Smell. An odor, or odour, is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive by their sense of smell. An odor is also called a smell or a scent, which can refer to either a pleasant or an unpleasant odor. While scent can refer to pleasant and unpleasant odors, the terms scent, aroma, and fragrance are usually reserved for pleasant-smelling odors and are frequently used in the food and cosmetic industry to describe floral scents or to refer to perfumes. In the United Kingdom and other Commonweath, English-speaking nations, odour refers to scents in general-without positive or negative connotations; but in the United States, and for many non-native English speakers around the world, odor generally has a negative connotation as a synonym for stink. An unpleasant odor can also be described as reeking or called a malodor, stench, pong, or stink. The perception of odors, or sense of smell, is mediated by the olfactory nerve. The olfactory receptor cells are neurons present in the olfactory epithelium, which is a small patch of tissue at the back of the nasal cavity. There are millions of olfactory receptor neurons that act as sensory signaling cells. Each neuron has cilia in direct contact with the air. Odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, initiating electric signals that travel along the olfactory nerve's axons to the brain. When an electrical signal reaches a threshold, the neuron fires, which sends a signal traveling along the axon to the olfactory bulb, a part of the limbic system of the brain. Interpretation of the smell begins there, relating the smell to past experiences and in relation to the substance inhaled. The olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Olfactory information is further processed and forwarded to the central nervous system, which controls emotions and behavior as well as basic thought processes. Odor sensation usually depends on the concentration available to the olfactory receptors. A single odorant is usually recognized by many receptors. Different odorants are recognized by combinations of receptors. The patterns of neuron signals help to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix. This does not correspond to the concentration or intensity of any single constituent. Most odors consists of organic compounds, although some simple compounds not containing carbon, such as hydrogen sulfide and ammonia, are also odorants. The perception of an odor effect is a two-step process. First, there is the physiological part. This is the detection of stimuli by receptors in the nose. The stimuli are recognized by the region of the human brain which handles olfaction. Because of this, an objective and analytical measure of odor is impossible. While odor feelings are very personal perceptions, individual reactions are usually related. They relate to things such as gender, age, state of health, and personal history. The ability to identify odor varies among people and decreases with age. Studies show there are sex differences in odor discrimination, and women usually outperform men. Pregnant women have increased smell sensitivity, sometimes resulting in abnormal taste and smell perceptions, leading to food cravings or aversions. The ability to taste also decreases with age as the sense of smell tends to dominate the sense of taste. Chronic smell problems are reported in small numbers for those in their mid-twenties, with numbers increasing steadily, with overall sensitivity beginning to decline in the second decade of life, and then deteriorating appreciably as age increases, especially once over 70 years of age. For most untrained people, the process of smelling gives little information concerning the specific ingredients of an odor. Their smell perception primarily offers information related to the emotional impact. Experienced people, however, such as flavorists and perfumers, can pick out individual chemicals in complex mixtures through smell alone. Odor perception is a primal sense. The sense of smell enables pleasure, can subconsciously warn of danger, help locate mates, find food, or detect predators. Humans have a surprisingly good sense of smell, correlated to an evolutionary decline in sense of smell.