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

While evidence points to climate change adversely impacting health and wellbeing, there remains a great need for more authoritative and actionable data that better describes the full magnitude and scope of this growing crisis. Given the uncertainty inherent to current detection and attribution studies, the improved specificity offered by the 10th revision of the International Classification of Diseases (ICD-10) coding of climate-sensitive health outcomes at the point of care may help to better quantify the connection between more intense and frequent extreme weather events and specific health sequela. With improved application of the available ICD-10 codes designed to capture climate-sensitive health outcomes, the ICD-10 system can function as a leading indicator. In this collaboration, publicly available ICD-10 code data was downloaded from Centers for Medicare and Medicaid Services archives and cross-referenced with 29 keywords (e.g. heat, hurricane, smoke, etc) determined by relevance to climate impacts on human health from consensus literature. We identified 46 unique ICD-10 codes for climate-sensitive health conditions. By highlighting the need for broader application of these codes and advocating for the development of new codes that better document the growing burden of climate-sensitive health outcomes, we hope to drive the development of more evidence-based, health-protective interdisciplinary climate action strategies across health systems.


Introduction
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][12][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][18][19][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.

Methods
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][36][37].
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.

Results
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.

Discussion
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.

Attribution in the clinical setting
Surveillance of public health threats including exposures to EWEs related to climate change is currently carried out by groups like the National Syndromic Surveillance Program-a collaboration among CDC, federal partners, local and state health departments-with help from ICD-10 inputs [37]. Such surveillance initiatives are critical in supporting patient surge and other planning related to environmental stressors [38,39]. In order to better elucidate the health burdens of climate change, reliable long-term datasets and refinement of techniques geared toward attribution are imperative [24]. We contend that it is within the scope of practice of clinicians to apply the codes presented here to patient charting and thus contribute to these datasets. Clinicians are uniquely positioned to interpret clinical context and attribute morbidity and mortality to a climate-sensitive exposure. We acknowledge that education-including the potential need for electronic health record (EHR) clinical decision support systems-would be essential for clinicians to apply these codes based on well-informed judgments. As noted by medical organizations including the American Medical Association, there also remains an urgent need to incorporate more fundamental teaching about climate change and health into existing medical school, residency, and continuing medical education curricula [40]. Moreover, resources and roadmaps do exist for training programs looking to modernize curricula in this way [41]. Lastly, we acknowledge that other members of the healthcare team, such as care management or social work, as well as those working in clinical informatics would benefit from similar education and can ultimately play equally important roles in gathering improved point of care data about climate health impacts. A concerted campaign to educate clinicians and the health professional workforce at large in the application of these codes would support broader efforts towards raising awareness of the health impacts of climate change for health professionals. Moreover, it is not the goal of the authors to replace the current system that relies on a retrospective application of attribution, but rather to augment this system with a climate-ready workforce of clinicians trained to apply these codes to patient's impacted by climate exposure. Such a system may improve real-time public health projections and forecasting, thereby supporting not only long-term data sets but efforts toward short-term disease and injury prevention.

Limitations of the ICD-10 system for capturing climate-sensitive health outcome
Surveillance of the health effects of climate change relies on data collected from clinical care settings. The quality of this surveillance correlates with the ability of the electronic medical record (EMR) data to accurately reflect the full scope and scale of potential harms. While human physiology and pathophysiology-and the corresponding diagnosis codes-will remain largely unchanged, the pathways of exposure to harm related to climate change are becoming more varied and better understood by global entities [42][43][44][45]. The ICD-10 system must be adapted to account for this change. Table 3 illustrates the fact that the current iteration of the ICD-10 coding system does not account for several key climate change driven exposure pathways, including wildfires, droughts, heatwaves, and secondary air pollution that forms in the atmosphere from precursors, such as ground-level ozone. Adverse health impacts from each of these exposure pathways are expected to increase as a result of climate change and our surveillance systems should be equipped with data that reflects the full spectrum of climate-sensitive health outcomes [37,45]. Expansion of the ICD coding repository, as demonstrated by the 2015 ICD-9 to ICD-10 transition, allows for improved capture of important parameters for accurate and specific healthcare assessment [46]. Moreover, criticism of ICD expansion arguing that it has adverse impacts on clinician productivity, reduces accuracy, introduces unnecessary complexity, and causes financial disruption have been largely unfounded [47][48][49][50][51].
Indirect physical and mental health impacts triggered by climate-sensitive events may be harder to capture using an ICD-10 system given their downstream manifestations. For example, the health effects resulting from crop failures from drought, reduced marine food capture, and the geographic expansion of vector-borne diseases will be less amenable to attribution by clinicians, particularly in the absence of strict decision support tools. Such long term climate sensitive health outcomes could be captured more effectively by expansion of the ICD-10 to account for the full lifecycle of health trajectories incited by climate events.

