When measuring odours, 2 different methods can be used depending on the characteristics of the Odours (1).

  -Physicochemical Methods: Gas Chromatography, FID, piezoelectric sensors (electronic noses), colorimetric tubes, etc.

  -Sensory methods: Olfactometry, Nasal Ranger, psychometric assessment, etc.

  The physical-chemical methods are suitable when the Odorous compound is known or when we pretend to monitor the emissions of certain installations in continuum to control the presence of gas leakages associated with odorous compounds. This methods are not very specific or sensitive nowadays.

  Although it is a relatively new technique, Olfactometry is presented as the future standard to measure odours until other devices, such as the electronic nose, are further developed.

  Before explaining what the Olfactometry is we are going to explain the process that is followed when a certain number of complaints from citizens is received.

  A number of steps are taken to attend and identify a bad odours complaint:

• Previous study, psychometric assessment: a certain number of questionnaires is distributed to the citizens in the study area. The statistical analysis ofthe responses provided by the citizens is a starting point to begin an odour study.
• Identification of the sources generating odours.
• Design of the sampling/measuring campaign
• Sampling of samples: Olfactometry and physicochemical techniques.
• Calculation of the odour emission for each source.
• Modelling: assessment of the impact on the surroundings of the installations. Odourmap.
• Conclusion and recommendations: Proposal of alternatives.

  Once the samples have been collected in the bag, we have to bring them to the laboratory. The analysis of these samples must be done in a time frame not less than 30 h (EN 13725). This period is critical as some physicochemical processes can alter the content of the bag causing a decrease or increase in the quantity of odour measured as well as a change in the intensity and the hedonic tone of the odour.

  (1) Bidlingmaier et. al, Odour Emissions from Compost Plants - Dimensioning Values for Enclosed and Open Plants (Translation), Rhombos-Verlag, Berlin, 1997