Human bodies breathe out 2 billion tonnes of CO2 a year, up 74% since 1985

Every person alive turns food into energy and breathes out the leftover carbon as carbon dioxide. Added up across humanity, that quiet chemistry comes to about two billion tonnes of CO₂ a year, according to the first global estimate built from measurements of people going about their ordinary lives.

The total grew by nearly three-quarters in three decades, mostly because there are far more of us. In raw tonnage, human breath now outweighs the fossil-fuel emissions of entire industrial nations, yet the scientists behind the figure insist it is not what is heating the planet.

The study, led by Javier Gonzalez of the University of Bath’s Centre for Exercise, Nutrition and Metabolism and published in The Lancet Planetary Health, puts humanity’s metabolic CO₂ at 1.18 billion tonnes a year in 1985 and 2.05 billion tonnes in 2015. More than 60% of that rise came from population growth. The rest came mainly from people getting heavier, since a larger body burns more fuel.

“We all produce carbon dioxide simply by turning food into energy, but until now we haven’t had a good idea how much that adds up to worldwide,” Gonzalez said. “What we’ve found is that the increase is mainly because there are more of us.”

Spread across the world’s population, the 2015 total works out to roughly 275 kilograms per person per year, about three-quarters of a kilogram a day once babies, children and older adults are averaged in. In 1985 the figure was closer to 240 kilograms. The world gained about 2.6 billion people over the same stretch, which is why the global sum climbed so much faster than the individual one.

How you measure the breath of billions of people

Nobody can strap a gas mask on humanity, so the team started with the most reliable way of measuring what a single body burns outside a laboratory: doubly labelled water. A volunteer drinks water in which some hydrogen atoms are replaced by heavy hydrogen and some oxygen atoms by heavy oxygen. Over the following one to three weeks, urine samples show how fast each marker leaves the body.

The trick lies in the difference. Hydrogen leaves almost only as water, while oxygen leaves both as water and as exhaled CO₂. The gap between the two disappearance rates reveals how much carbon dioxide the person produced while working, sleeping, eating and walking around as usual.

The researchers drew on 9,006 such measurements from 6,756 people aged from newborn to 101, held in the International Atomic Energy Agency’s doubly labelled water database. From those, they built a model of how CO₂ output scales with body mass and sex. They then fed in country-by-country population figures from the United Nations, along with body-mass data from the World Health Organization and the NCD Risk Factor Collaboration, and tracked the result year by year.

The approach matters because earlier figures leaned on shortcuts, such as average calorie needs or small groups of volunteers sealed inside measuring chambers. Doubly labelled water captures real days, including the walk to work and the restless night, which is why the authors describe their estimate as the first based on free-living measurements at scale.

Bigger than Russia’s fossil emissions, in tonnage

To feel the size of two billion tonnes, set it beside fossil fuels. In 2015, burning coal, oil and gas plus making cement released about 35.4 billion tonnes of CO₂ worldwide, according to Global Carbon Project data compiled by Our World in Data. Human metabolism added up to almost 6% of that.

Country by country, the comparison is starker. Russia’s fossil and cement emissions that year came to about 1.6 billion tonnes and Japan’s to about 1.2 billion, both below the CO₂ exhaled by humanity. India’s stood at about 2.2 billion. The proportion barely moved over three decades: in 1985, metabolic CO₂ was also close to 6% of a much smaller fossil total.

The geography follows people, not physiology. Asia produces the majority of humanity’s metabolic CO₂ simply because most people live there, and the study found that differences between men and women evened out at the population level.

Why your breath does not heat the planet like a coal plant

The carbon you exhale was in the air not long ago. A wheat field or a rice paddy pulled it out of the atmosphere through photosynthesis, it reached your plate as bread or rice, and your cells released it again. Over months to a few years, the loop closes. Scientists call this the fast carbon cycle.

Coal, oil and gas carry carbon that plants locked away millions of years ago. Burning them adds carbon to the air that had been out of circulation for geological time, which is why atmospheric CO₂ keeps climbing. “The important thing is that the carbon dioxide we breathe out is not the same as burning fossil fuels,” Gonzalez said. “We’re not saying that breathing is driving climate change.”

Feeding eight billion people does have a large fossil footprint, from fertiliser plants to tractors, refrigerated trucks and cleared forests. Those emissions are counted elsewhere, under agriculture, land use and energy. The new figure concerns only the final step, the body turning food into motion and heat.

So why count it at all? Because scientists who model the carbon cycle, and city networks that measure CO₂ in the air above streets, need every flow to balance. In dense cities, the breath of millions of residents is part of what the sensors pick up. The Bath team stresses the work is about better carbon accounting, not about telling anyone to exercise less or eat differently.

What the estimate does not settle

This is a modelling study, and its foundation has limits. The 6,756 volunteers took part in research run for other purposes, so they are not a random sample of humanity, and women outnumber men roughly two to one. The model predicts each person’s output from body mass and sex, which means differences in how physically active whole populations are get folded into averages rather than measured country by country.

The series also stops in 2015. With the world population now around 8.2 billion, the same per-person rate would put today’s total somewhere near 2.25 billion tonnes, but that is simple arithmetic, not a figure the study reports. Other gases fall outside its scope too. Some people’s gut microbes produce methane that leaves partly through breath, and earlier chamber experiments found that about a third of volunteers were methane producers.

The study also leaves open how much of the carbon in our food was itself shaped by fossil energy, through synthetic fertiliser for instance. That question belongs to agriculture’s carbon books, and it is where the climate impact of eating really sits.

Common questions about how much CO₂ humans exhale

How much CO₂ does a person breathe out in a day?

Averaged across all ages, about three-quarters of a kilogram, based on the study’s 2015 global total divided by the world population that year. An active adult produces more than that average, a small child far less. Over a year, the average comes to roughly 275 kilograms.

Does breathing contribute to climate change?

Not in the way fossil fuels do. The carbon we exhale comes from food, and the plants behind that food took it from the atmosphere recently, so breathing returns carbon that was already circulating. The climate cost of food comes from how it is grown, processed and transported, which is counted separately.

How do scientists measure the CO₂ a body produces?

The most reliable method outside a lab is doubly labelled water. A person drinks water tagged with heavy hydrogen and heavy oxygen, and urine samples over one to three weeks show how quickly each leaves the body. The difference between the two rates reveals how much CO₂ was produced.

Why did human CO₂ output rise so much since 1985?

Mainly because the population grew from about 4.9 billion to 7.5 billion between 1985 and 2015. Population growth explains more than 60% of the increase, and most of the remainder reflects rising average body weight.

What comes next

The study appeared online on 26 October 2026 and drew wider attention in early November. Its authors argue that human metabolism is a quantitatively important part of the fast carbon cycle and should be built into carbon-system models. United Nations projections have the world population peaking at about 10.3 billion in the mid-2080s, and the authors expect metabolic CO₂ to keep growing in step. Every one of those future people will breathe out carbon their food borrowed from the sky, and the job of keeping the books honest will only get bigger.

Reference: Gonzalez et al., “Accounting for global carbon dioxide production from human metabolism between 1985 and 2015: a modelling study,” The Lancet Planetary Health, 2026. DOI: 10.1016/j.lanplh.2026.101546

Tags: climate, carbon cycle, carbon dioxide, metabolism, population growth, The Lancet Planetary Health