2 Properties and Effects of Odour
2.1 What is odour?
Odour is perceived by our brains in response to chemicals present in the air we breathe. Odour is the effect that those chemicals have upon us. Humans have a sensitive sense of smell and can detect odour even when chemicals are present in very low concentrations.
Most odours are a mixture of many chemicals that interact to produce what we detect as an odour. Odour-free air contains no odorous chemicals. Fresh air is usually perceived as being air that contains no chemicals or contaminants that could cause harm, or air that smells 'clean'. Fresh air may contain some odour, but these odours will usually be pleasant in character or below the human detection limit.
Different life experiences and natural variation in the population can result in different sensations and emotional responses by individuals to the same odorous compounds. Because the response to odour is synthesised in our brains, other senses such as sight and taste, and even our upbringing, can influence our perception of odour and whether we find it acceptable or objectionable and offensive.
2.2 Physical properties and odour perception
How an odour is perceived and its subsequent effects are not straightforward. The human perception of odour is governed by complex relationships, and its properties need to be considered when assessing potential odour effects.
The perception of the intensity of odour in relation to the odour concentration is not a linear but a logarithmic relationship. The same relationship is known to occur for other human senses such as hearing and sensitivity to light. This means that if the concentration of an odour increases 10-fold, the perceived increase in intensity will be by a much smaller amount.
Interactions between mixtures of odorous compounds can also occur. These are known as synergistic effects. An example is where one odorous compound disguises or masks the presence of other compounds. As the odour concentration reduces through dilution, the nature of the odour may change as different compounds dominate the effect; for example, mushroom-composting odour has been observed to have a distinctly different odour character at source than when diluted downwind. The odour intensity experienced by an observer is, in general, not equivalent to the sum of the intensities of the component odorous compounds. The perceived intensity may be greater or less than the components depending on the synergistic effects of the compounds present.
Exposure to an odour can result in people becoming desensitised so that they can no longer detect the odour even though the odorous chemical is constantly present in the air. This is sometimes known as 'olfactory fatigue'. For example, people working in an environment with a persistent odour are often unaware of its presence and may not be aware that the odour is having an impact on the surrounding community.
Conversely, individuals may become sensitised to olfactory stimulants through acute exposure events or as a result of repeated exposure to nuisance levels of odours.
2.3 Effects of odour
Under the RMA, the main concern with odour is its ability to cause an effect that could be considered 'objectionable or offensive'. An objectionable or offensive effect can occur either where an odorous compound is present in very low concentrations, usually far less than the concentration that could harm physical health, or when it occurs in high concentrations. Where the offensive odour is caused by high concentrations, contaminants in the odour may also be causing direct health effects such as skin, eye or nose irritation, and these should be considered in addition to any potential odour impacts. Repeated or prolonged exposure to odour can lead to a high level of annoyance, and the receiver may become particularly sensitive to the presence of the odour.
Effects that have been reported by people include nausea, headaches, retching, difficulty breathing, frustration, annoyance, depression, stress, tearfulness, reduced appetite, being woken in the night and embarrassment in front of visitors. All of these contribute to a reduced quality of life for the individuals who are exposed.
People can develop physiological effects from odour even when their exposure is much lower than that typically required to cause direct health effects. This effect is sometimes termed 'odour worry' and is due to the perception that if there is a smell it must be doing physical harm.
2.4 Factors influencing odour effects
Whether an odour has an objectionable or offensive effect will depend on the frequency, intensity, duration, offensiveness [In this context, offensiveness is one consideration in whether there is an 'objectionable or offensive' effect occurring as a result of exposure to odour.] (or character), and location of the odour event. These factors are collectively known as the FIDOL factors and are described in Table 2.1.
Different combinations of these factors can result in adverse effects. For example, odours may occur frequently in short bursts, or for longer, less-frequent periods, and may be defined as having 'chronic' or 'acute' effects (section 2.5).
