The best instrument to measure odour is right in front of you. Your nose.
Some jurisdictions in the United States and Europe rely on human testers to monitor industrial odours. Testing is done through a controlled measurement procedure called olfactometry that determines relative concentration and unpleasantness.
The process is this: Odours are captured at their source in special bags and delivered to an air quality testing centre. There, human testers are subjected to alternating bursts of clean air and the odour. The concentrations of odour are halved with each successive exposure until the testers can no longer detect a difference between the two samples.
The precise strength of the smell is measured in “odour units” per cubic metre.
Sensory testing is not the only method to measure the strength of industrial odours. Some jurisdictions rely on chemical analysis or electronic noses. But nothing can touch the human nose for its ability to successfully detect a full range of smells.
Once testers establish the precise concentration of odour, compliance officers can determine whether the emission violates air pollution regulations.
Regulations differ between jurisdictions, but a common process to determine thresholds of an odour’s acceptability is to consider its source.
Some odours, like the smell of a landfill or a mushroom farm, can be unbearable in very small concentrations. Other scents — such as those related to cooking — can be quite pleasant until they reach a high concentration.
NASA sees odour testing as a crucial part of pre-trip planning. The space agency employs full-time sensory testers who ensure nothing is permitted on shuttles that could force astronauts to end their flights early. This scenario reportedly happened in 1976 when the Soviet Union’s Soyuz 21 flight returned to earth on account of a mysterious unbearable smell.