Dogs in the home change the air we breathe: new research has quantified for the first time what they “bring” into a space
In many households, a dog is a family member, a source of companionship and routine. But while the benefits of spending time with dogs for mental health are often discussed, scientists have only recently, under controlled conditions, measured how the presence of dogs changes the chemical composition, particle load, and microbial “signature” of indoor air. The results suggest that dogs are not just passive occupants of the home, but also active sources of emissions—from carbon dioxide and ammonia to larger particles and microorganisms—and potential “carriers” of chemistry otherwise associated with human skin.
What exactly the study measured and why it matters
Indoor air, where we spend most of our time, is shaped by a mix of outdoor air, ventilation, materials in the space, and—most of all—occupant emissions. For humans, it is already known that breath and skin emit gases, volatile organic compounds, particles, and microbes, and that ozone in the air can react with skin lipids, creating additional products and ultrafine aerosols. The question was: do dogs behave similarly and to what extent, and do small and large dogs differ?
To avoid speculation, an international team of scientists conducted experiments in a climate-controlled chamber with a volume of 62 cubic meters, with a controlled temperature around 24 °C and relative humidity around 50%. The space was ventilated with fresh outdoor air with multiple-stage filtration, and surfaces were systematically cleaned before the experiments to reduce the “noise” of existing pollutants. Under such conditions, it is possible to track changes that arise almost exclusively from the presence of people and dogs.
Small versus large dogs: differences visible in the numbers
Two groups of dogs were brought into the chamber: four small dogs (Chihuahuas) and three large dogs (Tibetan Mastiff, Newfoundland, and Mastiff). Each group stayed in the chamber together with its owner, and control experiments were also conducted in which only the owner was in the chamber, to isolate the dog’s contribution as precisely as possible.
Measured were carbon dioxide (CO2), ammonia (NH3), volatile organic compounds (VOCs), nanocluster aerosols of 1–3 nanometers, particles of 1–10 micrometers (total and fluorescent), and the concentration and composition of bacteria and fungi in the air. Special attention was paid to conditions with very low ozone and with elevated ozone concentration, because ozone drives additional chemical reactions on surfaces.
The key finding is that large dogs emit significantly more CO2, NH3, bacteria, and fungi than small dogs, while emissions of particles in the 1–10 micrometer range were similar. In other words, “gases and microbes” increase with size, but supermicrometer-range particles do not necessarily follow the same rule—likely because not only body mass is at play, but also activity, fur, behavior, and resuspension of dust from surfaces.
CO2 and ammonia: the dog as a source of emissions comparable to a human
In controlled experiments, CO2 and NH3 concentrations began to rise immediately after the dog and owner entered the chamber, and fell after they left—a typical emission signature of a living source. When the owner “activated” the dogs through short periods of walking and petting, levels rose further, suggesting that movement increases emissions or at least their dispersion in the space.
For large dogs, the average CO2 emission was about 12 liters per hour per dog, comparable to the typical emission range of a sedentary adult. For small dogs, CO2 emission was much lower, about 2 liters per hour per dog. With ammonia the trend is similar: large dogs emitted on average about 1.8 mg NH3 per hour per dog, and small dogs about 0.5 mg per hour, which also falls within the range typical for a seated adult.
In their interpretation, the scientists note that CO2 most likely comes primarily from breathing, while NH3 is linked to protein metabolism and emissions via the skin and fur. Interestingly, the NH3-to-CO2 ratio in dogs may differ from the human ratio, pointing to differences in diet, metabolism, and breathing patterns (dogs, especially small ones, often breathe faster, and under stress and heat switch to rapid panting).
Ozone, petting, and skin chemistry: why ultrafine “nanoclusters” appear in the air
One of the most intriguing points of the study relates to ozone. In afternoon sessions, ozone was introduced into the chamber to approximately 28 ppb, a level that can occur indoors when ozone-rich outdoor air enters through ventilation or open windows, especially in urban environments during episodes of elevated ground-level ozone.
Under elevated ozone conditions, experiments with dogs showed the formation of nanocluster aerosols of 1–3 nm, as well as the appearance of “ozonized” products of volatile organic compounds. An important detail is that dogs themselves do not produce squalene, a key lipid of human skin that reacts quickly with ozone and is known as a driver of the formation of such ultrafine particles. Therefore, the authors conclude that a likely mechanism is the transfer of lipids from human skin to dog fur during petting and contact, after which ozone in the air reacts with those lipids on the fur surface and creates new products.
This interpretation, the authors emphasize, remains the best explanation consistent with the chemical traces, but not a directly proven mechanism, because the amount of transferred skin oil and the intensity of contact could not be precisely quantified for each situation. Still, the very fact that ultrafine aerosols appear when ozone is present and when people and dogs are in the space suggests that interactions between pets and humans can become part of indoor chemistry.
Particles 1–10 micrometers: “puffs” of dust and biological material
For most households, the finding about larger particles may be even more important. Dogs proved to be a strong source of particles in the 1–10 μm range, with concentration spikes most pronounced during movement and interaction. This is a familiar picture to many: a dog shakes itself, moves through the space, or jumps onto the couch and a brief wave of dust is “felt in the air.”