Barriers and solutions for implementation
Efforts to improve provider utilization of ICD-10 codes beyond codes that describe the proximal patient outcome have been fraught with challenges. Z-codes, codes that identify nonmedical factors that may influence a patient's health status including a patient's socioeconomic situation, are an instructive correlate. Key challenges facing the widespread adoption of Z-codes as a tool for capturing a patient's social determinants of health (SDOH) include: (1) Z-codes are currently not tied to reimbursement and thus are less likely to be emphasized in the EMR; (2) lack of awareness among clinicians regarding the general importance and existence of Z-codes; and (3) heterogeneity of health systems including poor interoperability between EHR systems [14,47,51].
One of the greatest barriers to the implementation of ICD-10 codes designed to capture climate-sensitive health outcomes is the lack of financial incentivization for health systems to require or promote their use since the majority are not currently tied to reimbursement [52]. Hospital systems may be incentivized to prompt the use of E-codes because their use can help to ensure timely reimbursement from payers [53]. However, while some states have mandated E-code reporting, there is currently no national reporting requirement [13].
Traditional fee-for-service payment models are often criticized for limiting patient health to what happens within the clinic or hospital. However, the advent of newer value-based reimbursement models may incentivize health systems to incorporate social and environmental determinants of health in clinical encounters [54]. Just as there is growing consensus that social determinants play a critical role in health outcomes, healthcare utilization, and cost, environmental and climate-sensitive determinants are also essential to a holistic understanding of an individual's health and therefore should be incorporated into reimbursement models [55].
Education of clinicians on the importance and existence of ICD-10 codes that reflect climate-sensitive health outcomes represents another key opportunity to bolster implementation in the clinical setting. The American Health Information Management Association identifies education of clinicians as a critical step toward improving documentation of ICD-10 coding of SDOH and it follows that similar interventions are necessary for climate-sensitive impacts [54]. Implementation of SDOH-related codes has increased by allowing other types of health care clinicians beyond physicians and allied health professionals-such as social workers, case managers, and community health workers-to assign these codes to a patient's record [14]. The American College of Physicians has recommended that EMR's be leveraged as tools to more accurately record measures of individual and population health without adding to the administrative burden of physicians [56,57]. This may be achieved through computer assisted coding programs.

Consideration of cost
As the majority of the codes presented in this review cannot be used for determining reimbursement, advocating for their application in the clinical setting should be viewed as an attempt to more accurately reflect the climate exposures faced by patients and not as an attempt to 'upbill' patient charting [52]. This is an especially important point given the disproportionate burden of climate-sensitive health impacts on economically vulnerable groups [28,32,33]. However, some of the ICD-10 codes presented in this review, including all codes beginning with T, are billable codes. Given that most of the expected health harms from climate change can be tied to exposures, the authors argue for expansion of non-billable E-codes-that may accurately reflect the circumstances that surround an injury or illness-so that clinicians can contextualize a clinical encounter without passing on an undue financial burden to their patients. Expanding these codes and improving their implementation may help with efforts like the Agency for Healthcare Research and Quality's Health Cost Utilization Project, for example, thereby improving estimates of the financial burden of climate-sensitive EWEs [20,21,58].

Conclusion
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.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary information files).  Occupational exposure to other air contaminants Z574 Occupational exposure to toxic agents in agriculture Z575 Occupational exposure to toxic agents in other industries Z576 Occupational exposure to extreme temperature Pollen J301 Allergic rhinitis due to pollen Pollution Z77110 Contact with and (suspected) exposure to air pollution Z77111 Contact with and (suspected) exposure to water pollution Z77112 Contact with and (suspected) exposure to soil pollution Z77118 Contact with and (suspected) exposure to other environmental pollution Temperature T698XX A Other specified effects of reduced temperature, initial encounter T698XX D Other specified effects of reduced temperature, subsequent encounter T698XX S Other specified effects of reduced temperature, sequela T699XX A Effect of reduced temperature, unspecified, initial encounter T699XX D Effect of reduced temperature, unspecified, subsequent encounter T699XX S Effect of reduced temperature, unspecified, sequela Z576 Occupational exposure to extreme temperature Water Z586 Inadequate drinking-water supply T731XX A Deprivation of water, initial encounter T731XX D Deprivation of water, subsequent encounter T731XX S Deprivation of water, sequela