One of the chemical senses, specifically the sense of smell. Olfaction registers chemical information in organisms ranging from insects to humans, including marine organisms. For terrestrial animals, its stimuli comprise airborne molecules. The typical stimulus is an organic chemical with molecular weight below 300 daltons; about a half million such substances exist. A few inorganic chemicals can also stimulate olfaction, notably hydrogen sulfide, ozone, ammonia, and the halogens. For marine organisms, amino acids and proteins, which derive largely from the decomposition of organic matter, form particularly good stimuli.