The air we breathe in enclosed spaces plays an important role in our health and well-being. Far from being a filtered version of outside air, it has a specific chemical composition as well as its own load of particles, gases and microorganisms. Despite good ventilation, it constantly evolves through the simple presence of the occupants, the daily activities they carry out there or even the materials found there.
Although the impact of humans has been studied for a long time, no one has yet looked in detail at the emanations of our four-legged companions, although there are so many of them. In Switzerland, more than 550,000 dogs of all sizes breathe, snort, play or scratch in our homes. A team led by the Human-Oriented Built Environment Laboratory (HOBEL) at EPFL has for the first time quantified, under highly controlled experimental conditions, gases, particles of all sizes and the microorganisms they emit.
A laboratory room
To ensure reliable results, the researchers carried out their experiments in a unique climate chamber at EPFL Fribourg. Packed with high-precision instruments, this room was designed to reproduce an ordinary interior while eliminating external disturbances. Filtered air, constant temperature and humidity: each variation observed could thus be precisely attributed to the presence of dogs, and not to the environment.
“The most complicated thing was obtaining all the authorizations and respecting the necessary ethical standards so that dogs could stay in this space for 24 hours,” explains Dusan Licina. The animals had, for example, to know each other and be accompanied by a familiar person in order to limit stress. Two groups of dogs, three large and four small, were ultimately able to participate in the study.
Accompanied by the familiar human, the dogs alternated calm moments and phases of interaction – movements, light games, caresses – as in an ordinary living room, allowing scientists to observe, almost in real time, how the dogs modify the air around us, in conditions close to everyday life, but with the precision of a laboratory.
CO₂ and ammonia, like humans
Key indicators make it possible to measure indoor human pollution. Breathing releases CO₂, the skin emits small amounts of ammonia and volatile organic compounds, while skin cells, textile fibers and microorganisms flutter in the air. Added to this are complex chemical reactions, notably between the skin and molecules present in the air, transforming them into new compounds. The same criteria were taken into account to know the impact of dogs on the air we breathe.
Unsurprisingly, in absolute terms, our dogs emit as much CO₂ as humans. The study shows that a large dog, such as a mastiff or a Newfoundland, produces levels comparable to those of an adult at rest. For ammonia, again, the dogs’ production is generally similar to that of their masters.
But scientists have observed that in canines, the ratio of ammonia to carbon dioxide is higher than in humans. “This means that with equivalent breathing, dogs release significantly more ammonia. This difference is probably explained by their diet richer in proteins, their specific metabolism and their rapid breathing mode, used in particular to regulate their body temperature,” emphasizes Dusan Licina. But dogs go through long periods of naps, with slower breathing and interspersed with breaks: over the course of the day, they therefore breathe approximately as much air as a human, and their total ammonia emissions remain comparable.
Puffs of indoor pollution
The most significant impact of indoor air pollutants involving our furry companions is in suspended particles, those tiny solid or liquid fragments that float in the air without our knowledge. What dog owner hasn’t wondered what’s in their companion’s fur when they return from a walk? The study confirmed it: by stirring, scratching or simply being stroked, Médor releases large quantities of relatively large particles: dust, pollen, plant debris or even microbes.
With each movement, the instruments recorded real “puffs” of indoor pollution. Large dogs, in particular, emit between two and four times more microorganisms than humans in the same room. A large proportion of these particles are fluorescent: exposed to ultraviolet light, they emit a faint glow, a sign of their biological origin.
Not necessarily bad
“This increased microbial diversity is not necessarily bad news,” adds the professor. Some studies suggest that more varied exposure to microbes, particularly in children, could influence the development of the immune system. But the precise effects on human health are still poorly understood and can vary between individuals. From a scientific perspective, the measurements also help quantify how domestic animals act as mobile ‘vectors,’ transporting biological material indoors and redistributing it through their daily activities.”
Finally, the study highlights secondary chemical reactions. When a pollutant like ozone enters a home from outside, it does not stay intact for very long. In humans, it reacts rapidly with skin lipids, notably squalene, to form new chemical compounds (aldehydes and ketones) as well as very small particles.
Dogs do not produce squalene. But when we pet them, our skin residue gets deposited on their coat. In the presence of ozone, these substances react in turn, generating secondary chemical compounds and ultrafine particles. Despite all the caresses received during the study, the dogs produced on average 40% less ozone derivatives than humans. An interaction pathway largely neglected by indoor air quality models.
As Dusan Licina, professor at HOBEL, points out, the results, published in “ACS Publications”, provide “quantitative emission factors which can be taken into account to better evaluate the daily exposures of occupants, identify sources of pollution and guide actions aimed at improving the quality of living environments.”
