Dogs can smell not only illegal drugs in airports but the apple you aren't supposed to have in your backpack, not to mention the smallest particle of food under the refrigerator. But that's not all. Some dogs can sniff out human diseases.

A “biodetection dog,” according to Pennsylvania State University veterinarian Nancy Dreschel, DVM, PhD, can “pick up odors in the low parts-per-billion range” — the equivalent of smelling a teaspoon of sugar in a body of water the size of two Olympic swimming pools. Their sense of smell is said to be 10,000 to 100,000 times as acute as humans'.

How does a dog's nose know? According to PBS's NOVA, dogs have some 300 million olfactory receptors in their noses, compared to about 6 million in humans. And relatively speaking, the area of a dog's brain that is devoted to analyzing smells is 40 times larger than ours. Dogs also have a special "smelling chamber" called Jacobson's organ, which may be important in detecting body scents, according to the Alabama Cooperative Extension System.

Here are some of the medical conditions canines are able to detect.

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Cancer

Research has found dogs can learn to nose out several different types of cancer. In one study, five dogs trained to detect cancer based on the smell of a person’s breath identified breast cancer 88 percent of the time and lung cancer 99 percent of the time.

British researchers found dogs can be trained to detect bladder cancer by sniffing a person’s urine. Penn State's Dreschel described what happened at a conference she attended. "A scientist was training dogs to detect bladder cancer in humans by smelling their urine. She said a dog alerted them to a sample from a supposedly healthy person who was being used as a control." It turned out the person had bladder cancer, and the dog caught it first.

Other research has found dogs can detect melanoma, a deadly form of skin cancer, based on changes in the scent of urine and blood samples. And canines have been especially successful at detecting prostate cancer based on urine. In one study, prostate-cancer sniffing dogs were 93 percent effective at detecting tumors — much better than the prostate-specific antigen (PSA) test that's sometimes used for screening today, which has a high false positive rate.

Related: 10 Cancer Symptoms You Should Never Ignore

Low blood sugar episodes

Hypoglycemia can be dangerous, even deadly, for someone with Type 1 diabetes. When it happens, the person’s breath takes on a particular scent dogs are able to detect, according to Can Do Canines. That nonprofit trains diabetes assist dogs to keep a “nose” out for the smell of rapidly dropping or low blood sugar levels. Canines learn this skill as puppies and are taught in much the same way as drug sniffing and rescue dogs.

Diabetes assist dogs also are trained to alert the human they’re monitoring (or even the parents, in the case of a child with Type 1 diabetes) so the person can check his blood sugar level or eat something. The dogs wear special backpacks with pockets for medical information, a source of sugar and emergency contact information.

Related: Warning: You Might Have Diabetes and Not Know It

Seizures

People who have epilepsy don’t know when they’re about to have a seizure, but some dogs do. According to Canine Partners for Life (CPL), which trains service and companion dogs for placement in or near Pennsylvania, “Alerting to an impending seizure is an innate ability in some dogs.” A dog that shows he has this ability can then be trained to warn of an oncoming seizure by pawing, barking, circling, licking or making close eye contact. This gives their human partner time — often as much as an hour — to lie down or leave a crowded area.

No one knows for sure how canines predict seizures. On its website, the nonprofit organization Service Dogs For America states, “it’s our experience that most EMRD [Emergency Medical Response Dogs] dogs detect the subtle changes in a person’s odor, respiration rates, and behavior before the average human’s ability to do so.”

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