Pollution by Bad Odours, a Warning in the Face of Possible Environmental Problems

August 7, 2020

Table of contents


  • Odour pollution is a recurring source of complaint.
  • Incidents related to olfactory pollution can cause physical discomfort in people.
  • Although some emissions come from natural sources, others are related to specific industries or processes.
  • Establishing an early-warning perimeter system, based on sensors, for leaks and diffuse emissions can help to reduce odoriferous pollution episodes.


In May 2020, at the heart of the easing period after the coronavirus lockdown, Paris experienced an episode of bad-odour pollution. The night of May 10th to 11th, the emergency services were alerted to a strong smell of sulphur. As a result of and in light of the repeated warnings, the laboratory of the Police Prefecture took and analysed various air samples. But they were unable to identify any particular compound.

The event, which has various hypotheses, is an example of how quickly our olfactory system reacts to a stimulus. Therefore, olfactory pollution is a recurring complaint in urban and industrial areas. What solution does IoT technology offer for measuring and controlling this problem?


What is Bad Odour Pollution?

Odour is the sensation generated by an external stimulus that is perceived by our sense of smell. The UNE-EN 13725 standard defines it as ‘the organoleptic property perceptible by the olfactory organ when inhaling volatile substances’.

Bad odour, therefore, can be defined as a sensory response that tends to cause displeasure or rejection. However, this perception is highly subjective, given that what is agreeable or disagreeable depends on the individual and their olfactory detection capacity.

Perception of a bad odour does not always indicate the existence of a toxic substance. However, prolonged exposure to a nauseating odour may cause a response in the human body. Thus, in the case of ‘particularly offensive odours, exposure may lead directly to somatic reactions, especially gastric symptoms’ (1). For example, the incident reported in The New York Times that occurred in 2019 at the Marathon Petroleum refinery near Detroit (USA). The release of a pungent gas caused the population to be confined to their homes, given that the odour could produce ‘symptoms such as nausea, vomiting, headaches or difficulty breathing’.


Main Gases Renowned for their Odour

Some of the gases that cause the most problems relating to odoriferous pollution are:

  • Hydrogen Sulphide (H2S), with its characteristic rotten-egg smell. This affects human health even in small doses, causing eye, nose or throat irritation.
  • Ammonia (NH3), is a colourless gas with a very penetrating odour; in high concentrations, it can cause skin burns. Two common sources are the breakdown of waste and the use of nitrogenous fertilisers.
  • Volatile Organic Compounds (VOCs), are gases composed of hydrocarbons that at ambient temperature remain in a gaseous state. Some of these, like benzene, which has a sweet odour, are very dangerous for human health.
  • Sulphur Dioxide (SO2), is a gas characterised by its pungent irritating odour. It is perceptible at low concentrations (0.3-1.4 ppm). When it comes into contact with the mucous membranes of the eyes or nose, it can cause severe irritation and transform into sulphuric acid (H2SO4).

As we see in the following section, IoT technology facilitates its detection, warning of possible incidents that may put human health at risk.


How is Odoriferous Pollution Measured?

The odour is the result of the interaction between different volatile chemical substances. Additionally to those mentioned in the previous paragraph, there are others such as (2):

  • sulphuric compounds (sulphides, mercaptans…);
  • nitrogenous substances (amines, ammonium…); or,
  • volatile organic compounds (esters, aldehydes, etc.).

Their perception depends on factors such as:

  • amount of odour;
  • distance from the source;
  • meteorological conditions;
  • topography of the environment; or,
  • sensitivity and tolerance of the nearest inhabitants.

Thus, depending on the type of odour that requires measurement, different techniques will be used.


Sensory Techniques

Sensory techniques are based on information from people. After all, the olfactory sense exceeds the current instruments’ detection capacity. They are especially effective at identifying odours resulting from the mixture of different compounds. Thus, in evaluating an odour, other factors can also be taken into account, such as hedonic tone (how well or how poorly one smells something), intensity or frequency.

These techniques include, for example:

  • dynamic olfactometry;
  • field inspections; and,
  • the register of residents in the area that detect the offensive odour.


Analytic Techniques

Analytical techniques are appropriate when measuring a simple odour or a specific substance that is representative of a source. An example might be the monitoring of the compounds mentioned before with cost-effective sensors, like the ones offered by Kunak.

In addition to identifying specific compounds using sensor devices, another two analytical techniques are:

  • gas chromatography-mass spectrometry, which identifies and relates an odour with a molecule or mixture of molecules. Nevertheless, as Suffet and Braithwaite note (3), ‘the mere identification of a set of compounds in the air does not indicate which are the ones that contribute to the odour problems’.
  • the electronic nose, an instrument that, as highlighted by Eusebio, Capelli and Sironi (4) based on a definition recognised by the scientific community, ‘includes a set of electronic chemical sensors with partial specificity, and an appropriate pattern-recognition system, capable of recognising simple or complex odours’.

This video by the Rovira i Virgili University in Spain, shows, for example, the basic operation of an electronic nose.



Practical Example: Monitoring the Concentration of Hydrogen Sulphide as a Bad Odour Indicator

As we mentioned in the previous section, analytical techniques may be appropriate when measuring one or various predominant compounds associated with a specific activity.

A practical example is the monitoring of hydrogen sulphide, which is easily distinguished by its characteristic odour. H2S is a colourless, inflammable gas, perceptible in very low concentrations. In fact, the sense of smell can detect it from a range of between 0.0005 and 0.3 parts per million (ppm).

Some of these emissions may be natural, such as, for example, in volcanoes. But they may also be emitted as a result of certain industrial activities. Thus, some of the most common ones are refining petroleum, the paper industry or the purification of wastewater. And it is exactly this last activity, the purification of wastewater in Arazuri (Navarra), that we wish to highlight.

Since a few months ago, we are collaborating with the management entity of the wastewater treatment plant (WWTP) of Arazuri to implement a system to measure environmental variables, improve decision making and reduce the impact that bad odour can cause on the nearest houses (<1km).

The main objectives of the cooperation are:

  • To evaluate the H2S emissions in the different parts of the purification process, identifying possible critical points and the best locations to measure them.
  • To analyse which technologies are the most suitable for carrying out the monitoring.

Within the framework of this cooperation, Kunak is responsible for providing the Kunak Air system necessary to know the hydrogen sulphide emissions in real-time and in a precise manner. Similarly, the monitoring system is complemented by the measurement of other meteorological variables such as temperature, relative humidity, wind speed and direction, variables that greatly affect the dispersion of pollution and help to detect the source.


Monitoring of odour pollution at the Arazuri WWTP (Navarra, Spain).


Monitoring of odour pollution at the Arazuri WWTP (Navarra, Spain).


Monitoring of odour pollution at the Arazuri WWTP (Navarra, Spain).



Bad odour pollution episodes do not usually have a tragic end. That is to say, it is unlikely for a person affected by odoriferous pollution to die. Nonetheless, these events can affect the health of the population closest to the emanating source. Perimeter monitoring based on sensors can help prevent these types of incidents, a measure that may well improve the quality of life for the residents in the area.

Case Study: Odour pollution control system at the Arazuri WWTP

Did you like this article?

If you wish to obtain more detailed information on the solution that we have developed for the Arazuri WWTP, you can read the case study.