Human blood bicarbonate levels have been rising steadily since 1999, and the curve traces an almost perfect match with atmospheric carbon dioxide. That parallel is either one of the most striking coincidences in recent public health science, or the opening chapter of a story the medical community has barely begun to read.
The study behind this finding drew on more than two decades of blood chemistry data from approximately 7,000 Americans surveyed every two years. Over the study period, the average blood concentration of bicarbonate rose from 23.8 to 25.3 milliequivalents per liter, an increase of about 7 percent, or 0.34 percent per year. At the same time, two minerals essential to bone health and metabolic function moved in the opposite direction. The findings, modest in isolation, take on a different weight when placed against projections of where atmospheric CO₂ is heading.
Dr. Phil Bierwirth, a retired environmental geoscientist affiliated with the ANU Emeritus Faculty and co-author of the study, said that while the study does not prove direct causation, the consistent population-wide trends are difficult to ignore. “I actually think that what we are seeing is because our bodies are not adapting,” he said. “It appears we are adapted to a range of CO₂ in the air that may now have been surpassed. As CO₂ in the air is now higher than humans have ever experienced, it appears to be building up in our bodies.”
The Study: What Researchers Analyzed and Found
Associate Professor Alexander Larcombe, Head of Respiratory Environmental Health at The Kids Research Institute Australia, and his colleague Phil Bierwirth theorized that bicarbonate could act as a blood tracer of atmospheric CO₂ levels. They examined blood chemistry data from the US National Health and Nutrition Examination Survey (NHANES), which collected samples from approximately 7,000 Americans every two years between 1999 and 2020, to quantify any population-level shift in blood bicarbonate levels.
NHANES is an ongoing survey designed to measure the health and nutritional status of the US population. The continuous version of the survey was initiated in 1999 and is conducted by the National Center for Health Statistics, which is part of the Centers for Disease Control and Prevention. The survey utilizes a nationally representative sample of the US non-institutionalized, civilian population identified through a multistage probability sampling design. Larcombe described the dataset as “the most comprehensive (dataset) by far in terms of blood chemistry.”
The study, published in the peer-reviewed journal Air Quality, Atmosphere & Health, was produced by researchers from The Kids Research Institute Australia, in collaboration with Curtin University and the Australian National University.
The central finding concerns serum bicarbonate, a compound the body uses to carry CO₂ out of the bloodstream and to manage acid-base balance. Average blood bicarbonate levels increased by 7% since 1999, closely tracking the rise of atmospheric carbon dioxide over the same period. Alongside this rise, calcium and phosphorus levels in the cohort showed the opposite trend, with calcium dropping by 2 percent and phosphorus by 7 percent.
Blood pH, CO₂, and the Bicarbonate Buffer System
To understand why these numbers matter, it helps to know how the body regulates blood pH. In the absence of pathological states, the pH of the human body ranges between 7.35 and 7.45, with the average at 7.40. That range is not arbitrary. A pH at that level is ideal for many biological processes, one of the most important being blood oxygenation. Many of the intermediates of biochemical reactions in the body become ionized at a neutral pH, which makes the utilization of those intermediates more difficult.
The primary mechanism the body uses to hold blood pH within that window is known as the bicarbonate buffer system. The primary pH buffer system in the human body is the bicarbonate (HCO₃)/carbon dioxide (CO₂) chemical equilibrium system. When CO₂ dissolves in the bloodstream, it reacts with water to form carbonic acid, which then dissociates into bicarbonate and a hydrogen ion. Carbonic acid dissociates into bicarbonate and a hydrogen ion, and this reaction is one of the many buffer systems in the human body that resists dramatic changes in pH to allow a person to remain within the narrow physiological range.
As atmospheric carbon dioxide increases, humans have no option but to breathe in more of it, which increases blood acidity. The kidneys respond by compensating. According to CNN’s coverage of the study, this process involves the kidneys producing and retaining more bicarbonate, which plays a key role in controlling blood acidity. The 7 percent rise in serum bicarbonate observed across two decades of NHANES data reflects that compensatory mechanism operating at a population scale, not in sick patients, but in apparently healthy people living ordinary lives.
The concern is not that blood pH itself has shifted dangerously. The bicarbonate system is doing exactly what it is designed to do. The concern is that as atmospheric CO₂ continues rising, the buffer system is being pushed progressively toward the edge of its operating range.
Rising CO₂: Where the Atmosphere Stands Now
Humans evolved in an atmosphere containing roughly 280 to 300 ppm of CO₂. That concentration remained relatively stable across the 150,000-year history of Homo sapiens. The industrial era changed that. According to the fossil record, Earth’s atmospheric CO₂ levels remained relatively stable for at least the 150,000-year history of Homo sapiens, hovering around 280 to 300 parts per million.
Global average atmospheric CO₂ was 422.8 ppm in 2024, a new record high. The trajectory is accelerating. The average annual increase over the past decade has been about 2.6 ppm per year, with 2024 recording a 3.5 ppm rise. Looking further ahead, the UK Met Office forecasts the 2026 annual average CO₂ concentration at Mauna Loa to be 429.4 ± 0.6 ppm.
From the perspective of human physiology, this matters because the respiratory system has no filter for CO₂. Every breath taken outdoors, or in a room with poor ventilation, carries whatever concentration the atmosphere currently holds. The bicarbonate buffer is in an equilibrium state within the body under normal physiological conditions. If one substrate is increased, such as CO₂, the equilibrium shifts to generate more bicarbonate and hydrogen ions. The kidneys then adjust to restore pH balance, and the result is exactly the pattern the Larcombe-Bierwirth study found: rising bicarbonate, falling calcium, falling phosphorus, year after year.
