How Odour is Formed in Industrial Processes

Odour, or stench, generated in industrial processes is the result of chemical reactions and biological processes that release odour-generating substances and gases. These may include aldehydes, terpenes, sulfur-containing compounds, amines, volatile organic compounds (VOCs), and other gaseous substances. The intensity and character of the odour depend on the chemical structure of these substances and their odour threshold, which is the lowest concentration at which humans can perceive the scent.

The odour activity of different substances varies significantly. For example, sulfur compounds such as hydrogen sulfide (H₂S) have an extremely low odour threshold of approximately 0.5 ppb (parts per billion), whereas an aldehyde like Nonanal has an odour threshold of around 50 ppm (parts per million). This means that a single H₂S molecule can smell as strong as 100,000 Nonanal molecules. Such a difference in perceived odour intensity is remarkable and illustrates how certain substances can affect our environment and experience of smell even at very low concentrations.

The olfactory system is part of the human defense system, evolved over time to warn us of potential dangers. We are particularly sensitive to odours that signal threats, such as fire and spoiled food. Many substances with strong, unpleasant odours are linked to biological decomposition or chemical compounds that can be harmful when inhaled. Therefore, controlling and purifying odour-emitting substances is an important aspect of industrial operations.

Many industrial processes produce odour-emitting substances, especially those involving biological decomposition processes or chemical reactions with volatile substances. Examples of such processes include:

• Frying and roasting – Generate aldehydes, fatty acids, and ketones, which can have strong odour impressions.
• Biogas production – Involves anaerobic decomposition of organic material, resulting in hydrogen sulfide and other foul-smelling gases.
• Composting – Produces ammonia, hydrogen sulfide, and organic acids during the breakdown of organic waste.
• Wastewater treatment – Generates odours from sulfur-containing compounds and ammonia.
• Slaughterhouses and rendering – Produce amines, sulfur compounds, and fatty acids when processing animal materials.
• Waste management – Generates a mixture of VOCs, sulfur compounds, and organic acids depending on the type of waste.

Sulphur-Containing Compounds

Sulphur-containing compounds, such as hydrogen sulphide (H₂S) and methyl mercaptan, are strongly odorous substances that can cause discomfort and health risks when released into the environment, particularly in industrial settings. These compounds are primarily formed through the decomposition of organic material in oxygen-free environments, which can occur in sewage systems, biogas production, waste management, and other industries where organic material breaks down.

Hydrogen sulphide is one of the most well-known sulphur-containing gases. It has a characteristic smell of rotten eggs, making it easy to identify even at very low concentrations. H₂S is often produced through anaerobic (oxygen-free) decomposition of organic material, which is common in environments such as sewage systems, wastewater treatment plants, paper mills, and landfill sites. This gas can also be released during oil drilling and from the decomposition of organic material in sediments.

At low concentrations, hydrogen sulphide is usually not harmful to humans, but at higher concentrations, it can be very dangerous and cause serious health problems, including respiratory difficulties, headaches, dizziness, and nausea. At extremely high levels, hydrogen sulphide can be fatal due to its paralysing effect on the respiratory system. Therefore, it is crucial to closely monitor and control hydrogen sulphide levels in industrial environments.

Methyl mercaptan is another sulphur-containing compound that is often found in gaseous form and has a very strong and unpleasant odour resembling rotten vegetables or cabbage. It is also used in industry as an odour indicator for natural gas, as it is very easy to detect at low concentrations. Methyl mercaptan is formed in a similar way to hydrogen sulphide through anaerobic decomposition of organic material and can be found in sewage treatment, waste management, and biogas production.

Methyl mercaptan is highly toxic at high concentrations and can affect the central nervous system. Exposure can cause headaches, dizziness, respiratory difficulties, and nausea. To reduce the risks associated with these gases, industries where these substances may be present often implement advanced ventilation systems, gas detectors, and safety protocols.

Biological Processes

Biological processes play a central role in many industries, especially in the breakdown of organic material. In the food industry, particularly in meat processing and dairy production, microorganisms such as bacteria, yeast, and fungi can produce strongly odorous compounds when breaking down organic substances. These biological decomposition processes release a variety of chemicals, including amines, fatty acids, and other compounds that can generate unpleasant and sometimes penetrating odours.

