A population study suggests the human body may already be responding to changes in the composition of the atmosphere.
The air outside may be leaving a subtle signature in human blood. Researchers have identified gradual changes in a key carbon dioxide-related blood marker and warn that it could approach the upper limit of its healthy range within decades if current trends persist. Children and adolescents may be especially relevant because their developing bodies could face the longest cumulative exposure to rising atmospheric CO₂.
Researchers from The Kids Research Institute Australia, Curtin University, and The Australian National University (ANU) analyzed more than 20 years of U.S. population data for a study published in Air Quality, Atmosphere and Health. They found that several measures of blood chemistry shifted over time in patterns that closely followed increasing atmospheric CO₂.
Using the U.S. National Health and Nutrition Examination Survey (NHANES), the researchers examined blood test results from approximately 7,000 people every two years between 1999 and 2020.
Blood chemistry tracks atmospheric CO₂
Since 1999, average serum bicarbonate levels have increased by about seven percent. Serum bicarbonate is closely connected to the amount of carbon dioxide carried in the body. During the same period, average concentrations of calcium and phosphorus declined.
Those changes occurred as atmospheric CO₂ climbed from approximately 369 parts per million (ppm) in 2000 to more than 420 ppm today.
Associate Professor Alexander Larcombe, an author of the study, said the pattern may indicate that the body is already responding to the changing composition of the atmosphere.
“What we’re seeing is a gradual shift in blood chemistry that mirrors the rise in atmospheric carbon dioxide, which is driving climate change,” A/Prof Larcombe said.
The body may be compensating
Bicarbonate helps the body maintain its acid and base balance. As CO₂ rises, the body can retain more bicarbonate to keep blood pH stable, but maintaining that adjustment over long periods could have physiological effects.
“If current trends continue, modeling indicates average bicarbonate levels could approach the upper limit of today’s accepted healthy range within 50 years,” A/Prof Larcombe said. “Calcium and phosphorus levels could also reach the lower end of their healthy ranges later this century.”
The cause remains uncertain
Humans evolved when atmospheric CO₂ concentrations were approximately 280 to 300 ppm. Over the past decade, levels have increased by an average of about 2.6 ppm annually, while 2024 alone recorded a rise of 3.5 ppm.
Fellow author Dr Phil Bierwirth, a retired environmental geoscientist affiliated with the ANU Emeritus Faculty, emphasized that the study does not establish a direct causal connection. Even so, he said the consistent changes across the population deserve attention.
“I actually think that what we are seeing is because our bodies are not adapting,” Dr Bierwirth said.
“It appears we are adapted to a range of CO2 in the air that may now have been surpassed. The normal range maintains a delicate balance between how much CO2 is in the air, our blood pH, our breathing rate, and bicarbonate levels in the blood. As CO2 in the air is now higher than humans have ever experienced, it appears to be building up in our bodies. Maybe we can never adapt such that it is vitally important to limit atmospheric levels of CO2.”
Climate policy may need health markers
The findings point to a possible form of climate risk that extends beyond rising temperatures, severe weather, and sea level rise.
Associate Professor Larcombe said atmospheric CO₂ may need to be treated not only as an environmental measure but also as a long-term public health variable that should be monitored over time.
“We’re not saying people are suddenly going to become unwell when we cross a certain threshold,” he said.
“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.”
The researchers recommend monitoring atmospheric composition and human biomarkers alongside conventional climate indicators. Doing so could help clarify how gradual environmental changes influence human biology over many years.
Cutting CO₂ emissions remains essential for limiting global warming. The findings also raise the possibility that reducing emissions could matter for long-term human health, making the potential physiological effects of rising CO₂ relevant to future climate policy discussions.
Reference: “Carbon dioxide overload, detected in human blood, suggests a potentially toxic atmosphere within 50 years” by Alexander N. Larcombe, and Phil N. Bierwirth, 26 February 2026, Air Quality, Atmosphere & Health.
DOI: 10.1007/s11869-026-01918-5
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