Livestock farms are significant contributors to air pollution worldwide, primarily through the emission of odorous gases that result in unpleasant community odors. These emissions are characterized by low concentrations, complex compositions, and uneven distribution. Prolonged exposure to such environments can pose health risks to the public. Intensive livestock production is associated with the release of odors and various air pollutants, including ammonia, hydrogen sulfide, and organic amines. If not effectively addressed, these emissions can hinder the future development of the livestock industry due to their negative environmental impacts. Odor and gas production in livestock operations are complex processes influenced by bacterial degradation of organic matter, which can occur in animal barns, animal waste management systems, and during manure land application. Odor, in particular, is a significant concern for local communities, as it can cause complaints related to animal production [1]. Odor emissions from agricultural operations and food industries have several negative impacts. These emissions can cause annoyance, discomfort, and reduced quality of life for nearby residents. High levels of odor not only impact the health of workers but also contribute to conflicts between animal farms and nearby residents. Common health symptoms associated with odors include eye, nose, and throat irritation, headaches, and drowsiness. As people increasingly prioritize health and environmental protection, governments are compelled to establish appropriate regulations and guidelines to prevent odor nuisances from these sources.
Despite the importance of addressing odorous emissions, there is limited research available on odor emission factors and the relationships between odor properties specifically related to poultry operations. Long-term monitoring of ammonia (NH3), hydrogen sulfide (H2S), and greenhouse gases (GHGs) is also rarely conducted. Additionally, while the indoor air of animal barns contains hundreds of chemical compounds, no comprehensive indoor air quality index has been established to account for the combined effects of these pollutants on human health. Various factors, such as climate, animal species, waste management practices, and building ventilation control, can influence livestock odor emissions. Accurate measurement and analysis of malodorous compound concentrations are crucial for understanding the severity of odor issues and designing appropriate mitigation strategies [2].
Odor emissions are recognized as one of the main sources of air pollution globally, with sewage treatment plants, landfills, and food-processing industries identified as major contributors [3]. However, in recent times, the odor emissions from large-scale and intensive livestock farms, particularly poultry units, have gained increased attention. Unlike industrial emissions, odor emissions from livestock farms are characterized by their low concentrations, complex compositions, and uneven distribution. Prolonged exposure to such environments can cause anxiety, eye irritation, headaches, and respiratory problems in individuals. The growing public concern for air quality has led to an urgent demand for a clean and livable environment, which poses challenges for the development of the livestock industry.The perception of odor intensity is subjective and influenced by various factors, including individual sensitivity and environmental conditions [4].
Typical livestock farms comprise various areas, including breeding places, manure storage facilities, compost areas, and lagoons. The odors emitted by livestock farms contain numerous compounds, such as ammonia gas, hydrogen sulfide, and volatile fatty acids (VFAs). Ammonia, in particular, is the most common odor compound produced in poultry farms, with compost reaching ammonia concentrations as high as 4100 ppm. Composting sites, where moisture and ventilation are controlled, and microorganisms are used to degrade livestock manure, exhibit higher concentrations of odor compounds compared to other sources. During the composting process, temperature fluctuations correspond to changes in odor compound concentrations [5].
Odor concentration (OC) is typically determined using a detection threshold, which refers to the minimum concentration that elicits an olfactory response. Odor intensity (OI) quantifies the degree or magnitude of perceived odor and is often rated according to a predetermined scale system. To compare odor intensities, a reference substance with a specified magnitude is recommended. n-Butanol (C4H9OH) is commonly used as a reference due to its high purity, stability, relatively low toxicity, and unrelated odor profile to most other odors. Potential challenges in using the field olfactometer, such as its sensitivity, calibration, and the need for rigorous training of the panelist, are discussed. Suggestions for future research directions are presented, including device improvements, further validation studies in different environmental conditions, and exploring the device's potential for long-term odour monitoring [6].Table 1 illustrates a 5-point scale system used for measuring odor intensity.
Table 1
The 5-point Referencing Scale System as per ASTM standard
| Odour Intensity | Odour Strength |
| 0 | No Odour |
| 1 | Very Weak |
| 2 | Weak |
| 3 | Moderate |
| 4 | Strong |
| 5 | Very Strong |