Meat Processing

During meat processing, especially in cooling and storage, microorganisms can begin to break down proteins, fats, and carbohydrates in meat products. This process produces a range of compounds that can contribute to a strong odour. Commonly occurring substances include ammonia, hydrogen sulphide, and fatty acids. Ammonia and hydrogen sulphide are formed when bacteria break down proteins and sulphur-containing compounds in the meat. Hydrogen sulphide is the gas responsible for the smell of rotten eggs, while ammonia produces a sharp, pungent odour.

Fatty acids, such as butyric acid and hexanoic acid, are produced when fats break down and can generate a strong, rancid-like smell. During prolonged storage or insufficient cooling, these odours can become very intense, affecting both the quality of the product and the surrounding work environment.

Dairy Production

In dairy production, where milk and other dairy products are processed, similar biological processes can occur. Bacteria and microorganisms are essential for fermentation processes that produce cheese, yogurt, and other dairy products. During these processes, the carbohydrates in milk are broken down into lactic acid, and in some cases, these microorganisms can also produce fatty acids, amines, and other byproducts. The strong and often characteristic odours that can arise largely depend on the compounds produced during fermentation and storage.

A common odour in dairy production comes from amines, such as putrescine and cadaverine, which are formed when proteins are broken down by bacteria. These amines are often associated with rotten meat or decaying bodies, making them unpleasant to humans. Depending on which bacteria dominate the fermentation process, the types of amines and fatty acids can vary, leading to different smells in dairy products.

Fermentation and Composting

Fermentation and composting are additional biological processes that can produce strong and sometimes unpleasant odours. In food fermentation, such as during the production of beer, wine, sauerkraut, or certain cheeses, organic material is broken down by microorganisms under oxygen-free or low-oxygen conditions. This breakdown often releases ethanol, carboxylic acids, and ester compounds, which in some cases can produce a smell of rotten or yeasty material.

During composting, where organic waste is broken down into nutrient-rich soil, sulphur-containing gases such as hydrogen sulphide (H₂S) and methyl mercaptan can also be released. These gases have a strong odour and can create an unpleasant smell of rotten eggs, especially when the compost is not properly ventilated or when decomposition occurs rapidly under anaerobic conditions.

Composting of food waste, manure, or garden waste can also produce other odours, such as ammonia (which arises from the breakdown of nitrogen-rich materials like manure) and short-chain fatty acids (such as butyric acid), which can have a smell reminiscent of strong cheese or urine.

Biological Odours and Their Impact

The biological processes that generate odours in the food industry, including meat processing, dairy production, fermentation, and composting, affect both the environment and workplace conditions.

Health Risks: Prolonged exposure to strong odour compounds such as amines and hydrogen sulphide can cause headaches, nausea, and respiratory issues. In some cases, exposure to sulphur-containing gases like hydrogen sulphide in high concentrations can be life-threatening.

Environmental Impact: Strong odour compounds released into the surroundings can also negatively affect nearby communities and the environment. This may lead to public complaints and, in some cases, government regulations aimed at reducing odour pollution.

Control and Management of Biological Odours: To manage biological odours in the food industry, several measures are implemented, including:

• Ventilation and Air Purification: Effective ventilation systems that capture and filter odorous compounds are essential. In some cases, activated carbon filters or other air purification systems are used to remove odours from production areas.
• Cooling and Storage Temperatures: In meat processing and dairy production, proper cooling and storage are crucial to preventing decomposition and the formation of odorous gases.
• Composting Treatment: Composting processes can be optimised by improving airflow, which reduces anaerobic decomposition and minimises odour production.

In summary, the biological processes occurring in meat processing, dairy production, fermentation, and composting are significant sources of odour compounds. These processes generate a variety of substances such as amines, fatty acids, and sulphur-containing gases, which can cause unpleasant and sometimes hazardous odours. Effective management and control of these biological odours are essential for both health and environmental protection.

Oxidation Processes

Industries that handle metals, chemicals, and various types of materials often undergo numerous chemical processes that can generate strong and sometimes unpleasant odours. Oxidation is a key chemical reaction that plays an important role in many industrial environments and can contribute to the production of compounds such as aldehydes, ketones, and volatile organic compounds (VOCs), all of which have strong odour profiles. Additionally, certain cleaning and disinfection processes, where chemicals react with organic material, can also result in odour formation.

Oxidation in the Metal and Chemical Industry

Oxidation is a chemical process in which a substance reacts with oxygen, leading to changes in its properties. In the metal industry, oxidation of metals, especially at high temperatures or in contact with air, can produce several strongly odorous compounds. A classic example is rust formation on iron, which occurs when iron reacts with oxygen and water. Rust itself is not known for having a sharp odour, but the processes involved can release other substances.

