This introductory paper was presented at the Nose2008 conference. Later Ton commented me that he got contacted by the Italian Association of Chemical Engineering (AIDIC)  with a very short notice, so he had not much time left to prepare the paper.

Nonetheless, that morning, when the audience was hearing that speech about the history of a professional life and its relation with odours, it had the sense of being part of a very long history. A nice slice of this history belongs to Mr. Van Harreveld.

As we were hearing this speech, we realised that we were getting involved by the history of olfactometry itself, and as a part of it, we got inspired with this narration… At least I did!. So anyway, we enjoyed the rest of the day with other interesting papers on a very well organized conference. Meanwhile I made a note in my mind about reading the paper again.

I got to speak with Ton again later in Barcelona for the IWA meeting and I asked him for permission to publish this paper, not included, I guess due to its late birth, in the papers published by the organizers of the Nose2008. The permission was granted and here you have the paper.

 Now you can read it and enjoy it.

Carlos N. Diaz Jimenez

Are we there yet? Achievements and challenges in management of environmental odours

Anton Philip van Harreveld

Chairman of the Odournet group

ton@odournet.com

From a 30 year perspective as a consultant in odour management, the achievements in creating and improving tools for odour assessment and management are reviewed. The regulatory frameworks that have been implemented in various EU member states are discussed briefly. The main part of the lecture will touch on areas that still pose a challenge for achieving optimum odour management and regulation. The challenges identified concern sampling of odorous emissions, continuous monitoring of odour emissions, atmospheric modelling, accounting for the annoyance potential of odours, and the challenge of attaining uniform quality levels for odour assessment in the E

1. Introduction

It is a great honour and a privilege to be invited to present a reflection on the achievements and challenges for an audience of so many distinguished researchers and practitioners of management of environmental odours, at this International Conference on Environmental Odour Monitoring and Control, Nose 2008. I am very grateful to the organizers of the the Italian Association of Chemical Engineering for inviting me to present an appetizer to the more substantial programme ahead.

My involvement in this area started many years ago, just after graduating for a masters degree in environmental health in 1979, when I took on a temporary assignment to study odour and ammonia emissions from a mushroom substrate production facility in the South of the Netherlands. I must admit that I did it out of economic necessity rather than academic zeal, but it was an experience that has determined my professional life.

The mushroom compost facility was partly open, under a steel construction roof. It is very telling that the first suspicion that there was a problem with emissions to air arose when part of the roof collapsed, less then 5 years after being constructed. Upon inspection it became obvious that the steel bolts holding the structure had corroded away. Also, the residents over the nearby German border complained of bad odours. Tellingly, the Dutch residents did not complain as much, as they were familiar with the odour of a major employer in the area.

The study brought me into contact with the true pioneers of quantitative odour assessment for environmental purposes in the Netherlands. They were a small group consisting of researchers in psychometrics, and agricultural researchers who were starting to look into emissions from livestock operations.

These early experiences provided a steep learning curve in the application of techniques that were then available for the quantitative characterisation of odour emissions, their fate in the atmosphere and their potential impact on public health and wellbeing.

After struggling for a year to measure emissions and model their dispersion I reached some personal conclusions:

• Composting releases large amounts of ammonia, at this facility 10-15 kg/hour (!)
• Ammonia can quickly corrode away steel constructions
• Composting can be very smelly
• So smelly that people can smell a facility up to 15 km away
• When you are on the plant you smell ammonia, further away the ammonia smell is not present but another smell is.
• Measuring odours is not easy
• Odour researchers do not apply statistics as I had learned to do in environmental health studies
• There is a lot of room for improvement!

In the course of the project I had many frank discussions with the pioneers of odour measurement and assessment of impacts, and that led to more assignments. In short, I was hooked on smells. The six months project initiated a career of nearly 30 years of developing improved tools for odour management, while establishing a consultancy company, Odournet, that now employs over 40 specialists in odour assessment in 6 countries.

