Cancer does give off a smell of sorts, but not one most people can notice. Tumors change the chemistry of breath, urine and skin in ways too faint for the human nose, even though doctors have used smell as a diagnostic clue since Hippocrates around 400 BC.1Atlas of Science. Finding aroma clues in the human breath to diagnose diseases The best noses for cancer turn out to be a dog’s, a worm’s and a machine’s, and none of them is yet a test you can order.
Why Cancer Cells Change How the Body Smells
Every body gives off airborne chemicals. Cancer changes the mix because of how its cells run. They carry high levels of reactive, oxygen-based molecules that damage the fats in cell walls.2PubMed Central. Lipid Peroxidation Produces a Diverse Mixture of Saturated and Unsaturated Aldehydes in Exhaled Breath That Can Serve as Biomarkers of Lung Cancer—A Review That damage breaks the fats into a varied mix of sharp-smelling chemicals.3PubMed Central. Lipid Peroxidation Produces a Diverse Mixture of Saturated and Unsaturated Aldehydes in Exhaled Breath That Can Serve as Biomarkers of Lung Cancer—A Review In lung cancer, this process puts compounds such as pentane, isoprene and hexanal into the breath.4PubMed Central. Non-invasive detection of early-stage lung cancer through exhaled breath volatile organic compound analysis Cancer cells also burn sugar differently, which releases ketones including acetone.5PubMed Central. Non-invasive detection of early-stage lung cancer through exhaled breath volatile organic compound analysis
That signature differs by cancer type. Breath from people with lung cancer has measurably different chemical patterns from healthy people’s.6PubMed Central. Exhaled breath analysis for lung cancer Invasive melanoma releases odor chemicals different from those of harmless moles and healthy skin.7PubMed. Invasive melanoma in vivo can be distinguished from basal cell carcinoma, benign naevi and healthy skin by canine olfaction: a proof-of-principle study of differential volatile organic compound emission Urine from people with bladder cancer differed from others’ in ten specific odor chemicals.8PubMed Central. Investigation of urinary volatile organic compounds as novel diagnostic and surveillance biomarkers of bladder cancer
The chemicals reach the outside world by several routes. Those made in tissue pass into the blood, which carries them to the lungs, where they leave in exhaled air. The kidneys filter some into urine, and others evaporate directly from the skin over a tumor.
Most of these chemicals show up in healthy people too. Acetone and isoprene are both in healthy breath, so finding one of them says nothing on its own. What shifts in cancer is the combination and the relative amounts, more like a chord than a single note. Descriptions like “sweet” or “rotten” don’t fit cancer reliably. Diet, exercise, smoking and other illnesses also move these chemicals, so a pattern has to be compared against many healthy and sick people before it means anything.
Can People Smell Cancer
Some diseases do have odors a nose can catch, such as a musty breath odor in liver failure and fruity, ketone-scented breath in uncontrolled diabetes.9Netherlands Journal of Medicine. Smelling the diagnosis: a review on the use of scent in diagnosing disease Cancer’s odor chemicals, though, can be present at parts per trillion, far below what people notice.10IntechOpen. Canine Detection of the Volatile Organic Compounds Related to Cervical Cancer Cells
Stories of people smelling a loved one’s illness are memorable, but they are rare and unproven. People may also be noticing odors from wounds, infection or reduced hygiene during illness. None of these is a dependable sign of cancer, and relying on smell could delay real screening.
Odors that people do notice during cancer often come from treatment or daily life. Some chemotherapy drugs have an odor and can make a patient’s own sense of smell sharper or different. Foods such as coffee, leeks and garlic measurably change breath chemistry.11PubMed. Associations of diet and lifestyle factors with common volatile organic compounds in exhaled breath of average-risk individuals
If a strong new body or breath odor lasts, see a doctor about it as a symptom rather than assuming cancer. Mention other changes you have noticed, such as unexplained weight loss, a lingering cough, unusual bleeding or persistent pain, since these carry more weight than the odor itself. Bring a list of recent medicines, supplements and diet changes, because many of these alter body odor and may fully explain it. And keep up routine screenings on schedule either way, since screening finds cancers before they cause any smell or symptom.