When certain oils and fats undergo oxidation, for example in industrial processes where lubricants are used, chemical reactions can lead to the formation of aldehydes and ketones. These compounds often have strong and penetrating odours. Aldehydes such as hexanal and octanal may have a scent resembling rotten grass or fruit, while ketones like acetone have a characteristic, sweetish odour. Depending on the type of oil or fat being oxidized, different aldehydes and ketones may be formed, contributing to varying odour profiles.

Oxidation is also an important process in certain chemical industry applications, such as the production of dyes, polymers, and lacquers. When certain chemicals, such as solvents or organic compounds, undergo oxidation, various by-products may be released, potentially causing odour problems, especially if they are not properly managed or ventilated.

Cleaning and Disinfection

In many industries, particularly in food processing, healthcare, and laboratories, chemical disinfectants are used to clean surfaces and eliminate microorganisms. These disinfectants, which may be chlorine-based, oxidizing agents (such as hydrogen peroxide), or alcohol-based products, often react with organic material such as dirt, grease, or residues on surfaces.

The chemical reaction between disinfectants and organic material can release volatile organic compounds (VOCs). VOCs are a group of chemicals that quickly evaporate at room temperature and can have strong and sometimes unpleasant odours. One example of a VOC that can be released during disinfection is chloramine, which forms when chlorine reacts with ammonia or other nitrogen compounds in organic material. Chloramine produces a strong, sharp odour and can also be irritating to the eyes, respiratory system, and skin.

Other VOCs that may form during disinfection include formaldehyde and acetone, both of which have distinct, pungent odours. Formaldehyde is particularly well-known for its use in disinfectants and can be both irritating and toxic at high concentrations. When disinfectants are used in enclosed or poorly ventilated spaces, these VOCs can accumulate, creating an unpleasant working or environmental atmosphere.

Chemical Reactions during Cleaning and Their Odour Problems

When cleaning surfaces contaminated with oils, fats, or organic material, other chemical reactions can also occur. For example, cleaning agents that contain ammonia or sodium hydroxide can react with fatty acids, creating ammonium compounds or fatty alcohols, which can produce the smell of rancid fat or ammonia. These compounds can be very strong and leave a powerful impression on those in the environment.

Odour Management in Industry

To manage and control unpleasant odours generated through oxidation and chemical reactions in the metal and chemical industries, as well as during cleaning and disinfection, various measures are often implemented:

1. Ventilation and Air Purification: Effective ventilation is crucial to reduce the concentration of odorous gases in the air. Air purifiers with activated carbon filters or catalytic filters can be used to remove VOCs and other gases released during chemical reactions.
2. Encapsulation of Processes: Some industrial processes involving oxidation or disinfection can be encapsulated in closed systems or containers to minimize the spread of odorous gases to the environment. This is especially true for processes where strong chemicals are used, and the safety of both workers and the environment is a priority.
3. Controlled Use of Chemicals: By using chemicals and disinfectants with controlled composition and reaction rates, the risk of generating harmful VOCs or odorous compounds during cleaning and disinfection can be minimized. Alternative, less odorous disinfectants can also be considered.
4. Temperature Control: In many cases, temperatures can be adjusted to reduce the speed of oxidation. Lower temperatures can slow the reaction rate of oils and fats that would otherwise oxidize and release strong-smelling aldehydes and ketones.

In conclusion, oxidation and chemical reactions are central to many industries, where they can give rise to strongly odorous compounds such as aldehydes, ketones, and VOCs. During cleaning and disinfection, where chemicals react with organic material, similar odorous substances are often released. To control these odours, it is crucial to implement effective safety measures, such as ventilation, encapsulation, use of controlled chemicals, and temperature regulation.

Volatile Organic Compounds (VOC)

Volatile Organic Compounds (VOCs) are a group of chemicals that have high volatility, meaning they easily evaporate at room temperature. These substances are common by-products in several industries, especially in chemical manufacturing, pharmaceutical production, and oil refining, where they are released during various production steps. VOCs can include substances such as benzene, toluene, formaldehyde, and many other organic compounds that have a strong tendency to disperse in the air. Their presence in industrial environments can be both a health issue and an environmental challenge, making it necessary to manage and control their emissions.

