The presence of cannabis aroma is one of the most recognized odors in today's environment. Its odor is used for probable cause investigations by police and other agencies. Canines have been specifically trained to detect its odor and are routinely used to reliably supplement the human nose in the detection of the presence of cannabis odor. 

The plant has been around for centuries but its long-term medicinal effects have not been rigorously studied by current medical research. The claimed health benefits, or possible negative side effects (), () will likely be rigorously studied in the future. These studies will likely include monitoring of its proliferation of use. That monitoring will include, if its presence is consistent with terms of legalization, use and control. Therefore, the control and detection of marijuana odor is of importance. 

The biogenic monitoring of terpenes in ambient air () is actively in progress by various organizations. () In our laboratory we monitor ambient levels of terpenes as an indication of air quality. The presence of low or sub part per billion (ppb) levels of a-pinene, camphene, B-pinene, myrcene and limonene are always detected in indoor air. Limonene is generally the major terpene at levels three to ten times higher than the other common terpenes. This is because limonene is currently used in all types of commercial products as a substitute for other halogenated based solvents. The ambient air ppb levels of limonene now exceed previous biogenic part per trillion (ppt) concentrations in urban environments. 
Cannabis odor emissions are dominated by the presence of the more volatile terpenes as listed above. Specifically, when myrcene is the dominant terpene in air samples, it is a marker for the presence of marijuana material. () Even if there is a time with no reported odor, if myrcene equals or exceeds the limonene concentrations, it indicates a potential source of marijuana material. 

It is recommended that these air quality measurements be made using gas chromatography / mass spectrometry (GC/MS) analysis. GC/MS allows for the identification of other terpenes and compounds that are routinely present. It is not uncommon to detect well over a hundred volatile organic compounds (VOCs) in a typical air sample by GC/MS analysis. Knowledge of these other compounds identity often allows for identification of their source. Then, with VOC knowledge 
of the type of odor, its sources and concentrations, engineering control can be designed and implemented for the site. These engineering controls are now routinely being required of marijuana growing facilities and other significant sources of its odor. 

Author: Ron McCullen, Mayfly Odor Laboratory