the ICD coding system for surveillance of health harm related to EWE is inadequate [13, 20]. In the case of extreme heat, for example, research indicates that mortality estimates may represent a 50-fold underestimation of the true burden of disease related to excessive natural heat [18, 25, 26]. Thus, in their current iteration, ICD codes may not be the optimal tool for population-level surveillance.

Currently, detection and attribution (D&A) studies help to establish the causal relationship between greenhouse gas emissions, climate driven hazards, and observed health outcomes [24, 27]. The results of these studies offer probabilistic estimates demonstrating the likelihood of a given climate exposure resulting in a particular health outcome. However, given the wide range of inputs—including the fact that a climate-influenced extreme event is rarely the sole cause of a given health outcome—there exists considerable uncertainty and therefore often large confidence intervals [17, 24]. Such D&A studies rely on a retrospective analysis of environmental hazards and associated health impacts and are often separated from the event in both space and time. Retrospective attribution of specific health conditions to climate change is essential for quantifying population-level health impacts and trends.

With improved application of the available ICD-10 codes designed to capture climate-sensitive health outcomes and with the advent of new codes developed to better capture the full scope of health harms and health-related financial burdens related to climate change, the ICD-10 system can function as a leading indicator [28]. Moreover, improved capture of climate sensitive health outcomes may bolster the assessment of healthcare and community climate resilient adaptation efforts proposed by the WHO [29]. As of November 2021, there are over 70 000 possible ICD-10 codes a provider can choose from when documenting a patient encounter [30]. Of these available codes, a relatively small and manageable fraction denote impacts from environmental hazards, EWEs, and air and water pollution related to climate change and fossil fuel combustion [31]. To support investigative efforts in understanding how patients are being affected by climate change, accurate clinical data collection is key [24].

Current efforts can be improved by adding the climate-specific nuance and context available to clinicians at the point of care, or soon after a health problem surfaces. Clinicians, thus, play an integral role in capturing real-time clinical information about the environmental context in which a clinical encounter occurs. For example, under the current coding system, a patient admitted to the intensive care unit from the emergency department after suffering severe heat stroke during a heat wave may be classified as a case of 'altered mental status'—a classification that fails to capture health-relevant environmental exposure to extreme heat. More widespread utilization of codes that capture climate-sensitive health outcomes may elucidate where more codes can be developed and ultimately, help direct public health policies that improve equitable allocation of resources to those who need them the most. For example, prospective surveillance of adverse related heat outcomes could further validate the previously observed disproportionate impact of heat on marginalized communities [32, 33].

In this investigation, we examined the current ICD-10 codes available to all clinicians by CMS and extracted the codes capable of capturing climate-sensitive health outcomes. Health care clinicians find themselves on the frontlines of the climate health crisis and are uniquely positioned to respond to this historic challenge [34]. Equipping clinicians and decision-makers with clinically-enhanced climate data can inform responses and motivate expanded ambition to address climate threats.

The full list of current publicly available ICD-10 code data was downloaded from CMS archives in November 2021 [30]. All 72 748 entries were converted from.txt files to.xls files. Keywords related to climate change were then searched and identified in the dataset. Twenty-nine keywords were determined by relevance to climate impacts on human health from consensus literature, including the CDC, WHO, and the bibliography of the Fourth National Climate Assessment [1, 35–37].

The keywords used were the following in alphabetical order: climate, dehydration, disaster, drought, dust, ecoanxiety, environment, evacuation, fire, flood, heat, heatwave, hurricane, hyperthermia, migration, occupation, ozone, pollen, pollution, smog, smoke, snow, storm, temperature, tornado, vector, water, weather, wildfire. A total of 150 initial ICD-10 codes were captured using the 29 identified climate-sensitive keywords. These codes were then screened for relevance. Exclusion criteria for ICD-10 codes included the following:

• (1)  
  Any code that referenced an irrelevant clinical context (i.e.: 'V9329—heat exposure on board unspecified watercraft').
• (2)  
  Any code that explicitly excluded an environmental exposure (i.e.: 'R680—hypothermia, not associated with low environmental temperature').
• (3)  
  Any code with an ambiguous clinical application (i.e.: 'T678—other effects of heat and light').

Inclusion criteria included any code that provided a relevant context or clinical outcome related to a climatic event. The identified ICD-10 codes were then organized according to keyword.

A total of 46 unique ICD-10 codes were identified, expanded to 100 total codes when accounting for 'initial encounter', 'sequela' and 'subsequent encounter' denominations. These designations indicate when a patient was seen relative to the onset of their condition or injury. For instance, 'initial encounter' denotes the first point of contact for a specified issue. The 'sequela' and 'subsequent encounter' denote any visit or consequent issues after the initial point of contact. Eighteen of the twenty-nine keywords yielded associated ICD-10 codes in the search. These keywords are listed in table 1.

The 46 unique codes are outlined in table 2 according to keyword. The expanded 100 codes are located in the appendix table A1.

The remaining 11 of the 29 keywords yielded no ICD-10 code results. These are listed in table 3.

The ICD-10 coding system provides an opportunity for the collection and exchange of climate-sensitive health incidence data capable of supporting real-time characterization of the impacts of climate change on human health on local, regional, national, and international levels for all ICD participating member states under the WHO. The widespread application of more precise, climate-sensitive coding could help to inform more effective, and data-driven climate adaptation strategies. The majority of the codes identified in this review fall under the category of 'external cause of injury', or E-codes. Distinct from diagnosis codes or procedure codes, these codes are secondary codes–not tied to reimbursement—that capture specific details about an injury or health event and provide data valuable for injury research and evaluation of strategies for injury prevention [12]. Application of these codes during a patient encounter can help capture data points that would have otherwise been lost in a retrospective public health analysis.

4.1. Attribution in the clinical setting

4.2. Limitations of the ICD-10 system for capturing climate-sensitive health outcome

4.3. Barriers and solutions for implementation

4.4. Consideration of cost

Climate change poses threats to human health via important exposure pathways that can be qualified during a patient encounter. Public health surveillance platforms designed to capture these exposures are hamstrung by a health informatics system that has not yet adapted to accurately reflect the health harms posed by this growing crisis. Accurate and timely documentation of climate-sensitive health harms using ICD-10 codes can help make the dangers of the climate crisis more tangible and actionable by providing important real-time patient-centered data on the mounting human toll of this intensifying global problem. Physicians and other allied clinicians can improve health care recordkeeping for climate change-harmed patients through the application of the codes reviewed here. However, given the expected scope and far-reaching nature of climate change's impact, the ICD-10 system and specifically external cause codes should be updated to equip clinicians with evidence-based tools that can serve to inform climate action plans and interventions.

The author(s) received no financial support for the research, authorship, and/or publication of this article. The authors have no conflicts of interest to declare. All co-authors have seen and agree with the contents of the manuscript and there is no financial interest to report. We certify that the submission is original work and is not under review at any other publication.

All data that support the findings of this study are included within the article (and any supplementary information files).

The following table lists all ICD-10 codes identified in our search accounting for 'initial encounter', 'sequela' and 'subsequent encounter' denominations.