Human perception o f indoor air quality involves responses from the common chemical sense, the olfactory sense and responses to the hygro-thermal environment. The preceding discussion o f the human perception o f indoor air has the following points which are particularly important to the assessment o f indoor air quality in buildings;
1. Odour detection and response is very complex and very variable between
people and between methods used for testing, even in laboratory situations. Statistically based measurements have failed to produce precise rules to relate concentrations o f odorants or irritants and human perception with any real degree o f accuracy even in laboratory conditions. This questions the ability o f people to accurately measure levels o f indoor air quality in buildings.
2. Asking people to respond to a building’s indoor air quality requires that the
respondent understands what they are supposed to respond to i.e. odours, irritations, perceptions o f humidity and temperature etc. Asking people to judge, and therefore focus on, odour intensity, despite the variability in
results, is a well understood request.
3. Odour evaluations appear to depend on context. Ideally judges should be
‘blinded’ from the visual impact o f the building. It is difficult in occupant surveys o f indoor air quality to adopt this approach and respondents are therefore distracted by the visual impact.
4. The range effect will result in areas within a building o f relatively higher
odour levels possibly being judged as unacceptable when assessed against a general low level o f odour. Perhaps by increasing the background level o f odour the impact o f these pockets may be reduced as is the case with noise pollution.
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5. Adaptation to odours is complex both in the short and long term. In the
short term the sensitivity to odours changes very rapidly both rising and falling over several orders o f magnitude during the first few minutes. It is normally during this period that odour levels are measured by visitors. The exact time that measurements are taken is very difficult to control when entering a building and as such these measurements will have considerable inaccuracies. Long term adaptation will effect occupant’s judgements o f odours who will be lesss sensitive to ambient odours and only respond to any new odours that occur during the working day. However, these occupants will effectively act as visitors at the beginning o f each new day and on returning to their workplace after a short break outside.
6. The sense o f smell appears to have evolved to protect humans from
dangerous airborne chemicals. The response therefore to unfamiliar odours is to discover its source and make a value judgement to its potential danger. However, indoor air quality surveys often seem to require a more sophisticate response from the respondents such as degrees o f acceptability which this sense has not been developed to assess.
7. An odorants’ quality (or perhaps its familiarity) seems to be an important
feature o f air quality judgements. Furthermore, dilution o f odours does not always improve their perceived pleasantness. Therefore, increased ‘fresh air’ ventilation rates will not necessarily disperse odours.
8. Odour judgements are always made against a background o f ‘noise’. Signal
detection theory explains that people will use different criterion to report the presence o f odours depending on different levels o f motivation. For example, reports o f false negative results during indoor air quality
evaluations may arise from motivational influences such as payment,
pressures to report something rather than nothing etc. The responses from a trained and remunerated panels may differ from those o f the occupants or other groups o f indoor air quality assessors. Each o f these groups have different motivations which will be reflected in their voting patterns.
C h a p te r 2 - H u m a n P e r c e p tio n o f In d o o r A ir Q u a lity an d O d o u r
9. Humans appear to be fairly insensitive to changes in odour intensity above
the threshold levels as compared to other stimuli i.e. a high Weber fraction. This may suggest that large increases in fresh air ventilation may not result in any perceptible change in indoor air quality. However, these large increases in ventilation rates will have a large energy penalty.
10. Knowledge o f the human response to irritants is not as developed as the
study o f olfaction. However, it has been established that irritation thresholds are several magnitudes higher than their olfactory equivalents. Furthermore individual levels o f VOCs typically found in buildings tend to be below even the odour detection levels which suggests that irritation is unlikely to occur in many buildings. However, there is some evidence that below threshold irritants may combine to produce a noticeable effect.
This Chapter has focused on the research and techniques used to measure the human perception o f odours, irritants and hygro-thermal factors. Particular attention has been paid to specific techniques that have been used in laboratories and in field studies to establish how people respond to the quality o f air with varying ventilation rates. Several studies have been carried out to determine the minimum outside ventilation rate necessary for an acceptable indoor air quality. These studies are the subject o f Chapter 3.
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