Depending on the severity of the odour event, one single occurrence may be sufficient to deem that a significant adverse effect has occurred. However, in other situations the duration may be sufficiently low and the impact on neighbours sufficiently minor that the frequency of events would need to be higher before an adverse effect would be deemed to have occurred.
Table 2.1: Description of the FIDOL factors
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Frequency |
How often an individual is exposed to odour |
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Intensity |
The strength of the odour |
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Duration |
The length of a particular odour event |
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Offensiveness/character |
The character relates to the 'hedonic tone' of the odour, which may be pleasant, neutral or unpleasant |
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Location |
The type of land use and nature of human activities in the vicinity of an odour source |
The sensitivity of the receiving environment must be taken into account under the RMA and therefore should be considered as part of any odour assessment. The degree of sensitivity in a particular location is based on characteristics of the land use, including the time of day and the reason why people are at the particular location (e.g. for work or recreation). Different locations have different sensitivities to odour and can be classified as having high, moderate or low sensitivity.
The sensitivity that can be assigned to a range of different land uses is described in Table 2.2. This table should be considered as a guide only, and offers comment on issues that contribute to the assessment of sensitivity of the receiving environment. Reference should be made to the local regional plans, and to district/city plans where specific amenity values for various land-use zones may be defined. Regional council staff should be able to assist in working out the degree of sensitivity of the surrounding land use.
Table 2.2: Examples of sensitivity for different land uses
| Land use type | Sensitivity classification | Comments and reasons for classification | ||
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| High | Mod | Low | ||
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People of high sensitivity to odours can be exposed. People can be present at all times of day and night, both indoors and outdoors. Visitors to the area who are unfamiliar with an odour are likely to raise awareness of a problem. In cases of mixed land uses, where the residences are present with industry, the use may be judged to have the same sensitivity as residential depending on the circumstances. |
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Rural residential (low-density residential, minimum property size around 1 ha) |
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Lower population density, therefore less opportunity for exposure to odour. People of high sensitivity can be exposed at all times of the day and night. Rural-type background odours may be present but are usually lower intensity than in a rural zone. Residents tend to work in cities and return home at night or weekends and may not be desensitised to rural-type odours. Can be sensitive to non-rural-type odours (e.g. rendering plant or landfill odours). Overall high or low sensitivity, depending on the circumstances of the particular area. |
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Rural |
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Low population density means low opportunity for exposure to odour. People living in and visiting rural areas generally have a high tolerance for rural-type odours. May be highly sensitive to non-rural type odours (e.g. rendering plant or landfill odours). |
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Heavy industrial |
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A mix of odours is generally tolerated in industrial zones, as long as the intensity is not severe. People in these areas tend to be adults in good health and are more likely to tolerate some odour without finding it to be objectionable or offensive, particularly if the odour is associated with their employment (and source of income). Odours emitted from ground level sources tend to produce the greatest effects at night, when the occupancy of an industrial zone is low or nil, therefore the opportunity for exposure is low. |
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Light industrial |
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Characterised by a mix of small industrial premises and commercial/retail/food industry activities. The latter are often incompatible with industrial odour effects, hence the sensitivity is described as moderate even though occupation is likely to be low at night. |
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Light commercial/ retail/business/ education/ institutional |
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Similar in sensitivity to the high-density residential area, as it affects people of all ages and health status. Hospitals and schools tend to be land uses where people expect better-than-average air quality. Depending on the mix of development, human occupation may be low at night, which can moderate the sensitivity slightly, particularly if dispersion from the odour source is poorest at night. |
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Open space/ recreational |
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People tend to be more aware of air quality when undertaking outdoor activities and exercise, and sensitivity is heightened. People of all ages and health status can be present. People are more likely to be present during the day but events can also be held at night. People are often visitors from other parts of the city or country who are more likely to be sensitive to odours they are not used to. Sports fields may be moderately sensitive and need to be considered on a case-by-case basis. |
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Tourist/ conservation/ cultural/marae |
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Generally have high environmental or spiritual value and a low tolerance to exposure to odours. |
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Public roads |
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Generally low sensitivity because people using the roads are only present for a short period of time. |
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Other factors that may determine whether an objectionable or offensive effect from an odour emission is likely to occur are the presence of background odours, factors influencing perception, and the mental and physical state of the affected person. Cultural issues such as the presence of marae, mahinga kai, and waahi tapu should also be considered (Ministry for the Environment, 2001c).