Origin and Formation of VOC in Industry

In many industries, VOCs arise as by-products from the use of solvents, raw materials, fuels, and intermediates in various manufacturing processes. Examples include:

• Chemical Manufacturing: In chemical manufacturing, solvents are often used to dissolve, dilute, or extract substances. These solvents, which are often organic, can evaporate and release VOCs during storage, transportation, or when used in reactions. VOCs can also be released during the production of plastics, rubber, paints, and other chemicals.
• Pharmaceutical Production: In the pharmaceutical industry, solvents are used in several manufacturing steps, such as during the extraction of active substances, crystallization, and dilution of certain drug components. These solvents and intermediates, such as benzene and toluene, can evaporate and spread into the atmosphere.
• Oil Refining: In oil refining, VOCs are released during distillation, when crude oil is heated to separate the different fractions of fuels and other chemicals. This process releases substances like benzene, toluene, and xylene, which are known VOCs. Oil refineries are therefore a major source of VOC emissions.
• Other Manufacturing Processes: In many other processes, such as the production of textiles, adhesives, paints, and varnishes, solvents are used that can lead to VOCs, both during production and when the finished products are used.

Environmental and Health Risks of VOCs

VOCs are particularly harmful to the environment and human health, and their emissions need to be managed strictly:

• Environmental Issues: VOCs contribute to the formation of ground-level ozone, which is a key component of photochemical smog. This type of smog is a serious environmental issue, especially in cities with high industrial activity, and can negatively affect both plants and animals. Ground-level ozone is also a potent air pollutant that can lead to breathing difficulties and other health problems in humans.
• Health Risks: Exposure to VOCs can cause both short- and long-term health problems. In the short term, individuals may experience headaches, dizziness, nausea, and irritation in the eyes, nose, and throat. Long-term exposure, especially to substances such as benzene and formaldehyde, can increase the risk of more serious health problems, including cancer and damage to the nervous system. Some VOCs, such as benzene, are classified as carcinogenic.

Managing VOC Emissions

Since VOCs are both environmentally and health-harmful, it is crucial to control and reduce their emissions in industrial settings. Here are some methods used to handle VOC emissions:

1. Combustion Techniques:
   To reduce VOC emissions, combustion is used to transform VOCs into less harmful substances like carbon dioxide and water vapor. This is often done in incinerators or afterburners, where VOC vapors are directed into a high-temperature process that burns off the organic compounds.
2. Adsorption:
   In some cases, adsorption systems, such as carbon-based filters or zeolites, are used to capture VOCs from the air before they are released into the atmosphere. These systems can be effective in trapping VOCs from ventilated spaces or gases emitted during manufacturing processes.
3. Conditioning and Cooling:
   In certain industries, VOCs can be condensed by cooling or altering pressure, allowing them to be recovered or reducing their concentration before being released into the air.
4. Catalytic Oxidation:
   Another method is catalytic oxidation, where VOCs are oxidized at lower temperatures than combustion. This process often requires the use of a catalyst to accelerate the reaction, transforming VOCs into less harmful substances, such as carbon dioxide and water.
5. Phase Transition Processes:
   To reduce VOC emissions in industrial facilities, phase transition processes like condensation and absorption are also used. By cooling the gases or using solvents, VOCs can be removed from the air and treated safely.

Odorants and Particles

When industries handle processes that generate a mixture of gases and particles, it becomes crucial to first remove the solid or liquid particles before carrying out effective odour removal. The mixture of gases and particles is common in manufacturing processes where both solid materials and gaseous pollutants are released into the air, which can cause both air pollution and odour problems. Separating these particles from the gases is a critical part of the purification process to ensure a clean and odour-free work environment as well as to meet environmental requirements.
Particles generated in industrial processes can be of two main types: dry particles and liquid particles (such as oil mist). Depending on the process causing the emissions, the particles can vary in size and composition, which affects which purification technique is most effective.

Dry Particles

Roasting: During the roasting of materials, such as in the food industry (coffee beans, grains) or the metal industry, small dry particles are released that can spread in the air.
Grinding: Processes where raw materials are ground, such as in the cement industry or food processing (flour, sugar), generate large amounts of dry dust particles.
Frying: When frying food, particles are released from the surface of the food, which can form dust from the fried material.

Liquid Particles

Frying Oils: During frying, especially in the food industry, vapors from hot oils can condense and form small droplets that spread in the air.
Metalworking: In processes involving the use of lubricating oil, such as cutting or grinding metals, oil can spray out as a mist in the air. This is common in the automotive industry, manufacturing, and other metalworking processes.
Painting and Coating: When using solvents and paint, small liquid droplets or mist can spread as particles in the air.

Mixed Gases and Particles

In many industries where both gases and particles are generated, such as during frying, roasting, or milling, combined particle and gas filtration is necessary. To effectively handle both gases and particles, multifunctional filtration systems are sometimes used, which may include various technologies in series.

