A peer-reviewed study published in Air Quality, Atmosphere and Health on February 26, 2026, reports that average serum bicarbonate levels in American adults rose approximately 7% between 1999 and 2020, tracking in parallel with climbing atmospheric carbon dioxide concentrations. The researchers analyzed more than two decades of nationally representative blood chemistry data and found shifts in calcium and phosphorus as well, raising questions about whether the air people breathe is quietly altering the chemical balance of their blood. The findings have already drawn sharp disagreement from other scientists who argue that outdoor CO2 levels remain far too low to overwhelm the body’s built-in buffering systems.
Two Decades of Blood Data Show a Clear Trend
The new study draws on the National Health and Nutrition Examination Survey, a continuous program run by the CDC’s National Center for Health Statistics that collects physical exams and lab work from a representative sample of the U.S. population. According to the researchers’ analysis, population-average serum bicarbonate, a key marker of acid-base status in the blood, climbed to roughly 25.3 mEq/L by the 2019–2020 survey cycle. That figure sits well within the accepted clinical reference range of 23 to 30 mEq/L, yet the upward drift itself is what concerns the authors. They also reported that average serum calcium fell about 2% and phosphorus dropped roughly 7% over the same period, a combination they interpret as evidence of the body working harder to buffer incoming CO2.
The underlying dataset is large and well-documented. NHANES is maintained as a public resource by the CDC, and its laboratory files, questionnaires, and sampling methods are available through the program’s online portal. An earlier ecological study that examined NHANES serum bicarbonate trends from 1999 through 2012 used an adult sample of 33,546 participants and noted that instrument and platform changes across survey cycles required careful adjustment. The newer paper extends that timeline through 2019–2020, a period during which atmospheric CO2 continued to rise. According to NOAA’s Mauna Loa observatory record, annual mean CO2 concentrations climbed from roughly 369 parts per million in 1999 to about 393 ppm by 2012 and kept increasing through 2020. The researchers treat these atmospheric readings as an exposure variable and argue the blood chemistry shifts track them too closely to dismiss as coincidence.
What Bicarbonate Shifts Actually Mean for Health
Bicarbonate is the body’s primary chemical buffer against acidity. When CO2 enters the bloodstream through the lungs, it reacts with water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions. A healthy kidney adjusts bicarbonate retention to keep blood pH in a narrow band around 7.4. The study’s central argument is that a slow, population-wide rise in serum bicarbonate signals the body is compensating for a gradually increasing CO2 load, not that anyone is acutely ill. The concern is cumulative: if the trend continues along current emissions trajectories, the authors suggest the atmosphere could approach levels associated with adverse physiological responses within roughly half a century.
That projection deserves scrutiny. A 7% rise in bicarbonate over two decades, while statistically detectable in a dataset this size, still leaves the population average squarely inside the normal clinical window. The simultaneous drops in calcium and phosphorus could reflect dietary changes, shifts in vitamin D status, or other confounders that a purely ecological study cannot isolate. The researchers acknowledge these limitations but point to the consistency of the trend across multiple survey cycles and demographic groups as evidence that something systemic is at work. The findings carry particular weight for children and adolescents, according to a news summary of the work, because younger people face the longest cumulative exposure and their developing bodies may be more sensitive to gradual metabolic shifts.
A Sharp Scientific Disagreement
Not all researchers accept the premise that outdoor CO2 levels can meaningfully alter human blood chemistry. A 2023 paper published in Acta Physiologica directly challenged this hypothesis, arguing that the body’s respiratory and renal compensation mechanisms are more than adequate to handle the small incremental increases in ambient CO2 recorded over recent decades. In that view, the roughly 50 parts-per-million rise in atmospheric CO2 since the late 1990s is too small to push arterial CO2 or blood pH outside the range that healthy lungs and kidneys can automatically correct.
This disagreement is not merely academic. If the Acta Physiologica authors are correct, the bicarbonate trends in NHANES data likely reflect other population-level changes, such as rising rates of obesity, metabolic syndrome, or kidney disease, rather than a direct atmospheric signal. The 2014 ecological study published in Environment International had already noted a statistical association between CO2 emissions and the prevalence of obesity and diabetes in the United States, raising the chicken-and-egg problem: are people’s blood bicarbonate levels rising because they are breathing more CO2, or because metabolic diseases that independently alter acid-base balance are becoming more common? The new study does not resolve that question with controlled experimental data, and no clinical trials testing blood chemistry shifts under tightly controlled CO2 exposure conditions have been published to date.
Indoor Air May Be the Bigger Exposure Risk
One dimension of this debate that often gets overlooked is indoor air quality. While outdoor CO2 averages in the United States remain under 450 ppm, measurements inside classrooms, offices, and homes can routinely exceed 1,000 ppm and sometimes climb several times higher in crowded, poorly ventilated spaces. That means the “exposure” people actually experience is not the open-air concentration recorded at remote observatories, but a fluctuating mix of indoor and outdoor air that depends on building design, occupancy, and ventilation practices. If chronic CO2 exposure is capable of nudging blood bicarbonate upward, indoor environments are the most plausible setting for that effect to accumulate.
The authors of the new study do not claim to have directly measured indoor CO2, but they argue that rising outdoor levels set a higher baseline from which indoor concentrations start. In practical terms, a school building that once hovered around 800 ppm during a full class may now spend more of the day above 1,000 ppm even without any change in ventilation, simply because the air coming in from outside contains more CO2 than it did two decades ago. That framing helps explain why the researchers see a population-wide signal in NHANES data despite relatively modest shifts in outdoor measurements. It also dovetails with broader concerns about indoor air, where elevated CO2 has been linked in other research to impaired cognition and decision-making, even at levels far below those that cause acute toxicity.
What Comes Next for CO2 and Human Biology Research
Both sides of the debate agree on one point: more rigorous evidence is needed. Ecological associations between atmospheric trends and health metrics are inherently vulnerable to confounding, and neither the bicarbonate analysis nor its critics have yet produced definitive human exposure studies. Future work will likely draw on large biomedical databases, such as those catalogued through the National Library of Medicine, to cross-check bicarbonate trends against diagnoses, medications, and lifestyle factors that might explain the observed shifts. Carefully designed chamber experiments, in which volunteers are exposed to controlled CO2 levels for extended periods while their blood chemistry is monitored, could also help clarify how sensitive human physiology really is to chronic, low-level changes in inhaled CO2.
In the meantime, the new findings add a provocative layer to discussions about climate change and public health. If rising CO2 is influencing blood chemistry even subtly, it would represent a direct, physiological pathway by which fossil fuel emissions affect human biology, alongside more established risks such as heat stress and air pollution from particulates and ozone. If, instead, the bicarbonate drift mainly reflects the growing burden of metabolic disease, the signal is still a warning, one that points inward, toward diet, physical activity, and healthcare access rather than the composition of the air. Either way, the convergence of climate science, epidemiology, and physiology in this research underscores how tightly environmental and metabolic forces are intertwined, and why understanding that relationship will matter for decades to come.
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*This article was researched with the help of AI, with human editors creating the final content.