How Well Dogs Sniff Out Cancer
Dogs can smell odor chemicals at parts per trillion in body fluids,12IntechOpen. Canine Detection of the Volatile Organic Compounds Related to Cervical Cancer Cells which puts cancer’s faint signature within reach. Dogs have identified melanoma on patients’ skin and on samples hidden on healthy people.13PubMed. Volatile biomarkers from human melanoma cells Results across studies range widely:
- Lung cancer, breath samples: dogs caught about 99 of 100 cases and wrongly flagged about 1 in 100 healthy people, at every stage.14PubMed. Diagnostic accuracy of canine scent detection in early- and late-stage lung and breast cancers
- Breast cancer, same group: dogs caught about 88 in 100 cases.15PubMed. Diagnostic accuracy of canine scent detection in early- and late-stage lung and breast cancers
- Prostate cancer, urine: two highly trained dogs caught 98 to 100 of 100 cases.16PubMed. Olfactory system of highly trained dogs detects prostate cancer in urine samples
- Lung cancer, breath: another team’s dogs caught 71 of 100 and wrongly flagged 7 of 100 healthy people.17PubMed. Canine scent detection in the diagnosis of lung cancer: revisiting a puzzling phenomenon
- Aggressive prostate cancer, urine: dogs caught 71 of 100 and wrongly flagged about a quarter of others.18PubMed Central. Feasibility of integrating canine olfaction with chemical and microbial profiling of urine to detect lethal prostate cancer
Training makes much of the difference. Dogs trained on breath samples caught about 92% of lung cancers, versus about 50% when trained on tumor tissue.19PubMed Central. Sniffer Dogs Diagnose Lung Cancer by Recognition of Exhaled Gases: Using Breathing Target Samples to Train Dogs Has a Higher Diagnostic Rate Than Using Lung Cancer Tissue Samples or Urine Samples In one prostate study, a sample the dog “wrongly” flagged turned out to be cancer on repeat biopsy.20PubMed. Olfactory detection of prostate cancer by dogs sniffing urine: a step forward in early diagnosis Yet some dogs in training never reach a reliable level.21Open Access Journal of Veterinary Science & Research. Canine Scent Detection of Lung Cancer: Preliminary Results
“Caught” and “wrongly flagged” translate directly into what happens to a person. A missed cancer means someone who has the disease is told, in effect, that all is well, which can delay treatment when it matters most. A false alarm means a healthy person faces worry, follow-up scans and possibly a biopsy they did not need.
Most dog studies use small, carefully sorted sets of samples from known cancer patients and healthy volunteers, which is much tidier than a real clinic full of mixed conditions. Dogs get tired or distracted, so performance varies from one day to the next. Each dog needs months of training, which makes the approach slow and hard to scale. And a strong result in one lab is often not repeated by another team using different dogs, samples and methods.
Worms, Ants and Electronic Noses
Other animals respond to cancer odor too. Ants learned within three trials to find cancer cell odor for a sugar reward.22PubMed Central. Ants detect cancer cells through volatile organic compounds Tiny worms moved toward urine from people with breast cancer and away from healthy urine, with over 97% accuracy.23PubMed Central. C. elegans-based chemosensation strategy for the early detection of cancer metabolites in urine samples These animals are cheaper to train than dogs, though the work so far is early lab testing.
Machines imitate the nose as well. An electronic nose is a device with many sensors that each react differently to odor chemicals.24Journal of Cancer Metastasis and Treatment. ENose: a new frontier for non-invasive cancer detection and monitoring Their combined response forms a “smell fingerprint” that software learns to recognize.25Journal of Cancer Metastasis and Treatment. ENose: a new frontier for non-invasive cancer detection and monitoring One compact electronic nose sorted lung cancer breath samples correctly 94% of the time.26Microchemical Journal. VOC pattern recognition for lung cancer detection using a compact E-nose system A urine odor analyzer caught 88% of colorectal cancers but cleared only 60% of people without it.27PubMed Central. Detection of Colorectal Cancer (CRC) by Urinary Volatile Organic Compound Analysis
Unlike a dog, a device does not tire, and its readings can be recorded and checked afterward. But its software must first learn from samples from many patients, since its accuracy depends on what it has been taught.
Can You Get a Cancer Smell Test Today
No breath test is approved by US or European regulators for cancer diagnosis.28PubMed Central. Advanced strategy for cancer detection based on volatile organic compounds in breath Most studies involve fewer than 1,000 people, and results vary 15 to 30% between devices.29PubMed Central. Advanced strategy for cancer detection based on volatile organic compounds in breath Breath analysis is also hard to collect, analyze and store consistently,30PubMed Central. Common Strategies and Factors Affecting Off-Line Breath Sampling and Volatile Organic Compounds Analysis Using Thermal Desorption-Gas Chromatography-Mass Spectrometry (TD-GC-MS) so it has stayed mostly in research.31PubMed Central. The Use of Breath Analysis in the Management of Lung Cancer: Is It Ready for Primetime?
Daily life muddies the signal. Room air and the environment can cause around 30% false positives.32PubMed Central. Advanced strategy for cancer detection based on volatile organic compounds in breath Antibiotics and bowel preparation for colonoscopy changed breath chemistry in one study.33PubMed. Analysis of the effects of microbiome-related confounding factors on the reproducibility of the volatolomic test
Progress is underway. One lung cancer breath detector is in a fast-track US program, with a major trial due in 2025.34PubMed Central. Advanced strategy for cancer detection based on volatile organic compounds in breath Regulatory submissions for lung and colorectal cancer may come within 3 to 5 years, at under $200 a test.35PubMed Central. Advanced strategy for cancer detection based on volatile organic compounds in breath Even then, a smell test would most likely serve as a first filter before scans or biopsy, adding to current screening rather than replacing it.
For now, be wary of anyone selling a smell-based cancer test, and stick to proven screening. Mammograms can find breast cancer early when done at the ages and intervals your health service recommends. Colonoscopy or stool-based tests screen for colorectal cancer. Low-dose CT scans are recommended for some heavy smokers or former smokers at higher risk of lung cancer. Which tests apply depends on your age, family history and personal risks, so ask your doctor what is advised for you.