1. First Step – Particle Filtration: To begin the process of effective odour removal, it is necessary to first separate the solid particles from the gases. This can be done using bag filters, cyclone separators, or electrostatic precipitators, depending on the size and composition of the particles. By removing particles from the air, it helps prevent them from blocking or damaging the equipment used for gas filtration.
2. Second Step – Gas Filtration: Once the particles have been removed, the remaining gases, including those that cause odours, can be treated. This can be done using:
   • Activated Carbon: To adsorb volatile organic compounds (VOCs) and odour-causing gases.
   • Catalytic Oxidation: To convert harmful gases into less harmful substances through chemical reactions.
   • Combustion: If VOCs or other odour-causing substances are concentrated enough, combustion can be used to eliminate them.

Examples of Processes that Generate Mixed Gases and Particles

• Frying: In the food industry, where products like potatoes or chicken are fried, both small particles from the fried food and gases that may have an unpleasant odour (e.g., from the oil) are released. To manage this, both particles and gases must be addressed through efficient filtration systems.
• Roasting: During the roasting of materials like coffee or grains, both dust and gases that can have a distinctive odour are produced. These processes require dual filtration: first, a method to capture the dry dust, and then a process to eliminate odours and gases.
• Grinding and Polishing: In the metal industry, where metals are ground or polished, both metal particles and gases that may be toxic or odorous are released. Here, both particle separation and gas filtration are necessary to ensure a safe and odour-free working environment.

Spread of Emissions and Odours from Industries to Surroundings

Industrial emissions, especially odours and gases, can have a significant impact on the surrounding environment and human health. These emissions spread from industries and affect not only the nearby environment but also residential areas, agriculture, and recreational areas. To reduce negative effects and control the spread of pollutants, it is important to understand the underlying mechanisms that govern this dispersion.

Meteorological Conditions

Wind speed and direction are crucial for how pollutants spread in the air. Strong winds can quickly carry pollutants away and spread them over a larger area, which can reduce the concentration near the point of emission. Low wind speeds, on the other hand, cause pollutants to remain close to the source and concentrate near the industrial area.
• The wind direction is particularly important because it determines where the pollutants are transported. If the wind direction is constant, the pollutants can spread along a predictable path. However, if the wind direction varies, the pollutants can spread in different directions, creating more complex dispersion patterns.
• Temperature gradients can also affect dispersion. If the air temperature is stable or if there are inversions in the atmosphere (where cold air lies under warmer air), pollutants can get trapped at ground level and remain concentrated in one location. In an unstable atmosphere, where warm air rises, vertical mixing is promoted, allowing pollutants to spread in larger volumes at higher altitudes.

Topographical Factors

The topography of the area where an industry is located also plays a significant role in the dispersion of emissions. Geographic features such as mountains, valleys, and buildings affect how winds behave, and thus, how pollutants are spread.
• Mountains and Valleys: An area with valleys can create a “trap” for pollutants, where the winds cannot carry them away as efficiently. Pollutants can accumulate and concentrate in these areas, leading to higher local concentrations. In mountainous areas, the wind can be dampened or change drastically depending on how it interacts with the mountain walls, which can cause pollutants to spread further away from the source or concentrate in certain places.
• Buildings and Industrial Infrastructure: Tall buildings and other structures near industrial areas can create turbulence that either helps spread pollutants or causes them to circulate around the area. This turbulence can result in emissions spreading over a larger area or staying close to the ground.

Atmospheric Dispersion

Atmospheric dispersion is the process by which pollutants spread in the air and lose their concentration as they mix with surrounding air masses. Dispersion is influenced by several factors:
• Turbulence: The turbulence created by winds, weather conditions, or topography is crucial for how effectively pollutants mix and spread in the atmosphere. Strong turbulence helps break up pollutants and distribute them over a larger area. Low turbulence, on the other hand, means pollutants may remain concentrated near the emission source.
• Atmospheric Stability: A stable atmosphere prevents vertical mixing of air and pollutants, which can cause emissions to get trapped near the ground, leading to local concentrations of odour and pollutants. In an unstable atmosphere, where air is more mobile, pollutants can spread vertically and over larger areas, reducing concentration near the ground.
• Air Humidity and Temperature: Humidity and temperature can also play a role in how pollutants spread. Warm, humid air can cause particles to coagulate and fall to the ground more quickly, while cold air can create more stable conditions for concentrated dispersion near the ground.