2. Achievements in methodology and QA/QC

In the early years the focus was mainly on the improvement of instruments. In 1980 Bill Gates introduced MS-DOS and it became obvious that the combination of an olfactometer and a computer would be an improvement. Also, surprisingly, it was then not normal practice to calibrate the dilutions produced by the olfactometers. Once we started doing that, the conclusion was, again, that the existing dilution systems in practice did not perform as well as we would have hoped.

With the support of a few loyal clients we set out to produce better olfactometers. With some inventor friends, like Mathijs Teunissen van Manen (hardware) and his brother Floris (software), we started hooking up computers with air controls. A breakthrough was a chip that was developed in the USA for Reagan’s Star Wars military programme, which could rotate mirrors with fantastic precision to aim deadly laser beams. In Amsterdam we used it to set precision needle valves at extremely repeatable positions. By 1987 the first fully automated olfactometer was born: olfaktomat!

It also collected panel responses and did the calculations, reducing the time needed for one measurement from 4 hours to 1 hour. Now, in 2008, we perform 4 measurements/hour

While we were in the lab, as true boffins, the society around us became more environmentally aware. The Dutch public was complaining in large numbers over noise and odour and the politics of the day picked this up as an area to legislate.

In 1984 regulations were introduced in Holland introducing air quality criteria for odour, based on measurement at source and modelling of exposure. Remarkably, one of the reasons to introduce quantitative regulations was that industry wanted clear, transparent criteria, so they would not have to answer endless court cases brought by complainants. The limits were quite strict, in current terms 0,5 ouE•m-3 as a 99,5 percentile for new installations and 98p for existing installations. The application of these regulations led to increased discussions over license applications. The volume of odour studies increased, and as a result many odour laboratories opened. At one point there were 12 odour laboratories in a country of then 14 million inhabitants, roughly one lab for every million citizens.

Odournet, reflecting market conditions, evolved from an informal bunch of inventors to a more professionally structured consultancy company, a move that was accelerated when Frans Vossen joined Odournet in Amsterdam in 1989.

With all these data being produced it became clear we had a problem. The results often differed considerably between laboratories, but also when the same laboratory repeated the measurements. After a first proficiency test in the Netherlands in 1987 the need for standardization became obvious. Existing German and American standards were not considered effective, as inter-laboratory test in Germany did not show great results either.

A technical working group was formed by the Netherlands Normalisation Institute, now NEN, and the long an often painful process of improvement an application of strict metrological statistics and consideration of uncertainty began. In hindsight, the group did not invent anything really new. In a 1980 publication Andrew Dravnieks identified all sources of the unacceptable uncertainty that we found in practice [Dravnieks, A., Jarke, F., 1980].

The NEN group involved the Netherlands Measurement institute, NMi, to establish a traceable n-butanol standard and organise inter-laboratory tests. These tests were used to assess the effectiveness of procedural and instrumental improvements. In the end, this confirmed the key issues, identified years earlier by Dravnieks:

• Dilution system design and materials
• Nose/olfactometers interface
• Schedule of presentation
• Panel selection
• Number of panelists
• Panel techniques/ rules of behaviour
• Data treatment

All these issues proved critical. In the end, the overall measurement uncertainty was defined in NVN2820 by introducing strict statistical criteria for precision under repeatability and reproducibility conditions.

The NVNM2820 standard was the first olfactometry standard that linked the odour unit to a reference stimulus: 20 ppb n-butanol. This way the odour unit became traceable to SI units for mass and volume. This important step was later adopted in the European EN13725 standard, where the odour unit was defined in a more comprehensive structure of definitions.

For a more detailed history of the standardisation of olfactometry in Europe, see reference Van Harreveld et al, 1998.

