Thousands of years ago, dogs formed a close bond with humans and were widely considered the first animals to be domesticated. While the mental health benefits of canine companionship are well established, its effects on physical health remain less clear. Some studies suggest that children raised in households with dogs may have a lower risk of asthma and pneumonia, possibly due to early exposure to a diverse range of microbes. However, the scientific evidence is mixed, and any increased health risks tend to occur primarily in individuals already sensitized to allergens.
Researchers have recently investigated the pollutants emitted by dogs in indoor environments to gain a better insight into the potential effect of dogs on the indoor air quality and human health. Humans are known to contaminate indoor air with a wide variety of contaminants in the form of gases, particles, and microbes. Many of these emissions result from natural biological processes. For example, humans exhale carbon dioxide (CO2) and other volatile organic compounds (VOCs) through breathing, and release ammonia (NH3) through the skin. People also shed skin particles that carry microbes and dust.
Moreover, the airborne chemicals and ultrafine particles may be formed due to the chemical reaction of ozone with the skin oils. Because dogs share physiological characteristics with humans, scientists theorized that they may also emit similar pollutants, although the emissions could vary due to differences in skin structure, fur coverage, and sweat production.
Scientists tested this possibility through controlled experiments in a special climate chamber at the École Polytechnique Fédérale de Lausanne(EPFL) in Switzerland. The chamber was 1.44 ± 0.01 h–1 ventilated. The researchers monitored several pollutants, including CO2, NH3, VOCs, nanocluster aerosols, 1-10 μm particle size, and air bacteria and fungi. Dogs were divided into two groups based on size. There were four Chihuahuas in the small- dog category and three Tibetan Mastiff, Newfoundlands, and Mastiff in the large-dog category.
Individuals with each group were introduced to the chamber with 2-hour sessions, in which they were simulated to interact in a normal way, e.g., walking and petting. The morning sessions were conducted under low ozone conditions of approximately 2 ppb, and the afternoon sessions were done when the ozone level was raised to about 28 ppb to see the effect of ozone on the chemical emission.
The findings revealed that indoor pollutants are significant contributors to the health of dogs. Under conditions when dogs were introduced into the chamber in the presence of their owner, the CO2 and NH3 concentrations increased very quickly. An average-sized dog produced 12 L/h of CO2, which is similar to the average of a sitting adult (10 -19 L/h). Small dog, on the other hand, produced approximately 2 L/h. For ammonia, large dogs emitted about 1.8 mg/h per animal, whereas small dogs emitted about 0.5 mg/h, both within the range observed for a seated human (0.5–2.7 mg/h per person). Also released by dogs were different VOCs.
Small dogs released four identifiable VOCs in low-ozone conditions, such as pentanal, approximately 45μg/h per dog. Large dogs produced over ten VOCs, some at higher emission rates than those produced by their owners, including acetone and nonanal. A single unknown with a C5H11O2+ raised to approximately 60 μg/h/dog, which was approximately 15 times that of the owners. The structure of this compound is not clear, but researchers believe it could have some relation to metabolites associated with dog diets or skin chemistry.
With ozone rising to 28 ppb, VOC emissions, especially C5–C11 oxygenated compounds, increased, likely from ozone interacting with substances on dog fur or human skin. Dogs contributed significantly to airborne particles, with small dogs producing the most (0.61 mg/h), followed by large dogs (0.42 mg/h) and owners (0.39 mg/h), likely due to higher activity. They also elevated indoor bacterial and fungal levels, introducing mostly outdoor microorganisms. While dogs clearly alter indoor air chemistry and microbiology, health effects remain unclear, warranting further research on long-term impacts on indoor air quality and respiratory health.
Reference: Yang S, Wang N, Arnoldi-Meadows T et al., Our best friends: how dogs alter indoor air quality. Environmental Science & Technology. Published online February 1, 2026. doi:10.1021/acs.est.5c13324
