Nitrogen trihydride NH3,  colorless gas with a characteristic pungent smell (like vinegar or a very long-stayed urine). Household ammonia is a 5-10% solution of ammonia in water. It is also called Ammonium hydroxide and used as a general purpose cleaner for glass, porcelain and steel surfaces or to dissolve grease. NH3 is extensively used to manufacture fertilizers and a wide variety of nitrogen-containing organic and inorganic chemicals.

Inhalation of ammonia may cause olfactory fatique, eye injury, nasopharyngeal and tracheal burns, bronchiolar and alveolar edema, and airway destruction resulting in respiratory distress or failure.

Ammonia is a nitrogen waste compound that is normally excreted in the urine. It can be also detected in breath and skin emissions of healthy people. Main factors affecting the concentration of mouth-exhaled ammonia are: oral fluid pH, urea concentration, and urease activity. 
Ammonia is a major metabolite of glycine. In brain it is synthesized by glutamine synthaze to form glutamine which may serve to protect the brain from hyperamonemia in liver failure. 
Levels of ammonia might correlate with neurological symptoms.
Elevated blood ammonia can happen for a variety of reasons. It is fairly common in infants with certain genetic conditions such as argininosuccinic acid synthase and argininosuccinic acid lyase deficiencies (1 per 35,000 live births per year in USA). Carbamoyl phosphate synthetase I deficiency is an autosomal recessive inborn error of metabolism of the urea cycle which also causes hyperammonemia. In children, it may be related to Reye’s syndrome, while in adults, it may indicate kidney or liver damage or an underlying metabolic disease. In some cases, an elevated blood ammonia level will resolve on its own without treatment.

Strong, acrid, pungent odor easily recognizable in cleaning products like Windex, vinegar, cat urine, chicken manure and, for some people, sweat. Gaseous amines possess a characteristic ammonia smell, liquid amines have a distinctive "fishy" smell. 
Human breath may have an ammonia-like odor (also described as urine-like or "fishy") in people with chronic kidney failure, uremia and other stages of renal disease. It occurs due to nitrogen-containing compounds, namely amines dimethylamine (DMA) and trimethylamine (TMA). Disturbances in the metabolism of amines may be also observed in epilepsy (increased 3-methoxy-4-hydroxyphenylglycol in urine), Parkinson's and dementia (deficient turnover of amines in the brain). Ammonia-smelling urine could mean dehydration or bacterial infection. 

Bad breath, fetor oris or halitosis is a symptom when a noticeably unpleasant odors are exhaled in respiration. Types of odors depend on the food eaten (such as garlic, onions, meat, fish, and cheese), hydration, body mass, smoking, alcohol consumption and other exposures. Low calorie dieting leads to acetone-like odors. Diseases associated with unpleasant breath include liver (fishy), kidney failure (ammonia-like), cystic fibrosis (acidic). Aging breath could produce cardboard-like smell resembling stale beer. 
About 20% of the general population are reported to suffer from foul-smelling breath to some degree. 5‑72% of people feeling that they have bad breath, have no genuine halitosis. 90% of those who is diagnosed with halitosis have it because of their oral bacteria below the gum-line and on the back of the tongue. The remaining 10% is accounted for by many conditions, including disorders in the nasal cavity, tonsil stones, sinuses, throat, lungs, esophagus, stomach, pancreatic insufficiency, food sensitivities or metabolic disorders like TMAU. 

hydrocarbon present in coal tar and petroleum, cigarette smoke, used in chemical synthesis, motor fuels, solvents, detergents, pesticides, personal care items such as nail polish or hair color and many other substances. It has a sweet, aromatic, gasoline-like odor. Benzene is a carcinogen that causes leukemia as well as a number of other illnesses. Virtually the entire U.S. population is exposed to benzene, at least in small amounts -- at gas stations (it's in the gasoline), in diesel exhaust or from cigarette smoke, including second-hand smoke. Benzene is also a problem in a number of workplaces, including oil refineries, coal-coking operations at steel mills, chemical processing plants, rubber manufacturing plants and laboratories, where it is often used as a solvent for other chemicals. You can avoid one significant source of benzene by stepping away from the gas pump when refueling your car.

Benzene levels in breath increase after exposure to tobacco smoke, but there are additional sources of benzene in breath than ambient air. It is difficult to establish exact relationships with the number of cigarettes smoked, or with other benzene exposures such as diet. 

Colorless odorless highly flammable gas, the lightest and most abundant element in the Universe. 
Hydrogen is used in the production of synthetic ammonia and methanol, in petroleum refining. It is added to fats and oils, such as peanut oil, through a process called hydrogenation.

Hydrogen is the most abundant element and is present in Earth’s atmosphere at about 0.5 ppm (parts per million). It is produced naturally by gut bacterial degradation of oligosaccharides and is present in human breath. Humans produce hydrogen at about 50 mg/day. Hydrogen is found in aircraft and space-shuttle air at about 100 ppm. 