A key factor to ensure that the improvements benefited ‘the market’ was the requirement by the government that all laboratories needed accreditation according to ISO17025, audited by RvA/European Accreditation. That sounds normal now, but was a hotly contested challenge at the time. By 1994 about 10 laboratories obtained accreditation in the Netherlands. Part of the requirement of the Dutch accreditation body was participation in annual or biannual inter-laboratory tests, initially organised by the Netherlands Measurement Institute. Laboratories who did not comply had a big problem and were threatened by withdrawal of accreditation, which tends to focus the minds of lab managers.

The success of the Dutch standardisation was probably one of the motives behind forming a European standardisation working group, TC264/WG27 ‘Olfactometry’ in 1994. I learned a lot acting as convenor for this WG which in the end achieved consensus and drafted what is now EN13725:2003 Air Quality - Determination of odour concentration be dynamic olfactometry. This international standard replaced all national standards for olfactometry in the EU, and also found wide acceptance worldwide.

Aspects of sampling were not comprehensively covered in the NVN2820 and neither in the EN13725. In the Netherlands the consensus of the practitioners was compiled in a document ‘Measuring and Calculating Odours, of 1994. This was useful, but far from a rigorous and validated standard. A German VDI working group is now producing a draft standard on odour sampling, VDI3880. Validation through inter-laboratory testing has not occurred yet, although we know that considerable uncertainty can be introduced when sampling odours, especially when practiced on non-ducted sources.

Field measurements, for direct measurement of exposure levels, were developed in Germany, as the main reference method for odour policy. The VDI3940 standard describes this method which is now widely applied, not only in Germany. The method has been extensively validated and provides a very useful tool in those cases where a quantitative assessment of exposure based in direct observation is required. The great advantage of the method is its essential simplicity. Using observers smelling in the field is a testimony that appeals specifically to the legal community.

In Belgium and the Netherlands field observations have been developed for estimating emissions based on determining the extent of the perceivable odour plume downwind, in combination with reverse modelling. In Belgium a regulatory framework based on these plume field measurements is advancing.

The field measurements are currently the objective of a CEN European working group, CEN/TC264/WG27, convened by Mr Jürgen Kost, that will meet for its fourth meeting in Barcelona in October this year.

3. Regulatory developments

The activity in methodology development and standardisation was matched by regulatory developments in an ever expanding number of jurisdictions in Europe and in a number of countries outside Europe. To mention but a few examples:

• Germany • Netherlands

• France • United Kingdom

• Spain • Belgium

• Italy • Poland

Many of these regulations set some sort of air quality criterion for odour exposure, based on frequency of occurrence of perceptible odours or modelled concentrations that are associated with levels of annoyance in the resident population.

These criteria should be based, like any environmental health air quality standard, on sound epidemiological data in the form of dose-effect studies. Such studies were undertaken, but too few and too long ago. They were mainly carried out in the Netherlands, in the period 1984 to 1996[Miedema e.a, 2000]. Many criteria have been based on theoretical considerations and discussion that were, in my personal opinion, too much dominated by discussions on aspects of atmospheric modelling rather than ‘people data’ observed in the actual world.

The real world, however, speaks loudly and clearly when odour annoyance is an issue when undesirable odours enter our living environment. At levels that are (just) identifiable as an unwanted odour, all Europeans tend to get upset, and a small but significant fraction of them will register complaints with the local, regional or national authorities. We should not forget that avoiding complaints is the core issue of environmental odour management.

Odour is identity, and odours can contribute in positive and negative ways to our awareness of our sensory environment. Odours can contribute to our identity, but also be an affront and challenge our intimacy. The link between intimacy and privacy and protection from unwanted odours was recently confirmed in a legal sense in 1994 by the European Court of Human Rights, recognising the failure of a local authority to protect the residential intimacy of a resident, Mrs Lopez Óstra, from intruding odours as an infraction of her human right of protection of intimacy in the private residence[EHRC, 1994]. The court considered that the government had failed to protect the citizen and hence her rights under art. 8 of the Convention of Rome, 1950:

• Art 8.

