If you share your home with a dog, you’re not just getting muddy pawprints and a wagging tail - you’re also getting a different kind of indoor air.

Scientists at the Swiss Federal Institute of Technology Lausanne (EPFL) have measured, for the first time, the gases, particles and microbes dogs bring into our living rooms, and how they stir them up.

In tests inside an environmental chamber at EPFL's Fribourg site, researchers compared groups of large dogs (including a Mastiff, a Tibetan Mastiff and a Newfoundland) with a group of small dogs (Chihuahuas).

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They found that a big dog at rest breathes out roughly as much carbon dioxide as an adult human and emits comparable levels of ammonia, a gas released when proteins break down and that comes off skin and breath.

But the real shake-up is in particles. By doing all the normal dog things - shaking after a nap, scratching an itch, being stroked - our four‑legged friends kick up hefty amounts of dust, pollen, plant bits and microbes that settle on floors and furniture.

In tests, large dogs released two to four times more microorganisms than a human in the same room.

Dogs also act as little couriers, ferrying biological material from the park or pavement into the house on their coats and paws, then spreading it about as they move.

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A richer mix of everyday microbes indoors may actually help the immune system, especially in children, according to previous studies, so the microbial muddle that comes with a family dog could be part of why kids who grow up with pets sometimes have fewer allergies.

Ozone drifting in from outdoors reacts with oils on skin and fur, creating new airborne compounds like aldehydes and ketones. Humans produce a skin lipid called squalene that’s especially reactive.

Dogs don’t make squalene themselves but we leave skin residues on their fur when we pet them, and those residues can still react. E

ven so, on average, the dogs generated about 40% fewer of these ozone‑reaction products than humans did in the same setup.