Interestingly, small dogs on average had a higher mass emission of 1–10 μm particles (about 0.61 mg/h per dog) than large dogs (about 0.42 mg/h), which the authors associate with higher activity levels and resuspension from the floor. Compared with a human in the control experiment, dogs overall outperformed owners in emitting these supermicrometer particles.
Analysis of the size distribution showed that dogs emitted relatively more coarse particles above 5 μm, and fewer particles in the 2–5 μm range compared with humans. That difference may stem from different sources: for humans, particles are often linked to skin shedding and clothing fibers, while for dogs, fur, dander, particles that cling to the coat during time outdoors, and their release through movement or rubbing against surfaces play an important role.
A large fraction of detected particles was fluorescent, which in this field is often used as an indicator of biological material. The authors nevertheless warn that fluorescence is not automatic proof of living microbes: non-living particles can also glow due to specific compounds on the surface, but in combination with direct microbiological analyses, the data gain weight.
Microbes in the air: large dogs release more bacteria and fungi, but the picture is not black-and-white
In the microbiological part of the study, qPCR and sequencing methods were used to estimate the amount and composition of bacteria and fungi in the air. The results showed that large dogs on average emit more microorganisms than small dogs, and in more categories more than a seated human. Specifically, large dogs on average emitted about 2–4 times more bacteria and fungi compared with a sedentary adult human, with a particularly pronounced contribution from some bacterial groups.
Along with quantity, community structure also changed: the presence of dogs increased the richness and diversity of microbial taxa in the air. Some of these microbes correspond to species in the literature associated with dog skin, while a significant portion is clearly of environmental origin—consistent with the thesis that dogs also act as mechanical vectors bringing outdoor microbes indoors, especially when they regularly spend time outside.
An important nuance: higher microbial diversity is neither “good” nor “bad” by itself. Health consequences depend on a person’s sensitivity, allergies, asthma, age, ventilation quality, humidity level, and other factors. In the literature, for example, possible protective effects of early exposure to diverse microbes in childhood are discussed, but also the fact that risk can increase in people already sensitized to certain allergens. The authors therefore emphasize that their results do not provide direct conclusions about health outcomes, but map the emission source and the change in exposure.
What this means for households: ventilation, cleaning, and a more realistic picture of “sources” at home
Although this is a laboratory-controlled experiment, the finding is practical: pets, and especially dogs, can be a significant part of the indoor air quality equation. This does not mean households should give up dogs, but that it is necessary to realistically understand what happens indoors, especially in smaller apartments, in rooms with limited ventilation, or in homes where people with respiratory issues live.
From an indoor air quality management perspective, the study opens several clear directions:
- Ventilation is key to diluting CO2, NH3, and volatile products, especially in rooms where the dog and people spend most of their time.
- Dust management becomes more important in homes with dogs, because coarse particles are linked to fur and resuspension; regular vacuuming and wiping surfaces reduces the “reservoir” that returns to the air during movement.
- Humidity control and avoiding prolonged elevated relative humidity can be important due to fungal spores and overall microbiological dynamics.
- Ozone indoors is not something people often think about, but during urban episodes of elevated ground-level ozone it can become relevant. Under such conditions, chemical reactions on surfaces (including fur) can contribute to the formation of new products.
For professionals, the finding has broader implications as well. Ventilation designers and exposure models often start from humans as the dominant source in a space. This study suggests that in households with dogs one should also account for an additional “biological-chemical source” that is dynamic: it changes with dog size, activity level, contact with people, time spent outdoors, and cleaning habits.
Study limits and next questions
The authors openly note limitations: for ethical and logistical reasons, dogs entered the chamber with the owner, so the dog’s contribution was estimated by subtracting the “owner only” measurements from the “owner + dogs” measurements. Dog activities before the experiment (e.g., walking, environmental contact) could not be fully standardized, and some volatile compounds could not be unambiguously chemically identified without additional standards.
Despite this, the study provides the first quantitative framework that did not exist before. Next steps, according to the authors, include a broader range of breeds, control of hygiene, diet, and outdoor routines, and examining other pets. It will be especially important to clarify to what extent dogs act as “sinks” for ozone (because ozone breaks down on surfaces), and how emissions change in real apartments with furniture, carpets, and everyday sources such as cooking.
Sources:- Environmental Science & Technology (ACS) – a scientific paper on gas, particle, and microbe emissions from small and large dogs, including ozone experiments (link)- DTU Orbit – a publicly available version of the paper and publication metadata (link)- EPFL – summary and explanation of the study findings on dogs’ impact on indoor air (link)- Swissinfo – overview of key results and context (large and small breeds in the experiment, ozone and reaction products) (link)- Phys.org – report on microbiological findings and particle “puffs” during dog movement (link)- EPFL – earlier explanation of the mechanism of nanocluster aerosol formation due to ozone reactions with skin lipids (context for interpreting ultrafine particles) (link)
Find accommodation nearby
Creation time: 2 hours ago