• Everyone has the right to respect for his private and family life, his home and his correspondence.
• There shall be no interference by a public authority with the exercise of this right except such as is in accordance with the law and is necessary in a democratic society in the interests of national security, public safety or the economic well-being of the country, for the prevention of disorder or crime, for the protection of health or morals, or for the protection of the rights and freedoms of others.

Noise and odour, of course, have much in common in terms of environmental management. Both are factors of environmental aesthetics. Both odour and noise can be fun but also can be a nuisance. Both can impact on public health through stress related mechanisms, rather than direct physiological health damage. Both can cause bitter and `persistent complaints and raise the temperature of local social conflict to boiling point.

It is therefore remarkable that the European Union has introduced wide ranging objectives, with the directives to match, to provide European citizens with a common minimum level of protection from unwanted noise. The European Noise Directive 2002/49/EC of 25 June 2002 sets well defined objectives, a standardised set of models and tools is being developed ( see http://www.harmonoise.org ) and all cities and infrastructures need to be odour mapped, with action plans to follow.

Considering that odour generates complaints in equal proportion to noise, it is remarkable that the European Union has shied away from any EU wide environmental quality objective for odour. The only European policy that is relevant for odour management is the IPPC directive[EU Directive 2008/1/EC]. It is telling that the IPPC directive itself failed to mention the word odour. In subsequent implementation documents this was corrected, and it became clear that IPPC regulated sectors of industry did have to consider the impacts of odours specifically for the IPPC licensing process. However, the treatment of odour assessment and guidelines for odour control in the IPPC Technical Reference documents (BREF’s) remains very poor indeed. Odour is still a neglected sensory environmental impact in the environmental policies of the EU, while obviously relevant to the implementation of major EU initiatives in the management of solid waste and wastewater treatment.

So, what has been achieved in the 30 years I have been watching environmental odour management?

A lot. We can now:

• Measure odour with known uncertainty, in accredited laboratories
• Observe odour exposure in the field with a recognised methodology
• Assess levels of annoyance in a population sample using questionnaires
• Determine dose effect relations
• Design and build processes with adequate containment and extraction of odorous vapours
• Treat odour emission with an ever expanding range of odour abatement techniques

But does that mean we have the issue of assessment and management of environmental odours sorted? Not quite. There are some considerable challenges ahead.

3. Challenges

In a way it is good news: A lot of work remains to be done by our community of odour specialists. Some of the challenges are outlined below.

3.1 Sampling of odour emissions

Hopefully the introduction of the German VDI3880 standard, expected in 2008, will be an impulse to improve and expand the clauses on sampling in the EN13725 standard in a next revision.

Tomorrow my distinguished colleague, Professor Frechen, will speak on this issue in more detail in his plenary presentation. However, before moving on to other challenges I would like to mention the key issues waiting to be resolved.

Sampling of area sources is still an area without proper validation of methods. Although an excellent recent review paper would indicate that the wind tunnels, which sweep the sampled surface with clean air, rather than passive sampling devices, appear to be the better choice^{[Hudson e.a., 2007a 2007b]} we need more data. In particular research to determine:

• The optimum sweep rate, to represent open air conditions
• Optimum dimensions and aerodynamic properties, to represent open air conditions
• The relation between measured specific area emission rates and ‘open top’ real life emissions
• The relation between area source emission rate and meteorological conditions
• The relation between area source emission rate and liquid/soil conditions, even for as obvious a parameter as temperature.

Validation of a standard method in relation to ‘open top’ real world emissions is of vital importance for any future standardised method. This should even include an uncertainty estimate for this sampling method.

And of course, for implementation, it is vital to introduce accreditation with the essential QA/QC obtained by regular inter-laboratory tests by practitioners.

Such proficiency tests are still mission for ducted odour emissions as well, incidentally.

