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Chemistry of Breath

By Aurametrix

We exhale mostly the same stuff that what we inhale. There is only about 10% difference in chemical composition of exhaled and inhaled air. This difference  accounts for the state of our health and metabolism.

10 gases that make up clean air, in order of highest to lowest concentration, are: Nitrogen, Oxygen, Argon, Carbon dioxide, Neon, Helium, Methane (CH4), Krypton, Hydrogen, and Xenon.

The air we inhale is roughly 78% nitrogen, 21% oxygen, 0.96% argon and 0.04% carbon dioxide, helium, water, and other gases, by volume. The quality of air varies. EPA monitors five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Underwater divers often breathe slightly different oxygen-rich or helium-rich gas mixes.

We exhale about 4-5% more carbon dioxide and less oxygen than was inhaled, more vapors and additional trace gases: 5% water vapor, several parts per million (ppm) of hydrogen and carbon monoxide, 1 part per million (ppm) of ammonia and less than 1 ppm of acetone, methanol, ethanol and other volatile organic compounds. The exact amount of exhaled oxygen and carbon dioxide varies according to the fitness, energy expenditure and diet of that particular person.

Normal human breath contains a great number of alkanes and monomethylated alkanes.
Food and beverages can influence what chemicals we breath out. For example, allyl thiosulfinates (mainly allicin) are detected in breath of those who ate garlic.
Chemicals in breath also depend on the age, exercise, and health/disease status.
Most of the compounds that cause bad breath (hydrogen sulfide, methyl mercaptan, cadaverine, putrescine, skatole) are the waste products of anaerobic bacteria (more specifically Gram-negative anaerobic bacteria).

Here are a few examples of breath chemistry studies:
-  Dimethyl sulfide (and other sulfur compounds), acetone (ketone), 2-butanone and 2-pentanone (methyl ketones) are increased in breath of liver patients, while indole and dimethyl selenide are decreased. People perceive the odor as mousy, sweet fecal and fishy.
-  Sulfur-containing compounds such as methanethiol and hydrogen sulfide are present at significantly higher concentrations in the breath of lung cancer patients and flatus of colon cancer patients. These compounds are produced when glucose reacts with methionine or its metabolites homocysteine or cysteine. 
-  Camphor, Bicyclo[2.2.1]heptan-2-one,1,7,7-trimethyl-,(IS)-, 2-Cyclohexene-1-methanol,alph,alpha4-trimetyl-, p-menth-1-en-8-ol are among major VOC identifiers of primary lung cancer in breath. 
-  Oxidative stress that accompanies transplant rejection degrades polyunsaturated fatty acids in membranes by lipid peroxidation, which releases alkanes and methylalkanes excreted in the breath.
-  Components of petrole and their methyl-derivatives such as 4-methyloctane, methylpentadecane, methylxecadecane, butane, xecane, and pentane are indicative of heart disease. Some of these compounds are used by insects in chemical communication. Odors range from floral to cement and racing motorcycles
Alkanes ethane and pentane, constituents of fuels, are indicative of Asthma, COPD and Cystic Fibrosis
-  Breath pentane (an alkane with the formula C5H12, generated by peroxidation of cellular fatty acids) is a determinant of inflammation and has been correlated with inflammatory bowel disease

Derivatives of cyclic hydrocarbons - such as Cyclopropane, Limonene, Benzene, Longifolene, and simpler hydrocarbons  - Tridecane, Tetradecane, Pentadecane, along with fatty alcohols (octanol) were found in breath of breast cancer patients. Smell of these compounds ranges from lemony to gasoline fumes, rubber and stinky bugs
Naphthalene, 1-methyl-, 3-heptanone, methylcyclododecane, heptane, 2,2,4,6,6-pentamethyl-, benzene, 1-methyl-4-(1-methylethyl)-, and cyclohexane, 1,4-dimethyl-, monomethylated alkanes (dimethylcyclohexane, methylheptane, methylcyclododecane, tetramethylbenzene) are the most abundant in active pulmonary tuberculosis.
-  Volatile sulfur compounds such as hydrogen sulfide, methyl mercaptan, and dimethyl sulfide primarily produced by anaerobic bacteria are considered to be the major gases associated with breath malodor. Other malodor chemicals include ammonia, aliphatic amines, branched chain fatty acids, indoles, and phenols.