Practical Measures to Reduce Dispersion

To reduce the negative impact of industrial emissions on the surrounding environment, dispersion calculations or dispersion models are often used. These models simulate how pollutants will spread in the atmosphere based on current conditions such as wind direction, temperature, humidity, and topography. By adjusting different parameters, these calculations can provide a better understanding of how emissions affect the surrounding areas and where concentrations are highest.

Dispersion Calculation and Technical Solutions

Dispersion calculations are often carried out by technical consultants and environmental experts who use advanced computer programs to simulate and predict how pollutants will spread from an industrial source. The calculations take into account factors such as:
• Emission Height: Raising or lowering the emission point (e.g., the chimney) affects how high or low pollutants are released, and thus how they spread. A taller chimney can cause pollutants to spread over a larger area, while a shorter chimney can result in pollutants being concentrated more near the source.
• Odour Concentrations: By installing better treatment technologies, such as filters or adsorption systems, odours and pollutants can be removed or reduced before they are released. Adjusting the treatment technology can reduce the impact on surrounding areas, especially if the technical solution lowers odour concentrations at the emission point.

Measures to Reduce Impact on Neighbors

An important part of dispersion calculations is understanding and planning for how emissions will affect neighboring areas. If emissions are expected to impact residential or other sensitive areas, additional measures can be taken, such as:
• Placing factories or industrial facilities in strategic locations that minimize impact on populated areas.
• Using air filters and scrubbers to capture odours and particles before they spread into the atmosphere.
• Upgrading treatment systems to further reduce pollutant concentrations at the emission point.

Measurement of Odour Emissions from Industries

Odours are unique in that they are not only perceived differently by different individuals but also often consist of a complex mixture of many different molecules, which creates what we call the “cocktail effect.” This effect means that the odour arises from an interaction of hundreds or even thousands of different chemical compounds, making it difficult to isolate and quantify the odour accurately with conventional methods. Therefore, odours are challenging to measure objectively with modern instruments, and human assessment is often required to get an accurate estimate of how odours are perceived in reality.

Olfactometry

To measure and quantify odours, olfactometry is used, a method that relies on human sense of smell. Olfactometry involves a trained panel of odour assessors who are exposed to controlled air samples with varying concentrations of odorants. Panel members rate the odour based on intensity and concentration, providing a direct reflection of how the odour is perceived by people in reality.

Procedure for Olfactometry

In olfactometric measurements, air samples are collected from the emission source and mixed with clean air. These mixtures are then exposed to a panel of odour assessors. The panel uses an olfactometer, a device that controls the flow of air samples to each assessor’s nose. The assessors are asked to identify the dilution level of the mixture at which the odour becomes detectable (i.e., at what concentration it can be perceived). This method allows for a representative estimate of the odour’s intensity in relation to the concentration emitted from the source.

Advantages and Disadvantages of Olfactometry

One of the advantages of olfactometry is that it provides a direct perception of how odours are experienced by people in real life, which is crucial for understanding how industrial emissions affect the surrounding environment. Since the human factor is involved in the assessment, it offers a true representation of how the odour feels in the specific environment.
Disadvantages of olfactometry include the fact that results can be subjective, as people have varying sensitivities to odours, and perceptions can differ between individuals. Additionally, the panel needs to be properly trained to ensure reliable results. This can create challenges in obtaining a consistent measure of odour intensity between different people and occasions.

Alternative Methods – Chemical Analysis

To complement olfactometry and provide a more accurate picture of the odour compounds, chemical analysis can be used. This method involves identifying and quantifying the specific chemical compounds in the air responsible for the odour. Techniques such as gas chromatography (GC) and mass spectrometry (MS) are used to analyze and break down air samples, thus identifying the exact chemical components causing the odour.
Another alternative method is the use of electronic noses (e-noses), a technology that mimics the human nose’s ability to identify and assess odour compounds. These systems use sensors that respond to various chemical compounds and can provide a fast and effective measurement of odorants in the air. However, electronic noses are not yet as precise and sensitive as the human panel when it comes to identifying complex mixtures of odour compounds, such as the typical “cocktail effect.”

Dispersion Modeling – Prediction of Odour Spread

Once the odour levels from an emission source have been determined through olfactometry, the next step is to understand how these odours spread and affect the surrounding environment. Dispersion modeling is used to predict how odour emissions spread from an industrial source over a geographic area and what concentration of odour reaches various receptors.