3.2 Continuous monitoring of odour emissions.

For many years the promise of the electronic nose has been presented as if it was an operational reality. However, the continuous monitor for odour emissions is not an everyday reality in the odour consultant’s toolbox. Nor is the even more difficult application to measurement of ambient odours at just recognisable intensities, which are those relevant for impact.

Lack of sensitivity and cross sensitivity to irrelevant components as far as odour is concerned (moisture, methane) continue to confound the e-nose devices.

Progress has been made in production control, where e-noses can distinguish well defined and well rehearsed odours under well defined conditions. But for routine monitoring, we will not be buying one just yet.

3.3 Atmospheric dispersion modelling of odour impacts

Modelling did not appear to be the biggest source of uncertainty in odour management. Gaussian models appeared to work in the conditions found in Northern Europe, where the wind blows from all directions, and calms are rare. We learned to live with the results and experience taught us how to interpret results.

However, when one moves to the South of Europe, things change. The weather is certainly more pleasant, but the downside is that the inherent faults of Gaussian models become more prominent. The problems are:

• Low wind speeds and calms are far more prevalent. This is a significant problem, as Gaussian models do not model wind speeds of less than 0,5 m/s correctly. This means that in some cases 40% of the hours are modelled with an unsuitable model. This is all the more significant when considering that for odour studies the most unfavourable 1% or 2% of hours determine the location of the odour contours reflecting commonly used air quality criteria for odours.
• Wind directions are less uniformly distributed, because of land/sea of valley flow patterns. Often this means that specific conditions may occur at the same hours of the day, in a certain season. This may lead to increased exposure to odours at times when the population receptors are typically less tolerant (start and end of the day).

The introduction of more advanced modelling, using puff and Lagrangian models is therefore of the greatest urgency. These models are available, but very often the complex meteorological data and localised predictive models to determine the 3 dimensional wind field is another matter.

Odour management practitioners need to become familiar with these new model results and their interpretation. The uncomfortable truth is that the relation of exposure, determined with these advanced models, with their effect in terms of levels of residential population annoyance will need to be reassessed, in epidemiological dose-effect studies.

3.4 Accounting for the annoyance potential of odours

In some jurisdictions the odour annoyance potential or ‘offensiveness’ is taken into account to determine more or less strict air quality criteria for odours.

The Netherlands are an example of this approach. Although ‘target values’ for specific sectors of industry are mentioned in the National Emissions Guidelines [NeR], it is in the end up to the authority granting the license to set the air quality criterion for odour. In many cases this is the local authority.

To do so, measurements of hedonic tone are carried out, according to a method according to Dutch standard NVN2818:2005. The concentration at which a certain scale value for hedonic tone is reached (H=-2) is then taken as a guide value for the 98th percentile. Although the practice is widely applied, there is no justification for this one on one linkage of values. Nor is there any information on the uncertainty of the method, which lacks validation. In my personal opinion the methodology is faulty, using very small panels of 6 members applying scaling without comparison to a reference. Often panel members of EN13725 measurements are used, which could be a significant problem considering the rather extraordinary sensory history and context of these people. After all, the tests are done in a laboratory where they are exposed regularly to mainly very unpleasant odours. And context is very important when testing appraisal of hedonic tone, an inherently subjective attribute.

In Germany, the impact of the type of odour has been considered using field measurements. The main conclusion was that only odours that were ‘consistently rated as pleasant’ had a significantly lessened odour annoyance potential, in terms of a different dose-response characteristic.
Later studies found that in rural areas, cow odours were rated as less offensive than pig and chicken odours.

However, in my personal experience, any odour that is unwanted and uncontrollable entering in the residential environment can cause annoyance and nuisance. Although the effect will be more pronounced when exposed to more intense odours, the appraisal process and the potential link to stress is triggered as soon as the odour can be perceived and identified. Maybe we should learn from our colleagues in noise studies: Keep it simple, just set a simple dB exposure limit without too much fuss over whether the neighbours are playing Mozart or Metallica.

