Dogs are born sniffers. Humans have roughly five million olfactory cells in their noses. It sounds impressive, until you compare it to the 200 million cells in a typical dog's nose. Canines' sense of smell is generally 10,000-100,000 times superior to that of humans. Much more of their brains are devoted to processing smell, and they also possess more that code for olfactory ability and many more olfactory neurons than humans.

People have known about and taken advantage of dogs' sense of smell for centuries, even breeding some dogs to be scent hounds used in tracking and hunting. In recent years, dogs have been trained to sniff out explosives, drugs, bodies and other scents.

Anecdotes about dogs "sensing" when their owners were sick before any diagnosis was made may sound crazy at first. But in the last decade, several scientists have put dogs' noses to the test in controlled laboratory experiments — diseases give off odors that, at least theoretically, dogs can smell. Malignant tumors exude tiny amounts of chemicals called alkanes and benzene derivatives not present in healthy tissue. If a dog can identify chemical traces in the range of parts per trillion, is it really crazy to think they can detect cancer, even before people know they're sick?

The first scientific test of canine cancer-detecting, to my knowledge, was in 2004. James C. Walker, of the Sensory Research Institute at Florida State University, and colleagues trained two dogs to detect melanoma tissue samples hidden on the skin of healthy volunteers. The dogs were trained and tested with methods normally used for bomb- or drug-sniffing dogs. One dog "confirmed" the presence of melanoma on five patients, and even detected cancer in a sample that was initially deemed negative, but subsequent histopathological examination revealed to contain melanoma in a fraction of the cells.

Scientists again trained dogs to sniff out lung cancer in a more recent study published in 2011. A group of German researchers wanted to know if dogs could discriminate between breath samples from lung cancer patients, chronic obstructive pulmonary disease (COPD) patients, and healthy volunteers, and whether the presence of tobacco in the samples made a difference. The dogs correctly identified 71 samples with lung cancer out of 100. They also successfully detected 372 samples that did not have lung cancer out of 400. It seemed the dogs were able to detect lung cancer independently from COPD and tobacco smoke.

A 2004 study in which dogs were trained to detect bladder cancer in humans by smelling their urine had a smaller success rate, but is notable for an unexpected result. Carolyn M. Willis of Amersham Hospital in Great Britain and colleagues trained six dogs. One dog failed completely, but two picked out the positive samples 60 percent of the time. The surprise came when one of the non-cancerous control samples caught the interest of the dogs. The medical staff assured the disappointed trainers that the sample was from a healthy person, but because the dogs consistently identified this sample as "positive," it was sent back to the hospital for further tests. On re-examination the person was found to have cancer on his kidney and bladder cancer. The dogs caught it before anyone else.

In a 2011 study from Japan, a Labrador retriever trained to sniff out colorectal cancer was at least 95 percent as accurate as a colonoscopy when smelling breath samples and 98 percent correct with stool samples. The dog was especially effective at detecting early-stage cancer and could also discern polyps from malignancies, which a colonoscopy cannot do.

Early detection is paramount in many cancer treatments. For some diseases, like prostate cancer, the blood tests currently used are notoriously inaccurate. Could man's best friend become a tool in early screening? Whether actual dogs will be making future diagnoses is uncertain, but it is clear they possess a pretty powerful tool we are only beginning to understand and appreciate.