Models and Tools

Dispersion modeling uses advanced mathematical models to calculate and simulate the spread of pollutants, including odours, from an emission source. Some of the most commonly used models are:

• AERMOD: One of the most widely used models for predicting the spread of pollutants and odours in the atmosphere. It takes into account meteorological data, topography, and other factors to provide accurate simulations of how emissions spread over an area.
  • CALPUFF: A flexible model that can be used to predict the long-range transport of odorants and other pollutants, especially under complicated meteorological conditions.
  • ADMS: Another popular model used to simulate emissions from industrial sources and to assess the impact on surrounding areas.

The dispersion models use data such as wind speed, temperature, terrain, and emission characteristics (e.g., stack height) to calculate how odorants spread. This method allows for visualizing odour concentrations and identifying areas where the odour may become disruptive to the surrounding environment.

Measurement and Dispersion in Connection with Emission Permits

Emission permits for a facility regulate the specific levels of emissions, including odour levels, that are allowed by law. The odour limit can be set either directly at the stack or at the nearest neighbor, depending on the permit applicable to the facility. In some cases, limits may be established for the concentration that reaches residential areas or other sensitive areas, making it important to measure and model emissions and dispersion accurately.

Both olfactometric results (to measure odour intensity at the emission point) and dispersion models (to understand how odours spread and their concentration in the surrounding area) may be used to ensure that emissions comply with the applicable permit.

Support in the Process

To ensure that emissions and dispersion do not exceed legal limits, it is often necessary to seek help from experts, such as environmental consultants. They have experience in interpreting and managing permits, and can provide advice on how to best ensure that industrial emissions meet the required standards. In many cases, these experts are also responsible for applying for or revising emission permits and assisting in the process of reducing emissions and their dispersion.

How does the human sense of smell work?

Introduction

The olfactory system is a complex network responsible for the human sense of smell. This system allows us to detect and interpret various odours in our environment, influencing our taste, emotions, and memories.
Groundbreaking knowledge about the olfactory system was developed by Richard Axel and Linda Buck, in a scientific publication from 1991. Their research was awarded the Nobel Prize in Physiology or Medicine in 2004. More information can be found on the Nobel Foundation’s website.

Anatomy of the Olfactory System

The olfactory system is a complex sensory system that allows us to identify and interpret odours in our environment. It consists of several key components that work together to detect, transmit, and process odour signals to the brain.

Nasal Cavity

The olfactory process begins in the nasal cavity, where inhaled air contains odour molecules. The structure of the nasal cavity is designed to optimize airflow and maximize the contact between odour molecules and olfactory receptors. The mucous membrane, which lines the inside of the nasal cavity, contains mucus that helps dissolve the odour molecules and transport them to the olfactory receptor cells.

Olfactory Epithelium

The olfactory epithelium is a specialized tissue located in the roof of the nasal cavity. It contains three main types of cells:

1. Olfactory Receptor Cells (Olfactory Receptor Neurons, ORN) – Neurons that have receptor proteins on their cilia, which extend into the mucus layer where they can bind to specific odour molecules.
2. Supporting Cells – Provide structural support and contribute to the metabolic process by secreting enzymes that help break down and eliminate odour molecules.
3. Basal Cells – Stem cell-like cells that can regenerate new olfactory receptor cells in the event of injury or normal cell death.

Olfactory Receptor Cells

Each olfactory receptor cell has specific receptors that respond to particular odour molecules. When an odour molecule binds to a receptor, a G-protein-coupled signaling system is activated, leading to the production of secondary messengers, such as cAMP. This results in the depolarization of the cell membrane, generating an electrical signal. The signal is then transmitted via axons to the olfactory bulb.

Olfactory Bulb

The olfactory bulb is a structure located above the nasal cavity that serves as the primary processing center for odour information. Here, the axons from ORNs form synapses with mitral and tufted cells in specific spherical structures called glomeruli. Each glomerulus collects information from ORNs with the same type of olfactory receptors, helping to organize and encode the odour information before it is sent to higher brain centers.

The Process of Odour Detection

When an odour molecule binds to a receptor on the cilia of the ORNs, a cascade of intracellular events is activated, leading to the production of cyclic adenosine monophosphate (cAMP). cAMP opens specific ion channels in the cell membrane, allowing the influx of sodium and calcium ions. This leads to a depolarization of the cell and the generation of an action potential.
The electrical signal travels along the axons to the olfactory bulb, where it is relayed and processed in spherical structures called glomeruli. Each glomerulus receives signals from ORNs with the same type of olfactory receptors, enabling a first level of selective processing and organization of the odour information.
Mitral and tufted cells in the olfactory bulb then relay the processed signal to several brain regions, including:
• Piriform Cortex – Where primary analysis of odour patterns occurs.
• Amygdala – Important for emotional and associative processing of odours.
• Hippocampus – Links odours to memories and learning.
• Hypothalamus – Regulates autonomic and hormonal responses to odours, such as appetite and behavior.
• Orbitofrontal Cortex – Where conscious perception and cognitive interpretation of odours take place.