Another factor is how to account for odours that are very similar in character to the background, such as biofilter residual odours that resemble soil odour background. Or odours that belong to the context of an area.

3.5 Attaining uniform quality levels for odour assessment in the EU

The infrastructure of an accepted EU standard for olfactometry, combined with a European accreditation regime and the availability of accredited inter-laboratory tests is in place. In principle these key elements of QA/QC should ensure that uniform levels of quality would be attained throughout the European Union.

However, there are concerns that the application of accreditation and performance requirements is not sufficiently uniform yet in all nations.
In some nations, such as Germany, there is still a confusing mix of state recognition of laboratories, which may or may not coincide with ISO1725 accreditation under the auspices of European Accreditation.

In addition, there is also concern that the importance of attaining the performance criteria of Accuracy and precision in proficiency tests is not considered a prerequisite for accreditation by all accreditation bodies.

In the 2005 proficiency test for olfactometry, organised by OLFAtec, Germany, only 12 laboratories participate in the expanded programme, which allows assessment of the lab performance against the EN13725 criteria. Of those 12, only 2 laboratories fully complied with the EN13725 performance criteria[Maxeiner, 2006].

In the most recent 2007 edition of the international proficiency test, 29 laboratories participated in the extended test, of which 12 fully complied with the EN13725 performance criteria[Maxeiner, 2007].

Although this result clearly confirms that the performance requirements can in principle be met, it also confirms that there are many laboratories who claim to work according to EN13725, under accreditation, while they can not demonstrate compliance with performance criteria that are key to this standard.

Regrettably only very few laboratories publish their results. I am pleased to say that the 3 laboratories of Odournet complied and the results aro on our website. At least 1 UK and 2 German laboratories complied. I must assume that at least 2 or 3 other Dutch labs complied, as their accreditation has not been withdrawn. So 8 or 9 labs are accounted for. That leaves 3 or 4 more unidentified labs that have passed. But how does this relate to accreditations?

On a total of 36 accredited laboratories in the EU, plus 6 in Australia, this outcome is simply not good enough. It means, for example, that more than half the German accredited laboratories have not demonstrated that they can attain the performance criteria of EN13725 in an international proficiency test. This poor result implies that the accreditation bodies in different member states appear to be applying different levels of requirement, which contravenes the basic principle underpinning the mutual recognition of accreditations by European Accreditation member organisations.

Let me put it in very clear terms: a laboratory for olfactometry that can not demonstrate its compliance with the performance requirements of EN13725 can not claim to be accredited to ISO17025 under a scope that claims a method in conformity to EN13725.

This issue will need to be addressed with some urgency, it is a challenge we can meet relatively easily.

4. Conclusion

In conclusion I can say that the past 30 years have seen the management of environmental odours evolve from a highly innovative, unknown area to a standard practice in assessment of environmental impacts.

Methodology and approach has been standardised in a number of key areas, in particular by the introduction of the olfactometry standard EN13725. Hopefully the CEN standard for field evaluations will contribute to reduce unwanted variation in direct exposure measurements using field panels.

Although the German VDI3880 is a step ahead in the area of standardising the practice of sampling odours, there is still a significant challenge ahead in validating and optimising area and fugitive source sampling.
Major challenges still exist.

Gaussian plume models clearly fall short of expectations in locations with prevalent calms and low wind speeds, and areas with complex topography, cold flows etc. A major advance in models is required. The modules are in principle available, but the wind field modelling required is still a bottleneck. Once these barriers have overcome, we the results need to be reassessed in epidemiological studies to determine the relation between the calculate dose and the level of resident population annoyance.

Dose effect studies are required in more jurisdictions to justify and validate air quality criteria for odour.

The way in which odour annoyance potential is accounted for in setting specific air quality criteria for industry sector odours is still largely unvalidated. Methods that are now being used and accepted lack justification and validation.