The indoor environment compounds this exposure. According to Yale Climate Connections, at CO₂ levels above 1,000 ppm, common effects include sleepiness, reduced productivity, and significant declines in cognitive performance. Modern buildings with limited ventilation can reach those levels during occupied hours. The Larcombe-Bierwirth study did not account for this variable, which means the actual indoor CO₂ contribution to blood chemistry changes remains unquantified.
Projections: What Happens if Trends Continue
Associate Professor Larcombe stated that if current trends continue, modeling indicates average bicarbonate levels could approach the upper limit of today’s accepted healthy range within 50 years. Calcium and phosphorus levels could also reach the lower end of their healthy ranges later this century.
Extending the trendlines, bicarbonate reaches the edge of the healthy range by approximately 2076, with calcium and phosphorus following by around 2100. These are projections, not certainties. They assume current emission trajectories hold and that no compensatory biological adaptation occurs. They also assume that the changes observed in the US NHANES cohort reflect a global pattern, which has not yet been tested in other populations.
The findings are especially relevant for children and adolescents, whose developing bodies will experience the longest cumulative exposure to rising atmospheric CO₂. A child born today will be in middle age when, on current projections, average bicarbonate levels approach the upper boundary of the healthy reference range. The physiological implications of prolonged borderline-high bicarbonate, combined with chronically declining calcium and phosphorus, are not yet understood.
Limitations: What the Study Cannot Yet Tell Us
The study did not take into account other potential influencing factors including people’s diets, medications, kidney function, rates of obesity, or the amount of time people spend indoors, where CO₂ levels tend to be higher. These are substantial confounders. Kidney disease, for instance, directly alters bicarbonate regulation. Dietary shifts toward or away from acid-forming foods affect blood pH. Both have changed across the US population over the same 21-year period the study examined.
The authors stress that the link is only correlational, and that measurement methods varied from cycle to cycle. Analytical consistency across decades of survey data is difficult to guarantee, and any systematic change in laboratory calibration or sample handling could artificially inflate or deflate apparent trends.
The researchers emphasize that this does not mean atmospheric carbon dioxide has been proven to cause the observed changes. Many environmental, dietary, and lifestyle factors can influence blood chemistry. However, the consistency across more than twenty years of nationwide health data convinced the team that the relationship deserves much closer scientific investigation, particularly as carbon dioxide levels continue rising year after year.
Associate Professor Larcombe said: “We’re not saying people are suddenly going to become unwell when we cross a certain threshold. But this suggests there may be gradual physiological changes occurring at a population level, and that’s something we should be monitoring as part of future climate change policy.”
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A New Category of Climate Risk
The health consequences most commonly associated with rising CO₂ are indirect: heat-related illness, vector-borne disease expansion, and the downstream effects of extreme weather. This study proposes a more direct pathway, one in which the gas itself, rather than its climatic consequences, acts as a physiological stressor at population scale.
The researchers say the findings point to an emerging dimension of climate risk, one that extends beyond heat waves, extreme weather, and sea-level rise. Rather than viewing rising CO₂ solely as an environmental issue, Associate Professor Larcombe says it may also need to be considered a long-term public health variable requiring ongoing monitoring.
Carbon dioxide should now be viewed not only as an environmental and climatic factor but also as a direct influence on public health, providing a compelling medical reason to accelerate efforts to reduce emissions.
The implication for how health agencies track population wellbeing is significant. Currently, atmospheric CO₂ concentration is treated as an environmental metric, tracked by bodies like NOAA and the Met Office. Blood bicarbonate is tracked as a clinical marker of kidney function, respiratory health, or metabolic disease. This study asks whether those two measurements should be considered together, interpreted as a linked system rather than separate domains of data.
What This Means for You
The Larcombe-Bierwirth study does not announce a health crisis. Blood bicarbonate levels in the NHANES population remain within the accepted healthy range. No one is being harmed by their blood chemistry today because of atmospheric CO₂. What the study documents is a directional signal, a slow, consistent drift across two decades of population-wide blood data that mirrors the rise of CO₂ in the atmosphere with unusual fidelity.
The specific numbers matter. A 7 percent rise in serum bicarbonate and a 7 percent fall in phosphorus across a nationally representative sample over 21 years is not noise. It is a trend that, if it continues at the same rate and atmospheric CO₂ keeps accelerating as projected, places average bicarbonate at the upper edge of today’s healthy reference range within 50 years. Calcium and phosphorus reach the lower edge of their healthy ranges later this century. For children alive now, that timeline is not abstract.
The practical implication for most people is not a change in individual behavior. You cannot modify how much CO₂ you absorb from ambient air by adjusting diet or supplementation. The paper’s core argument is aimed at policymakers and health researchers: atmospheric CO₂ concentration should be added to the list of variables that public health systems track longitudinally, and the blood pH and CO₂ relationship deserves dedicated, long-term study with confounders properly controlled. Whether that call is acted on may depend less on the science than on the institutional appetite to frame a climate variable as a medical one.
Disclaimer: This information is not intended to be a substitute for professional medical advice, diagnosis, or treatment and is for information only. Always seek the advice of your physician or another qualified health provider with any questions about your medical condition and/or current medication. Do not disregard professional medical advice or delay seeking advice or treatment because of something you have read here.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.
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