Can Dogs Smell Sickness?

The Medical Power of a Dog’s Sniff

Table of Contents

The Medical Power of a Dog’s Sniff

You probably already know that dogs have an excellent sense of smell. As soon as you open a tin of biscuits, your pet dog comes running! Humans have about 5 million scent receptors, but dogs have 220 million. Their sense of smell is 10,000 times greater than ours.

This means their nose is powerful enough to detect substances in parts per trillion. An example of this would be smelling 20 drops of pee in an Olympic-sized swimming pool.

A dog’s nose is 10,000 times more powerful than our noses
A dog’s nose is 10,000 times more powerful than our noses, Credit: Wikimedia/Stockimagery

Why is a dog’s nose so much better than a human nose?

Dogs originated from wolves. When they were wild, they relied on their sense of smell to understand their environment. Smell was very important to wild dogs. Not only did it give them current information about their environment, but it also gave them historical information. They could smell who had recently been in their territory or which way a prey animal went. They could smell out food and potential mates from miles away and smell danger approaching. 

When a dog inhales air through its nose, the air follows one of two pathways. About 12% of the air goes directly to the olfactory (smelling) region where odor molecules are deposited. They accumulate there without being breathed out. The rest of the inhaled air goes down the windpipe and into the lungs. A dog can sniff 300 times per minute. This means that the nose and its olfactory cells are constantly being supplied with new odors. 

Interestingly, dogs initially sniff with their right nostril. If the odor they are smelling is a familiar and pleasant smell, like food, they switch to using their left nostril. If the smell is something new or dangerous, such as a predator approaching, they continue to sniff with their right nostril. This is because different sides of the brain are responsible for different behaviors. The right side triggers the fight-or-flight response. This means an unfamiliar smell coming into the right nostril needs to be directed to the right side of the brain for processing. 

How can dogs’ ability to smell well help us?

Dogs’ noses have been used to help people in many ways. Sniffer dogs are used to sniff out various things like drugs, explosives, and lost people. More recently, dogs are being used in medicine. Because of their incredible sense of smell, dogs can detect subtle changes in human scent caused by disease. They can smell out illnesses such as cancer.

It turns out that cancer and other diseases or infections have a smell. Chemicals called volatile organic compounds are produced by cancer cells. These chemicals have a scent that can be detected by dogs. Dogs can be trained to detect these compounds in the same way they’re trained to detect drugs or explosives.

Some sniffer dogs are trained to look for explosives
Some sniffer dogs are trained to look for explosives, Credit: Wikimedia/The U.S. Army

Dog owners who, unbeknown to them, have cancer often report their dogs behaving differently. Dogs sometimes stay very close to their owners if they suspect something is wrong. They may repeatedly sniff an area on the owner’s body or lick their skin. Sometimes this behavior can be so persistent that owners suspect something is wrong. It is only when they go for a medical check-up and are told they have cancer that they realize their dog had sensed it all along.

What diseases and illnesses can dogs detect?

Dogs that are specially trained to detect disease are called bio-detection dogs. Given the incredible ability of dogs and their noses, they can be used extensively in the medical industry. They can sense cancer from urine, feces, sweat, or blood. In 2006, scientists trained dogs to detect cancer from people’s breath!

When sniffing people’s breath to detect cancer, the dogs were 88% accurate with breast cancer and 99% accurate with lung cancer. They could detect the presence of cancer across all 4 stages. So, even the really early stages of cancer were picked up by the dogs.

Incredibly, dogs have also been shown to detect malaria. They correctly sniffed out socks that had been worn all night by children infected with malaria. Furthermore, dogs can detect Parkinson’s disease before symptoms even appear. Early detection of diseases such as cancer and Parkinson’s is crucial for treatment. People can manage their illness much better and even recover from it, if it is picked up early on.

During the recent Covid-19 pandemic, scientists were quick to train sniffer dogs to detect coronavirus in people. The dogs were able to detect the infection before standard PCR tests could pick it up. 

Medical detection dogs

Medical detection dogs can be used in the home. People with a medical condition such as diabetes can have a dog that can alert them of imminent danger. Diabetes is a condition in which people require insulin to control their blood sugar levels. In some patients, sugar levels can drop very quickly without much warning. This can cause disorientation and fatigue and even seizures and loss of consciousness. A medical detection dog can smell the blood sugar level on their owner’s breath and, much like a guide dog, alert them in time.

Medical detection dogs alert people when they are in danger from a medical condition
Medical detection dogs alert people when they are in danger from a medical condition, Credit: Wikimedia/Cullen328

This ability in dogs can be used all over the world. It is far cheaper to train sniffer dogs to detect diseases than use some of the laboratory equipment currently available. In most cases, dogs also have a better ability to detect the disease early on, sometimes earlier than conventional tests. 

Because of the effective and non-invasive technique used to examine breath, scientists are looking to develop a breathalyser test for cancer.

Using their exceptional sense of smell, dogs can help us out in many different ways. They really are “man’s best friend.”


Scent receptors: these are found on the ends of nerves in and around your nose. They pick up smells in the air and pass the information along your nerves to your brain

Volatile organic compounds: these are gases that are released from certain solids or liquids

Malaria: a parasite that is passed from mosquitoes to humans. It causes fatigue, fever and vomiting. In severe cases it can cause seizures and death

Parkinson’s disease: a disease in which parts of the brain become damaged over time, which initially effects movement

Diabetes: a condition that causes someone’s blood sugar levels to become too high

Insulin: a hormone that helps to absorb sugar from the blood

Flesch Kincaid Grade Level: 7.4

Flesch Kincaid Reading Ease: 66.2

Gupta, P., & Pushkala, K. (2021). Corona (SARS-CoV2) and Dogs: Foes and Friends. Journal of Cell and Tissue Research, 21(1), 7025–7028.


Kokocińska-Kusiak, A., Woszczyło, M., Zybala, M., Maciocha, J., Barłowska, K., & Dzięcioł, M. (2021). Canine olfaction: physiology, behavior, and possibilities for practical applications. Animals, 11(8), 2463.


Krisch, J.A. (2014) The New York Times. “Training Dogs to Sniff Out Cancer”,the%20research%20will%20result%20in%20useful%20medical%20applications. Accessed 4th October 2022


Pirrone, F., & Albertini, M. (2017). Olfactory detection of cancer by trained sniffer dogs: a systematic review of the literature. Journal of Veterinary Behavior, 19, 105–117.


InSitu Foundation (2015) Can Dogs Smell Cancer? Accessed 4th October 2022


  • Kate Lewis, Ph.D.
    I studied for my BSc Zoology degree in Swansea University, UK in 2009 and competed my PhD in Agriculture in 2014 at Aberystwyth University, UK. I'm passionate about science, in particular zoology. I've worked in research for over 10 years. This work is usually broad and extensive. It has involved finding solutions to problems associated with farming, antibiotic resistance, animal and human nutrition, and reducing the impact of agriculture on climate change. I have had the pleasure of working both in the UK and overseas. My most enjoyable project to date, has been working along the African Rift Valley. I love talking about these topics and trying to convey my enthusiasm for them through scientific writing. Writing for Smore Magazine allows me to do that.

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