Dogs are 300 t0 10,000 times more sensitive to odors than humans. Humans have 10 million and dogs 1 billion olfactory receptors. Police dogs are used for police officers to sniff out whatever it is they need to because there sense of smell is so strong , that they would find something even if it is hidden .
In San Anselmo, California, a woman named Nancy Best noticed her dog licking and sniffing at her right breast very often . She later found out that she had breast cancer . Research has shown that the release of chemicals in tissue including cancer is a different smell than the chemicals released of normal tissue. Dogs sense of smell is so strong that they are able to smell it out . There are multiple studies that show dogs accurately detecting cancer by smelling a person's breath or urine sample . A study in 2011 with a Labrador retriever, showed he correctly identified 91% of breath samples and 97% stool samples for patients with colon cancer. I think this is pretty amazing .
Source : www.m.webmd.com/a-to-z-guides/features/pets-amazing-abilities