-  The concentrations of several of the C4 to C20 alkanes and methylated alkanes increases with age. Unsaturated aldehyde 2-Nonenal is also associated with aging. The odor is perceived as somewhat fatty and earthy with notes of roots of the iris plants, cucumber, buckwheat and aged beer.
-  A fruity odor to the breath is due to excess acetone. This is a sign of ketoacidosis, which may occur in diabetes. Acetone is a byproduct of fat catabolism related to circadian rhythmisity and its concentration increase overnight, reaching maximum levels 2-3 hours before lights on.
-  Breath that smells like feces can occur with prolonged vomiting, especially when there is a bowel obstruction.
-  Ammonia-like odor (also described as urine-like or "fishy") is present in people with chronic kidney failure , uremia and other stages of renal disease. It occurs due to nitrogen-containing compiunds, namely amines dimethylamine (DMA) and trimethylamine (TMA). Ammonia is 
by far the most significant abnormality, especially if renal patient were treated with CAPD (Continuous ambulatory peritoneal dialysis).
- “woody, musky odor” is associated with Parkinson's disease. It is probably caused by higher concentrations of 
eicosane, hippuric acid and octadecanal and lower-than-average levels of perillic aldehyde.

-  Breath isoprene concentrations depend on short-term physical activity and increase overnight.
-  Isoprene is also associated with problems in cholesterol metabolism. It smells like new rubber tubing or oak forest and repels insects.
-  Bacteria-specific volatiles in the breath can tell about sinus infection, although signatures of different bacteria greatly differ.


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  • Preti G, Thaler E, Hanson CW, Troy M, Eades J, Gelperin A. Volatile compounds characteristic of sinus-related bacteria and infected sinus mucus: Analysis by solid-phase microextraction and gas chromatography-mass spectrometry. J Chromatogr B Analyt Technol Biomed Life Sci. 2009 May 21.
  • Phillips M, Altorki N, Austin JH, Cameron RB, Cataneo RN, Kloss R, Maxfield RA, Munawar MI, Pass HI, Rashid A, Rom WN, Schmitt P, Wai J. Detection of lung cancer using weighted digital analysis of breath biomarkers. Clin Chim Acta. 2008 Jul 17;393(2):76-84. Epub 2008 Mar 3.
  • Phillips M, Cataneo RN, Condos R, Ring Erickson GA, Greenberg J, La Bombardi V, Munawar MI, Tietje O. Volatile biomarkers of pulmonary tuberculosis in the breath. Tuberculosis (Edinb). 2007 Jan;87(1):44-52. Epub 2006 Apr 25.
  • Phillips M, Cataneo RN, Cheema T, Greenberg J. Increased breath biomarkers of oxidative stress in diabetes mellitus. Clin Chim Acta. 2004 Jun;344(1-2):189-94.
  • Phillips M, Cataneo RN, Greenberg J, Gunawardena R, Naidu A, Rahbari-Oskoui F. Effect of age on the breath methylated alkane contour, a display of apparent new markers of oxidative stress. J Lab Clin Med. 2000 Sep;136(3):243-9. 
  • Kokoszka J, Nelson RL, Swedler WI, Skosey J, Abcarian H. Determination of inflammatory bowel disease activity by breath pentane analysis. Dis Colon Rectum. 1993 Jun;36(6):597-601.
  • Moorhead K, Lee D, Chase JG, Moot A, Ledingham K, Scotter J, Allardyce R, Senthilmohan S, Endre Z. Classification algorithms for SIFT-MS medical diagnosis. Conf Proc IEEE Eng Med Biol Soc. 2007;2007:5178-81.
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  • Phillips, M,Cataneo, R.N., Saunders, C, Hope, P., Schmitt, P., Wa, J.  Volatile biomarkers in the breath of women with breast cancer 2010 J. Breath Res. 4 026003 
  • JJBN van Berkel - 2010 - There's something in the air 
  • Kelly M Paschke, Alquam Mashir, and Raed A Dweik, Clinical applications of breath testing, F1000 Med Rep. 2010; 2: 56.
  • Esquivel E, Chaussain M, Plouin P, Ponsot G, Arthuis M, Physical exercise and voluntary hyperventilation in childhood absence epilepsy, Electroencephalogr Clin Neurophysiol 1991 Aug; 79(2): 127-132.
  • William D. McArdle, Frank I. Katch & Victor L. Katch, Exercise Physiology: Energy, Nutrition, and Human Physiology, Lippincott Williams & Wilkins, 2006
  • Average respiratory rates:

    * Birds: 5–100+ breaths per minute
    * Dogs: 10–30 breaths per minute (up to 200)
    * Cats: 20–30 breaths per minute (up to 300)
    * Newborns: 44 breaths per minute
    * Infants: 20–40 breaths per minute (over 50 if sick)
    * Preschool children: 20–30 breaths per minute (over 40 if sick)
    * Older children (5-8): 16–25 breaths per minute (over 30 if sick)
    * Healthy Adults: 12–20 breaths per minute (up to 70 if stressed or exercising)
    * Adults with Asthma: 24-30 breaths per minute (12-15 l/min)
    * Adults with Heart Disease : 28-32 breaths per minute
    * Adults with Diabetes: 20-40 breaths per minute
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    Check also Aurametrix Olfactics blog