Dogs are renowned for their keen senses, particularly their sense of smell. A dog can smell a person (particularly one they recognize) from up to 40 feet away, but this depends on the conditions surrounding the dog and the person they’re trying to smell.1
Certain factors affect how far away a dog can smell a person. Wind speed and direction, for example, affect how far away a dog can “catch” a scent and how much of that scent is carried.
Wind Speed and Direction
If a dog is trying to smell a person and is “downwind” (meaning the wind is blowing from the person in the dog’s direction), that person’s scent will be pushed toward the dog by the wind. The scent molecules will be carried toward them this way, and the opposite is true if the wind is blowing away from the dog toward the person.
The speed of the wind is also a factor since the scent spreads out slowly as molecules in the air. If the breeze is gentle, more scent molecules can fan out into the air, meaning more will be available to the dog. Conversely, if the wind is fast and strong, the molecules will be moved into a stream that is less likely to reach the dog’s nose.
The type of rain makes a difference when a dog is tracking a human scent. If the rain is heavy and oppressive, this can move scent molecules down and press them close to the ground, meaning a dog would have to be “nose to the ground” to find the scent trail of a person.
On the contrary, if there is a light rain, this can actually help to stir up scent molecules and “refresh” the smell, meaning that a dog is more likely to smell the person from further away and will have an easier time finding a scent trail.
It may seem obvious, but distractions and other overbearing smells can also affect how far away a dog can smell a person. If in a controlled environment, a dog is likely to be able to smell the person’s scent from a maximum distance (around 40 feet). If out in the field, such as during search and rescue, there will be a cacophony of other scents and smells to filter through, which can distract a dog.
Can Dogs Smell Illness in People?
Dogs have an excellent sense of smell. It’s so good that dogs can detect when a person is sick. Many dogs have been known to act strangely around their owners without being provoked, leading to the owners getting medical attention and being treated for various illnesses that hadn’t begun to show symptoms yet, such as cancer.
There have been years of research detailing how dogs can do things, and dogs are now able to be trained to detect illness in their owners, ranging from incoming seizures to drops in blood glucose and even narcolepsy and COVID-19.
Dogs can do this due to their incredibly sensitive sense of smell. Certain illnesses, such as cancer, give off specific scent molecules that people exude in their breath, sweat, or urine. These can be picked up by dogs trained to signal when they smell them, making them incredibly useful as service dogs in the community.
Dogs trained to detect seizures, for example, can not only smell minute differences in their owner’s natural smell but are also clued into their behavior. Scent changes and almost imperceptible changes in behavior trigger the dog to signal when a seizure is imminent, allowing the person to get to a safe place before the episode occurs.
What Breed of Dog Have the Best Sense of Smell?
All dogs have a certain number of scent receptors called olfactory bulbs. Sense of smell seems to be a dog’s main sense, so all breeds have an incredible olfactory system, including olfactory receptors and a vomeronasal organ. A human’s sense of smell isn’t comparable since dogs have between 200–300 million scent receptors in their noses, much more than a human’s 400.
However, one breed comes out as the top dog, and that’s the Bloodhound. The Bloodhound was bred for hunting and tracking and has been used for tracking people since the middle ages. Everything about this breed is made to amplify scent, from their creased faces that direct scent molecules to their noses to their long ears that prevent wind from scattering nearby scents.
This gives them an incredible gift for tracking and finding humans. The record for the longest time a scent was followed occurred in 1954 in Oregon when a Bloodhound followed the scent trail of a missing family found dead over 330 hours after they went missing. Because of this, the Bloodhound is often used as a search and rescue dog or a cadaver dog.
Dogs have extraordinary senses of smell that have been used historically to track humans for centuries. The farthest a dog can smell a human is usually around 40 feet away, but this usually depends on environmental factors such as the wind speed, wind direction, and weather. Some breeds are more skilled at tracking scents, such as the Bloodhound, and dogs are now being used more than ever to detect toxic chemicals, bodies, and even disease.
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