The Brain’s Role in Odour Perception

Olfactory Cortex and Hippocampus

The olfactory cortex is responsible for the initial processing and conscious awareness of odours, while the hippocampus plays a crucial role in memory formation and associating odours with memories. The brain sorts the response from the olfactory receptors and thus forms an olfactory impression. Evolution has made humans particularly sensitive to smells that indicate danger, such as those linked to spoiled food or fires. Since different individuals have different sensitivity to olfactory receptors, the same odour can be perceived with varying intensity depending on the person’s genetic makeup and prior experiences.
The olfactory cortex interacts with other parts of the brain to integrate odour impressions with cognitive functions such as decision-making and emotional responses. This enables us to identify odours, determine whether they are pleasant or unpleasant, and link them to specific events or situations.

Amygdala

The amygdala plays a central role in generating emotional reactions to odours. Odour signals that reach the amygdala can trigger strong emotional responses, such as pleasure from the scent of freshly baked bread or disgust from the smell of rotten food. This process is evolutionarily important because it helps us quickly identify and respond to potentially dangerous substances in our environment. The amygdala is also closely connected to the stress and reward systems, which means that certain odours can evoke nostalgic or comforting feelings.

Factors Affecting the Olfactory System

Genetics

Genetic variations influence the number and types of olfactory receptors, which can alter sensitivity to odours. Humans have approximately 400 functional genes for olfactory receptors, but which ones are active and how sensitive they are varies between individuals. This explains why some people are more sensitive to certain smells than others. Some genetic mutations can also lead to complete or partial anosmia, which is the inability to perceive certain odours.

Age

The sense of smell deteriorates with age due to a reduced number of receptor cells or changes in the brain. As we age, the regenerative ability of olfactory receptor cells decreases, leading to a gradual decline in the sense of smell, particularly after the age of 60. This can affect quality of life by reducing the ability to taste and identify dangerous odours, such as smoke or gas leaks.

Health Conditions

Various diseases and medical conditions can negatively affect the sense of smell:

• Infections and inflammations – Colds, flu, and sinusitis can temporarily reduce the sense of smell due to swelling and blockage of the airways.
• Neurological diseases – Conditions like Parkinson’s and Alzheimer’s have been shown to affect the sense of smell early in the disease progression.
• Trauma and head injuries – Injuries to the brain, especially in areas like the olfactory bulb or olfactory cortex, can cause permanent or temporary smell impairment.
• Polyps and nasal congestion – Growths in the nose can block the olfactory epithelium and prevent odour molecules from reaching the receptor cells.
• Chemical exposure – Long-term exposure to strong chemicals and pollutants can damage olfactory receptors and cause lasting deterioration of the sense of smell.

The function of the sense of smell is thus affected by both innate factors and external circumstances. Changes in the sense of smell can significantly impact an individual’s life, from the enjoyment of food to the ability to detect dangers in the environment.

The Importance of the Olfactory System

Taste Perception

The sense of smell plays a crucial role in the experience of taste, as odour molecules reach the olfactory epithelium via the retronasal route, where they contribute to the complex perception of flavors. Without the sense of smell, the taste of food and drinks is significantly reduced, which can affect appetite and the enjoyment of meals.

Safety and Survival

The sense of smell serves as an important warning mechanism by detecting hazardous substances such as smoke, gas leaks, chemicals, and spoiled food. A decrease in the sense of smell can therefore pose an increased risk in everyday life, such as when a person is unable to detect a fire in time.

Social and Emotional Connections

Scents play an important role in social interactions and emotional well-being. They can evoke strong memories and feelings, strengthen relationships through recognition of family members and partners, and influence attraction. The sense of smell also contributes to emotional responses, where certain scents can provide a sense of comfort and nostalgia, while others may evoke discomfort.

Conclusion

The olfactory system is a remarkable and complex network that enables our ability to experience and interpret scents. From the nasal cavity to the brain, various components work together to identify, process, and respond to odor stimuli. The sense of smell influences our taste, memories, and emotions, making it an indispensable part of our perception and survival.