Patients with advanced stomach cancer or pyloric obstruction may have bad breath resembling the smell of rotten eggs. This is because Helicobacter pylori, which inhabits dental plaque and tartar in the mouth, produces urease together with other bacteria, breaking down urea in the mouth and forming ammonia-smelling substances.
Helicobacter pylori is one of the main reasons it is recognized by the World Health Organization as a Class 1 carcinogen. The stomach is connected to the mouth, so even if the food is fresh and healthy, it can still be “secondarily contaminated” by bacteria such as Helicobacter pylori in the mouth. These bacteria can enter the stomach with food, damage the stomach lining, cause various stomach diseases, and may lead to stomach cancer over time.
Lung infections, bronchitis, lung abscesses, and even lung cancer can cause the mouth to have a rotten fish smell. This odor is usually caused by mucus buildup in the lungs. It is worth noting that smokers often have a distinctive “smoker’s breath,” and if they also have lung disease, their bad breath may be more pronounced.
Diabetic patients may have a breath that smells like rotten apples. This is because, when blood sugar is too high, the body breaks down fats to produce ketones, and alpha-ketoglutaric acid emits a sour smell.
If the exhaled gas carries this smell, it may mean that the concentration of ketones is very high, nearing or reaching the level of diabetic ketoacidosis, which requires immediate medical attention.