odorless

Odorless refers to the lack of a detectable smell by the typical human sense of smell. In practice, whether a substance is considered odorless depends on the concentration of volatile odor-active compounds and the sensitivity of the observer.

Odor perception is triggered when odorants exceed their odor threshold, the minimum concentration at which an average person can detect an odor. Many substances are inherently odorless (for example, water, nitrogen, oxygen, carbon dioxide, and several noble gases) or become odorless when highly diluted in air or mixed with other compounds. Conversely, some materials have strong or distinctive odors at low concentrations, but can become effectively odorless when diluted far below their detection threshold.

Odorless does not equate to safety. Some odorless substances can be toxic or flammable; for example, carbon monoxide and many industrial solvents are odorless, making detection by smell unreliable. Product labeling often uses "odorless" to indicate an absence of added fragrances but does not guarantee absence of all odor or risk; anosmia or temporary loss of smell can render odors undetectable.

Measuring odor involves olfactometry and human panels; instrumental sensors cannot replicate human smell perfectly. In industry, odorless solvents may be described, but require ventilation and detectors. Detection thresholds vary with humidity, temperature, and the presence of other chemicals, and regulatory labeling may differ by jurisdiction.