People breathe out gases like methane (CH4) and nitrous oxide (N2O), stronger greenhouse gases than carbon dioxide. When we eat food, and our bodies turn it into methane and nitrous oxide, it adds to global warming. While animals like cows release a lot of methane when they digest food, human breath also warms the atmosphere. Although the total mass of humans is similar to that of domesticated animals globally, few studies have looked into how much methane humans release, and none have specifically studied people’s breath.
A new study published in PLoS ONE investigated methane and nitrous oxide emissions in human breath from 104 volunteers in the UK population to better understand what drives these emissions and to quantify national-scale estimates.
Nitrous oxide (N2O) in human breath is thought to come from reducing nitrates in food and water by certain bacteria in the gut and mouth. There’s also evidence that bacteria can turn nitric oxide (NO), produced in our bodies, into N2O. Eating vegetables rich in nitrates can lead to higher levels of N2O in breath for up to 4 hours.
The number of studies on human N2O production is small. Thus, there is no information on geographical or ethnic variation in breath concentrations, although an increase in concentration with age after childhood has been observed.
This study, 328 breath samples were collected from participants, and details such as age, sex, diet, and smoking habits were noted. Additional information about the time of day, recent activities like eating or exercising, and food intake was available for 248 samples. For breath analysis, participants breathed into Tedlar® gas-sampling bags after taking a deep breath and holding it for 5 seconds, exhaling about 80% of their lung capacity into the bag.
The increase in CO2 concentration in participants’ breath ranged from 26.5 to 63.4 parts per thousand, following a typical distribution. On average, participants exhaled CO2, and although the data leaned slightly towards higher values, it was mostly balanced around the average.
This study identified 31% of participants as methane producers (MPs). The percentage of MPs was higher among older age groups, with 25% of people under 30 being MPs compared to 40% in the 30 and above age group. Females (38%) were more likely to be MPs than males (25%), but the overall concentrations emitted from both MP groups were similar.
Scientists noted, “All participants were found to emit N2O in breath, though none of the factors investigated explained the differences in emissions. Dietary preference was not found to affect CH4 or N2O emissions from breath in this study. We estimate a total emission of 1.04 (0.86–1.40) Gg of CH4 and 0.069 (0.066–0.072) Gg of N2O in human breath annually in the UK, the equivalent of 53.9 (47.8–60.0) Gg of CO2.”
“In terms of magnitude, these values are approximately 0.05% and 0.1% of the total emissions of CH4 and N2O reported in the UK national greenhouse gas inventories.”
Journal Reference:
- Dawson B, Drewer J, Roberts T, Levy P, Heal M, Cowan N (2023) Measurements of methane and nitrous oxide in human breath and the development of UK scale emissions. PLoS ONE 18(12): e0295157. DOI: 10.1371/journal.pone.0295157