Odour perception is often related to the source of an odour and whether the activity causing it is considered acceptable in a particular location. An odour associated with a natural source, such as mudflats or geothermal activity, may be accepted whereas a similar odour from an industrial activity may not.
Perception and acceptability are also affected by whether people believe an odour contains harmful chemicals. In such cases a person is more likely to consider the odour to be objectionable or offensive - even dangerous - despite the likelihood that the concentration of the chemical in the odour is too low to cause direct health effects.
Perception is also an important factor where the activity generating the odour is considered offensive in nature or is culturally offensive. This can cause an adverse reaction in the people who detect odours from such activities regardless of other factors (e.g. cremation and sewage treatment).
High levels of background odour in an area can desensitise people to a specific odour, and the addition of other similar odours may go unnoticed. Conversely the cumulative effects from additional odour may result in the odour becoming unacceptable. The likely effect depends primarily on the nature of the odours and the location in which they are occurring. If the nature of the odour is quite different to the background odour, then the background odour will probably not affect the perception of odour from a new odour source.
Sensitisation can also occur where an incident with significant adverse effects changes a person's threshold of acceptability for an odour. This can result in a high level of complaint over the long term and a general distrust within the community of those perceived as responsible for the odour. Conversely, desensitisation or adaptation is a reduction in the perceived odour intensity and/or effect following repeated exposure. This can occur on a short-term basis from olfactory fatigue. Adaptation is a long-term process that can occur when communities become increasingly tolerant of a particular source of odour, which is primarily a psychological response to the situation. For example, where odours are associated with a local industry that is considered to be important for the wellbeing of the local community and the industry maintains a good relationship with community members, then adaptation to the odour effects can occur over time.
Recommendation 1
Odour assessments should take into account:
2.5 Classification of odour effects as chronic and acute
Objectionable and offensive effects from odour can occur from low-intensity, moderately unpleasant odours occurring frequently over a long period, or from high-intensity, highly unpleasant odours occurring infrequently. These effects relate to different combinations of the FIDOL factors and can be termed 'chronic' and 'acute' effects respectively. It is useful to know what type of effect predominates, although odour effects will often result from a combination of acute and chronic odours. Knowledge of the predominant effect is useful for discussing and selecting the appropriate tools to assess and mitigate odours. Chronic and acute effects are encompassed in the definition of 'effect' under the RMA, which refers to temporary, permanent and cumulative effects (section 3.3).
The most significant effects of odour emissions from processing and manufacturing industries will typically be chronic effects. The main odour discharge in processing and manufacturing is normally a continuous or semi-continuous emission, and the main emission sources are often controlled and quantifiable, but there may be a low-level residual odour present for much of the time. Cumulatively, the low-level odour may have an adverse effect even though no single odour event considered in isolation could reasonably be assessed as objectionable or offensive. For chronic odour effects a longer-term assessment of the frequency and character of odour impacts is required.
Acute odour effects are those that can be considered objectionable or offensive on a single occasion. Acute effects typically arise from abnormal or upset conditions such as a malfunctioning biofilter, or infrequent activities such as re-opening old areas of fill at a landfill site. Acute odour impacts are usually from highly variable and/or uncontrolled discharges and are typically very difficult to quantify. The significance of an effect or a potential effect will often depend on the management practices employed.
Recommendation 2
Odour assessments should consider whether the odour discharge is likely to cause chronic effects (low-intensity odour occurring frequently over a long period), or acute effects (high-intensity odour occurring infrequently), or both.
