Parod, in Encyclopedia of Toxicology (Third Edition), 2014 HumanĪmmonia has an odor threshold ranging from 1 to 5 ppm. Most of the absorbed ammonia is excreted in the urine as urea, with minimal amounts excreted in the feces or expired air. Ammonia that reaches the circulation is distributed throughout the body where it can be used in protein synthesis or as a buffer.
It is unlikely that a significant amount of the ammonia contacting the skin is absorbed. Due to first-pass metabolism in the liver, little ammonia from the gut reaches the systemic circulation, and toxicologically significant amounts of ammonia in blood (>1 μg ml −1) probably occur only in severe disease states where the metabolism of ammonia by the liver and the excretion of metabolites by the kidney are compromised. The brain can also convert ammonia to glutamine. Almost 100% of the ammonia produced endogenously in the human digestive tract (60 mg kg −1 day −1) is absorbed and metabolized in the liver to urea and glutamine. Ammonia or ammonium ion is well-absorbed by the gastrointestinal tract. This absorption process may be adaptive or saturable because most of the ammonia inspired during longer-term exposures (10–27 min) is exhaled with ∼4–30% being retained within the upper airways and available for systemic absorption. During short-term (⩽2 min) exposures to ⩽500 ppm ammonia, most (83–92%) of the inspired ammonia is retained within the upper airways. Dissolved ammonia and the less permeable ammonium ion exist in a dynamic equilibrium that serves to retard the absorption of ammonia into the circulation depending on the complexity of the intervening tissues. Unionized ammonia can freely diffuse through tissue cells but forms ammonium hydroxide upon contact with tissue water.
Parod, in Encyclopedia of Toxicology (Second Edition), 2005 Toxicokinetics