How can dogs sense when people are sick?

Dogs can sense when people are sick through dogs' sense of smell and their ability to detect changes in people's behavior. Dogs have a remarkable sense of smell. Bloodhounds have almost 50 times as many scent receptors as humans. That translates to a sense of smell that's up to 100,000 times better than ours.

Researchers in Germany followed a program that trained dogs to detect the smell of a lung cancer waste product. The German dogs could smell a person's breath and correctly identify lung cancer 93% of the time. Similarly, a Japanese pooch sniffed the breath and stool samples of more than 300 people and correctly identified which people had bowel cancer 98% of the time. Other studies demonstrate that dogs can detect early stage breast cancer, melanomas and bladder cancer with an accuracy rate of 88 to 97%.

How is this possible? Malignant tumors exude tiny amounts of volatile organic compounds (VOCs) that aren't in healthy tissue. Dogs can sniff out each one in concentrations as dilute as parts per trillion. The dogs' ability to smell VOCs may lead to a new test to detect cancer. If it gets inexpensive enough, maybe we'll all have a breath analysis once a year to spot early, otherwise undetectable, disease.

Cancer isn't the only disease that stinks. Medical dogs can smell a change in blood sugar levels and the presence of ketones (toxic acids in the bloodstream that signal low insulin or high blood sugar), and then alert their owners or others to a potential diabetic seizure. Dogs can also be trained to detect changes in behavior, recognize the onset of high blood pressure, a heart attack and epileptic seizures, and get a person the help they need.