The study, published in Air Quality, Atmosphere and Health, examined blood-test results from roughly 7,000 people in the US National Health and Nutrition Examination Survey between 1999 and 2020.

Researchers from The Kids Research Institute Australia, Curtin University and The Australian National University (ANU) found that the changes occurred as atmospheric CO2 levels increased from about 369 parts per million (ppm) in 2000 to more than 420 ppm today.

Study author Associate Professor Alexander Larcombe said the findings suggest the human body may already be responding to changes in atmospheric composition.

“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.

Bicarbonate plays an important role in maintaining the body’s acid-base balance, and the body can retain additional bicarbonate as CO2 levels increase to help keep blood pH stable.

However, the researchers said maintaining this response 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 researchers said the findings may be particularly relevant to children and teenagers because their bodies are still developing and younger generations are expected to experience greater lifetime exposure to elevated atmospheric CO2.

Atmospheric CO2 concentrations were approximately 280 to 300 ppm during human evolution, while levels have risen by an average of about 2.6 ppm annually over the past decade.

In 2024 alone, atmospheric CO2 increased by 3.5 ppm.

Fellow Author Dr Phil Bierwirth, a retired environmental geoscientist affiliated with the ANU Emeritus Faculty, stressed that the study does not establish a direct cause-and-effect relationship.

However, he said the consistency of the changes across a large population warrants 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.”

The researchers said rising atmospheric CO2 could represent a climate-related health risk distinct from more established threats such as heatwaves, extreme weather and sea-level rise.

A/Prof Larcombe said increasing CO2 should be considered not only an environmental concern but also a potential long-term public health factor.

“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 recommended monitoring atmospheric composition alongside biological markers across populations to assess how gradual environmental changes affect human biology over decades.

They also said potential physiological effects from rising CO2 should be considered in future climate policy discussions alongside its established environmental consequences.

The researchers said reducing CO2 emissions remains essential for limiting global warming and could also have implications for long-term human health.

Associate Professor Larcombe is part of the Wal-yan Respiratory Research Centre, a partnership between The Kids Research Institute Australia, Perth Children’s Hospital and Perth Children’s Hospital Foundation.