Since its introduction as a diagnostic test in 1969 by Levitt and Calloway, the breath hydrogen test has gained in popularity for the diagnosis of intestinal disaccharidase deficiency (intolerance to lactose, fructose, or sucrose), being more sensitive than blood glucose based tests In selected conditions like celiac disease, it may be preferable to enzyme analysis of biopsy because it measures the response of the entire intestinal tract not just a small area which may not demonstrate enzyme deficiency in these cases. It is especially useful in blind loop syndrome (BLS), commonly referred to in the literature as small intestinal bacterial overgrowth (SIBO) or bacterial overgrowth syndrome (BOS). Diagnosis can be made if there is rise of more than 10 or 20 ppm after ingesting a certain substance.
Hydrogen could constitute from 0.000005 to 50% of human gas. 
The paper, published in 1969, found that: 'Flatus gases varied widely within dietary groups but much more gas was generated with diet S (Gemini-type: shrimp cocktail, chicken and vegetables, butterscotch pudding, apple sauce, etc) than with F (a bland formula). 
'In the first 12-hour collection, subjects fed S passed 3 to 209 ml of rectal H2 and 24 to 156 ml from the lungs.

Organic compound with the formula CH₂=CCOOCH₃, CAS 80–62–6 Used in n chain-addition polymerizations, in the manufacture of methacrylate resins and plastics (e.g., Plexiglas; advertising signs and displays, lighting, plumbing and bathroom fixtures; lacquer, and enamel resins; concrete additives, and prosthetic medical devices - such as hip and knee joint prostheses). It can be detected in small concentrations in human breath - Streptococcus pneumonia infections, for example, increase its concentrations up to 1420 ppb.
Methyl methacrylate is a flammable and reactive chemical, fire and explosion hazard. Breathing it in can irritate the lungs causing coughing and or shortness of breath. Higher exposures can cause a build up of fluid in the lungs and pulmonary edema. It can affect liver and kidneys, damage nervous system, causing numbness, “puns and needles”, and/or weakness in the hands and feet, and may cause a skin allergy. High exposure can cause dizziness, irritability, difficulty with concentration and reduced memory. Workplace exposure limits are 100 ppm over 8 hrs shifts or 50 ppm if longer term exposures. 

alcohols with a sharp musty sweet chemical odor like rubbing alcohol
There are two isomers of propanol:

• 1-Propanol, n-propanol, propan-1-ol, ethylcarbinol
• 2-Propanol, Isopropyl alcohol, isopropanol, propan-2-ol

1-propanol is a primary alcohol like ethanol and it smells more like grain alcohol (ethanol-like), while 2-propanol is a secondary alcohol and it smells a little sweeter and more pungent. 

1-Propanol is metabolized by alcohol dehydrogenase to propionic acid (vinegar-like smell) and presents with metabolic acidosis and elevated anion gap, whereas 2-propanol is metabolized by alcohol dehydrogenase to acetone (fruity, nail-polish-like smell) and presents with rapidly developing (within 3-4 h after exposure) ketosis and ketonuria but without metabolic acidosis. 2-propanol smells more like rubbing alcohol (Rubbing alcohol, hand sanitizer, and disinfecting pads typically contain a 60–70% solution of isopropyl alcohol in water) resembling more of acetone, and could be hazardous to health.

​Propanol in breath might correlate with acetone. It is present in the breath samples of cystic fibrosis (CF) patients infected by Pseudomonas aeruginosa (PA). 

There could be multiple sources of propanol. For example, propane-utilizing bacterium Gordonia sp. strain TY-5, oxidizes propane gas to 2-propanol that is further oxidized to acetone. 1-propanol is a by-product formed from certain amino acids when potatoes or grains are fermented to produce ethanol. 

Offensive smell resembling rotten eggs or other sulfury off-flavors.

Sulfur mineral itself has no scent. The odor of rotten eggs actually comes from hydrogen sulfide (H2S) which is a by product of sulfur - it can be produced in water (including humidity in the air). Volcanoes and hot springs could produce H2S via the hydrolysis of sulfide minerals. The odor can be detected at 2 parts per bilion (ppb).

Ninety-five percent of pollution related sulfur oxide emissions are in the form of sulfur dioxide (SO2), a heavy, colorless gas with an odor like just struck matches. 

Most thiols (compounds containing an SH functional group) smell like variations on garlic or skunk. Sulfides have the structure R-S-R′, and are therefore the sulfur analogues of ethers. Hydrogen sulfide smells like 'rotten egg', while Dimethyl sulfide has a smell that is usually described as 'rotting cabbage', 'cooked cauliflower' or just plain 'unpleasant. Dimethyl Disulfide smells like rotting meat. Methanethiol (or methyl mercaptan  - CH3SH) smells like rotten cabbage or urine after eating asparagus. 

Sulfur-reducing bacteria, which use sulfur as an energy source, are the primary producers of large quantities of hydrogen sulfide. These bacteria chemically change natural sulfates in water to hydrogen sulfide. Staphylococcus and Corynebacterium spp. are the most abundant organisms colonizing moist areas of the body and emitting sulfury odors. 

colorless gas with a fishlike odor at low concentrations changing to ammonia-like odor at higher concentrations. Trimethylamine (TMA) exposure can lead to transient increase in resistance to antibiotic stress associated with medium alkalinization, and could also contribute to the evolution of antibiotic tolerance in bacterial populations. Long-term inhalation of low concentrations or short-term inhalation of high concentrations has adverse health effects.
People with fish-odor syndrome (trimethylaminuria) have an impaired version of the enzyme flavin-containing monooxygenase 3 (FMO3). This is the enzyme that converts trimethylamine to non-odorous trimethylamine N-oxide. Trimethylamine in breath can be also used in diagnostics of renal and liver diseases, and for real-time monitoring of haemodialysis efficiency. 

One of the most important dietary sources of trimethylamine is choline found in most protein foods, especially eggs, liver, kidney, peas, beans, peanuts, soya products and cruciferous vegetables (brussels sprouts, broccoli, cabbage and cauliflower).