20 things dogs can smell that humans can't

A dangerous drop in blood glucose can change the chemical profile of a person's breath and sweat. Medical alert dogs can be trained to recognize glucose-related odor cues and signal their handlers, potentially giving people with diabetes an opportunity to respond to hypoglycemia. However, studies have found substantial variation in how reliably individual dogs alert to changes in blood glucose.

Cancer can alter the volatile organic compounds (VOCs) released through breath, urine, blood, sweat, and other biological samples. Researchers have trained dogs to pick out samples linked to several cancers, including lung, breast, and prostate. In one study of more than 900 men's urine samples, two trained German shepherds flagged nearly every prostate cancer sample and rarely misidentified a healthy one. Canine scent detection is still a research tool, not a replacement for established diagnostic tests.

Trying to act calm doesn't necessarily fool a dog's nose. Acute stress changes the compounds in human breath and sweat. In a 2022 Queen's University Belfast study, four trained dogs picked out samples taken after a stressful task with nearly 94% accuracy. They aren't literally smelling an emotion; they're detecting the physiological changes that come with it.

Humans may not notice bed bugs until bites or other evidence appears, but the insects produce distinctive odors that trained dogs can learn to locate. Pest-control teams sometimes use detection dogs to pinpoint infestations, including bugs concealed inside furniture, walls, and other places that are difficult to inspect visually.

Every person leaves behind a complex chemical signature through skin cells, sweat, breath, and other biological material. Search-and-rescue dogs use those traces to find missing people, and trailing breeds can follow one person's scent long after that person has moved on. Experienced bloodhounds can work trails up to two days old.

Seizures may carry a distinctive odor. In a 2019 study published in Scientific Reports, trained dogs distinguished samples collected during epileptic seizures from other samples taken from the same patients, suggesting that a seizure-specific scent may exist. The study was small, and more research is needed before seizure-alert dogs can be considered a reliable warning system.

Bomb-detection dogs are trained on odors tied to specific explosive materials. They can pick up extremely small traces in luggage, vehicles, buildings, and cargo. Electronic screening hasn't replaced them: TSA has reported more than 1,000 canine teams working at airports and major events like the Super Bowl.

Detection dogs can learn the odor signatures of specific controlled substances and find traces people would never pick up unaided. Handlers train dogs to associate each target odor with a reward, then to signal (often by sitting or freezing in place) when they find it.

Truffles grow underground and give almost no sign of themselves above the surface. A trained dog finds ripe ones by scent alone. The Lagotto Romagnolo started out as an Italian water retriever. Truffle hunters took the breed on after Italy's marshes were drained in the 1800s, and it's still closely tied to the job.

Damage hidden inside a building can still leave a scent trail. Detection dogs trained on dry rot (a fungus that destroys timber) show inspectors where it's growing, so they can narrow the search before tearing into walls and floors. Building surveyors in the U.K. and Europe have used the technique for years.

Termites communicate and navigate using chemical signals, and their colonies can remain hidden inside walls or wooden structures. Trained dogs can recognize odors associated with the insects and help inspectors focus on suspicious areas that appear perfectly ordinary to a homeowner.

That long inspection of a fire hydrant isn't idle curiosity. Urine and other scent marks carry chemical information about the dogs that left them, and dogs recognize their own. In a 2017 Barnard College study, dogs spent longer investigating their own scent when researchers had added an extra odor to it, suggesting they knew the original as theirs. A neighborhood walk is, in part, a message board.

Reproductive hormones send powerful chemical signals between dogs. Male dogs can pick up the scent of a female in estrus from well beyond the range at which a human would notice anything, which can trigger persistent tracking, roaming, or marking.

A mouse behind a wall may be silent and invisible to the people in the room, but it still leaves scent trails a dog can follow. Terriers and other ratting breeds were developed to hunt rats, mice, and other small animals, which makes them an effective early-warning system when an unwanted visitor moves in.

Specially trained cadaver dogs can find the compounds produced during decomposition, including remains that are buried, hidden, or underwater. Working from a boat or shoreline, some dogs can locate submerged bodies. One University of Huddersfield researcher saw dogs work with fair accuracy at depths of about 50 feet. Their role is different from that of search-and-rescue dogs looking for living people.

Electronics have a scent, and some police dogs are trained to find it. Electronic storage detection dogs key on triphenylphosphine oxide, a chemical that coats devices with memory storage. That lets them find hidden USB drives, phones, and memory cards that investigators might miss during a search. The Connecticut State Police, which developed the training, put its first such dog to work in 2012.

Malaria parasites appear to change how an infected person smells. In a proof-of-concept study presented in 2018, researchers collected socks worn by schoolchildren in The Gambia, shipped them to the U.K., and trained dogs to sniff them. The dogs correctly identified 70% of infected children and 90% of uninfected ones. Researchers called the results promising but said larger studies are needed.

Spotted lanternflies lay egg masses that look like smears of dried mud on bark, which makes them easy for people to miss. Virginia Tech researchers trained volunteers' pet dogs to find them. In real-world field trials, the dogs found the egg masses 61% of the time, and teams that passed the program's tests found live egg masses with 92% success.

Conservation dogs help researchers find elusive animals or the traces they leave behind. Depending on their training, they search for scat, nests, feathers, carcasses, or the animals themselves, covering ground where a visual search would be painfully slow. A 2024 review of conservation detection dog research found that dogs outperformed humans and other survey methods in 91% of the studies that compared them.

Every trip out the door leaves a record. Clothing and skin pick up tiny traces from other people, animals, foods, buildings, and places along the way. Humans may smell nothing unusual, but a dog greeting its owner at the door is reading a chemical scrapbook of the entire day.