General Overview
The air is made up of mainly three gasses; nitrogen, oxygen, and argon. The air also contains a wide variety of other gases. Some of these gases contain odours from naturally occurring and man-made processes. The South London Air Monitoring system is measuring odour, although odour is not generally related to health impacts, it can cause an unpleasant response for people.
The system in use uses various sensors to create an odour intensity reading. The values for the following can be seen on the system; (a) odour Intensity, (b) Hydrogen Sulphide and (c) Volatile Organic Carbon. There are other sensors in the unit that contribute to the odour intensity such as ammonia, but the values are not visible on the system as the information they provide is only used to create the odour intensity value.
H2S, NH3 and VOCs are commonly used as key indicators for many types of industrial oduor. Sulfides are typically produced in anaerobic conditions and can indicate process upsets in aerobic processes. Ammonia is often produced in agricultural processes, agricultural activies are also present in the south London area it has been included to contribute to the odour profile in the area. VOCs are produced directly by some processes and also indirectly when organic matter degrades.
Measurement of these substances is also likely to indicate presence of similar compounds (sulfides, amines, organic compounds) that can also cause odour issues.
Odour Intensity
The system monitors the values certain gases in air to create a reading for odour intensity. The odour intensity scale is one to four:
1
When the odour intensity reaches one the odour is discernable to humans, you may or may not be able to pinpoint the actual smell at this point.
2
When the odour intensity reaches two, the smell is noticeable and you can describe some of its characteristics, such as it smells like rotten eggs. This odour does not linger but may come and go.
3
When the odour intensity reaches three the smell is very apparent and will linger.
4
When the odour intensity reaches four, the smell is strong, and a human would be able to describe it easily.
Hydrogen Sulfide
Hydrogen sulphide is present in ambient air naturally and it is also produced from man-made processes. It is a colourless gas that has the smell of rotten eggs. It is found naturally in petroleum natural gas and volcanic gas; it is also produced by human and animal waste. Industrial processes such as food processing, petroleum refineries, paper mills, tanneries and coke ovens also produce hydrogen sulphide.
The hydrogen sulphide monitor on the site has colours built in to show different levels.
Odour Thresholds
GREEN: 0-1 ppm (hydrogen sulphide may not be detectable)
ORANGE: 1 – 1.5 ppm (Most people able to identify hydrogen sulphide)
RED: 2 and above (Hydrogen sulphide is detectable)
References:
https://pubchem.ncbi.nlm.nih.gov/compound/Hydrogen-sulfide
https://www.pdo.co.om/hseforcontractors/Health/Documents/HRAs/ODOR%20THRESHOLDS.pdf
VOC’s
Volatile organic compounds are a large group of organic compounds that can be found in ambient air inside and outside the home. They occur naturally and can also be manmade. They are produced by human activities such as fossil fuel combustion, steel making, petroleum refining, food processing and agricultural activities.
The VOC monitors in the ambient sensors are set to change colour at the following levels.
References:
https://www.ontario.ca/document/air-quality-ontario-2017-report/air-toxics-select-vocs
Ammonia
Ammonia is a clear colorless gas with a distinct odour, often we can associate the smell to that of glass cleaner as it is often present in these types of cleaners. Ammonia is produced naturally by bacteria and the breakdown of organic matter. Ammonia is used in industrial processes as a refrigerant and also as a fertilizer which is very important with food production.
Odour Thresholds
GREEN: Odour Threshold – 0-5.0 ppm (ammonia may not be detectable)
ORANGE: 6-20 ppm (Most people able to identify ammonia at these levels)
RED: 20 and up (Ammonia would be detectable)
References:
https://pubchem.ncbi.nlm.nih.gov/compound/Ammonia
https://www.pdo.co.om/hseforcontractors/Health/Documents/HRAs/ODOR%20THRESHOLDS.pdf