Laboratories for olfactometry are increasingly operating in a QA/QC framework according to ISO17025, claiming in their scope to work in conformity with EN13725. However, there is insufficient proof that the key performance requirements of EN13725 are indeed attained by accredited laboratories. The accreditation bodies must ensure that laboratories in all member states are all required top satisfy these criteria in international proficiency tests.

Much has been achieved. The fact that you are all here these days as a professional community discussing odour management is a visible testimony to that statement. I hope this meeting and the contacts between professionals will be fertile, and help to meet the challenges ahead.
Perhaps turning the remaining challenges into achievements will contribute to improving the balance in political efforts to reduce the impact of odours and noise to the number of complaints each of these environmental vectors generate among European citizens.

5. References

• Dravnieks, A., Jarke, F., (1980), Odor Threshold Measurement by Dynamic Olfactometry: Significant Operational Variables, Journal of the Air Pollution Control Association, December 1980, Air Pollution Control Association, Pittsburg, USA, dic 1980
• EHRC, (1994), Case 16798/90, Lopez Óstra vs. Government of Spain, European Court of Human Rights. http://cmiskp.echr.coe.int
• Maxeiner, Bjoern, (2006), Olfactometric Interlaboratory Comparison Test 2005, in: Proceedings of conference WEF/AWWA ODORS AND AIR EMISSIONS 2006, USA, may 2006
• EN 13725:2003 - Air Quality - Determination of odour concentration be dynamic olfactometry, CEN.
• EU Directive 2008/1/EC of the European Parliament and of the Council of 15 January 2008 concerning integrated pollution prevention and control (Codified version)
• Hudson, N., Ayoko, G.A., (2007a), Odour sampling 1: Physical chemistry considerations, Bioresource Technology 99 (2008) 3892-3992, Elsevier, , jun 2007
• Hudson, N., Ayoka, G.A., (2007b), Odour sampling. 2. Comparison of physical and aerodynamic characteristics of sampling devices: A review, Bioresource Technology 99 (2008) 3892-3992, Elsevier
• Maxeiner, Bjoern,(2007), Ringversuch Olfaktometrie 2007, Ringversuch zur dynamischen Olfaktometrie nach DIN EN 13725:2003, VDI-Berichte Nr. 1995, 2007, VI, Düsseldorf, ene 2007
• Miedema, H.M.E., Walpot, J.I., Vos, H., Steunenberg, C.F., (2000), Exposure-annoyance relationships for odour from industrial sources, Elsevier Atmospheric Environment 34 (2000) 2927-2936, Amsterdam
• NeR, The Netherlands Emission Guidelines (NeR) now available in English on internet, including industry specific BAT measures and exposure criteria for odour. Published by: Infomil. Availability: http://www.infomil.nl/aspx/get.aspx?xdl=/views/infomil/xdl/page&ItmIdt=164452&SitIdt=111&VarIdt=82
• NVN 2818:2005 Geurkwaliteit - Sensorische bepaling van de hedonische waarde van een geur met een olfactometers (Eng: Odour quality – Sensorial determination of the hedonic tone of an odour using an olfactometers), NEN, Delft
• NVN 2820, 1995. Luchtkwaliteit: Sensorische geurmetingen met een olfactometer. Netherlands Institute for Standardisation, Delft.
• Van Harreveld, A. Ph (Ton), Heeres, P., Harssema, H., (1998) A review of 20 years of standardisation of odour concentration measurement by dynamic olfactometry in Europe., J. Air & Waste Manage. Assoc. 49, 705-715, Pittsburg, USA, jun 1998
• VDI 3940:2006 - Measurement of odour impact by field inspection - Measurement of the impact frequency of recognizable odours - Grid measurement., Beuth Verlag.
• VROM, (1984) Lucht indicatief meerjaren progamma lucht 1985-1989 (English: Indicative
  Long-Term Programme for Air Quality 1985-1989), Minsitry VROM, The Hague,
  Netherlands, ISBN 90 12 04764 1