Androstenone

Androstenone (5α-androst-16-en-4-one) is a steroid found in both male and female sweat and urine. It is also found in boar's saliva. Androstenone was the first mammalian pheromone to be identified. It is found in high concentrations in the saliva of male pigs and, when sniffed by a female pig who is in heat, results in the female assuming the mating stance. Androstenone is the active ingredient in BoarTaint, a commercial product sold to pig farmers to test sows for timing of artificial insemination.

Properties
Depending upon who is doing the smelling, it is reported to be an unpleasant, sweaty, urinous smell, a woody smell, or even a pleasant floral smell. The differences in perceived odor can be accounted for by the genetic differences in the OR7D4 receptor which detects the chemical. People who possess 2 proper genes for OR7D4 tend to describe the odor for the steroid as likable to urine. Those with only 1 gene typically described the odor as weak or were not able to detect it. Those with mutated copies of the gene described the odor as pleasant or sweet.

According to the National Geographic Smell Survey, 30% of humans cannot detect androstenone unless the concentration is extremely high. In small amounts, the odor is hardly detectable by most people. This may be due to a polymorphism in the receptor gene that codes for the androstenone receptor. However, the range of abilities to detect the odor vary greatly. It has been proven that the odor can be detected by people in levels in a range of 0.2 parts per billion and 0.2 parts in 100 million. Several groups report, however, that some individuals who initially cannot smell androstenone can learn to smell it by repeated exposures to it.

As a Pheromone
In humans androstenone also has been suggested to be a pheromone, however, scientific data to support these claims are scant. There is more promising data for a closely related compound, androstadienone. The claim of being a human pheromone may originate from the well known fact, as mentioned above, that almost 30% of the population is unable to smell the odor. They have a so-called specific anosmia to the odor, i.e. they are unable to smell that specific odor but have besides that a normal sense of smell. However, this should by no means be seen as indicative for being labeled as a pheromone, as it is true of over 80 olfactory compounds.

Commercial Use
Some commercially-available substances are advertised using claims that the products contain human sexual pheromones and can act as an aphrodisiac. However these often lack credibility due to an excessive marketing of pheromones by unsolicited e-mail. Despite claims to the contrary, the fact remains that no defined pheromonal substance has ever been demonstrated to directly influence human behavior in a peer reviewed, published study.