The presence of dogs changes the gas, particle and microbial composition of the surrounding air. This impact has now been quantified, thanks to a recent study led by researchers from EPFL.
Unseen but all around us, the air we breathe in enclosed spaces is crucial to our health and well-being. Indoor air is not simply outdoor air that has been run through a filter: it has its own chemical makeup and a unique combination of particles, gases and microorganisms. Because indoor air has many sources of its own, concentrations of many pollutants can be as high as-or higher than-outdoor levels, especially during everyday activities like cooking or cleaning. The composition of indoor air, even in well-ventilated spaces, depends on the room's occupants and what they're doing, as well as any objects located there. And while the impact of human occupants on air quality has long been a subject of research, no one has ever taken a close look at the role of pets - even though many of us have them: in Switzerland, over half a million dogs of all sizes breathe, shake themselves off, play and scratch themselves in our homes.
In a new study, a team of researchers - led by EPFL's Human-Oriented Built Environment Lab (HOBEL) -measured the gases, particles of different sizes and microorganisms that dogs give off, through experiments run under tightly controlled conditions. "We simply brought some scientific clarity to a number of factors that, until now, were not fully understood," says Dusan Licina, a professor at HOBEL. "When assessing what building occupants are exposed to daily, we can now take these factors into account. These results provide quantitative 'emission factors' that can be incorporated into indoor air quality and exposure models, improving how we simulate real homes where people and pets share the same space. These findings will also help better understand sources of pollution and determine how we can improve the quality of our living environment."
PhD student at MPIC Mainz © Hobel Lab
More protein and ammonia
Key indicators can be used to measure indoor pollution affecting humans. We release skin cells, clothing fibers and microorganisms into the air; our breathing generates CO₂; and our skin gives off low levels of ammonia gas and volatile organic compounds. Complex chemical reactions also take place, such as when air molecules touch the skin and are turned into new compounds.
The researchers looked at these same factors when assessing the impact of dogs on the air we breathe. Unsurprisingly, the study shows that, in absolute terms, dogs give off about as much CO₂ as humans do: a large dog, like a mastiff or a Newfoundland, can produce as much CO₂ as an adult human at rest. And ammonia, better known for its bitter smell and its caustic effects, is actually a common human and animal byproduct. Whether released through the skin or exhaled through breathing, this gas serves as a discreet indicator of the biological activity of the body that emits it. It is produced in very small quantities when protein is digested, and is involved in chemical reactions when it comes into contact with the air. Here again, dogs produce about as much as their masters do.
From a scientific perspective, the measurements also help quantify how pets act as mobile 'carriers,' transporting biological material indoors and redistributing it through everyday activities
The researchers discovered that the ammonia-to-CO₂ ratio is higher in dogs than in humans. "In other words, a dog exhaling the same amount of CO₂ as a human will be producing significantly more ammonia. This difference is probably a function of their more protein-rich food, their unique metabolism and their fast breathing, which is one of the ways they control their body temperature," says Licina. But dogs spend more time sleeping, with slower, sometimes irregular breathing. Ultimately, over the course of a day, dogs breathe about as much as humans do and emit around the same amount of ammonia.
Dog hair, dust and puffs of particles
When it comes to air pollutants, dogs make their biggest impact through the tiny solid and liquid particles that they send up into the air. What dog owner hasn't wondered what their pet picked up in their fur while out for a walk? Here again, the study results can enlighten us. When shaking themselves off, scratching themselves or simply being petted, dogs release sizeable quantities of relatively large particles: dust, pollen, plant debris and microbes. Every time the dogs in the study moved, sensors picked up "puffs" of indoor pollution, with large dogs giving off two to four times more microorganisms than the humans in the same room do. Many of these particles are fluorescent: when exposed to ultraviolet light, they glow ever so slightly, betraying their biological origin. "This high level of microbial diversity is not necessarily bad news," says Licina. "Some studies indicate that exposure to a variety of microbes can boost the development of the immune system, particularly among children. But the precise impact on human health is still poorly understood and can vary from one person to the next. From a scientific perspective, the measurements also help quantify how pets act as mobile 'carriers,' transporting biological material indoors and redistributing it through everyday activities."
The effect of petting and ozone
The study also sheds light on secondary chemical reactions. A pollutant like ozone doesn't remain intact for long after entering a house. When it comes into contact with human skin, it reacts quickly with the fats, such as squalene, and forms new chemical compounds - aldehydes and ketones - as well as very small particles. Although dogs don't produce squalene, we leave our own skin residue on their fur when we pet them. That residue reacts with ozone, creating chemical byproducts and ultrafine particles. The dogs participating in the study, despite all the petting they received, produced an average of 40% less ozone derivatives than humans do. An interaction pathway that indoor air models have largely overlooked.
Researchers from the Max Planck Institute for Chemistry, the Finnish Institute for Health and Welfare and the Technical University of Denmark also participated in this study.
An environmental chamber and a human companion
To ensure their findings were reliable, the researchers ran their experiments in a highly controlled environmental chamber - a unique facility at EPFL Fribourg in Switzerland. The chamber, loaded with high-precision instruments, was designed to replicate an ordinary interior while eliminating outside interference. Because the air was filtered and the temperature and humidity were held constant, every change in the air quality could be attributed specifically to the dogs rather than environmental factors.
"The hardest part of this was to get all the authorizations we needed and meet the ethical standards," says Licina. For example, the animals had to be familiar with each other and accompanied by someone they knew, in order to reduce stress. In the end, the study population consisted of two sets - 3 big dogs in one group and 4 small dogs (chihuahuas) in another group.
Together with their human companions, the dogs alternated between periods of rest and periods of interaction - moving around, playing gentle games and being petted. This enabled the researchers to observe how the animals affect the surrounding air, almost in real-time and under nearly real-world conditions. The environmental chamber served as an ordinary living room for the dogs and a high-precision lab for the researchers.
