The future of odour detection

Rick Gould investigates the development of electronic noses, which could help to detect odour pollution.

Environmental regulators often receive complaints about odours – but when they visit the site, it is common for the smell to have vanished into thin air. Even if an odour is obvious, it can be difficult to detect with an instrument. Some people might notice very little, while others may be visibly distressed. This is the challenge: odours can be as elusive as they are complex – and even harder to measure. Until relatively recently, legislators often classed odours as a nuisance rather than a pollutant. This, however, is now changing, as there is increasing evidence that odours can be harmful to human health, while the science of measuring odours is improving rapidly.

The most effective tool

Complaints about odours have risen in parallel with industrialisation, urban growth and intensive agriculture, but most environmental legislators and policymakers do not consider smells to be as harmful as other pollutants, even though evidence linking odours with potentially damaging effects has started to emerge.

Researchers in North Carolina, for example, found a correlation between complaints about odours from intensive pig-farms, levels of hydrogen sulphide in the air and spikes in blood pressure among the people reporting them.

Because of the way the brain processes olfactory data, monitoring odours is much more complex than measuring many chemical pollutants. A single smell can consist of hundreds of chemicals: more than 600 contribute to the aroma of coffee.

During an investigation commissioned by the Environment Agency, the consultancy RPS found that odorous substances from an intensive poultry-rearing installation included carboxylic acids, ketones, aldehydes, aliphatic hydrocarbons, aromatic hydrocarbons, alcohols and reduced sulphur compounds such as hydrogen sulphide.

Furthermore, scientists believe that people are capable of recognising at least 10,000 distinct aromas, although we do not know how the brain processes this information. Despite advances in measurement technology, there is currently no single instrument that can match humans’ capability for detecting multitudes of chemicals and combining their olfactory effects into a single pattern. This is why the nose is still the most effective tool we have for monitoring odour.

A standard measure

The European standards body CEN has published one standard reference method (SRM), EN 13725, for measuring odour. Unlike other SRMs for measuring pollutants, EN 13725’s core detection method is the human nose, rather than measuring instruments. It employs a technique called ‘dynamic olfactometry’: a trained panel of people will sniff samples of air, which are diluted in stages within a device called an olfactometer until only half the panel can detect an odour. This degree of dilution is designated as the threshold of detection. The technique became popular with environmental regulators worldwide from the 1970s, with many developing their own olfactometers.

The problem was that these varying approaches were not always comparable or reproducible; there was no traceable reference standard to calibrate olfactometers, for example. Uncertainties around measurement prompted the need for a unifying SRM for olfactometry – so, in the 1990s, CEN mandated a work programme to develop EN 13725, which was published in 2003 and has significantly improved olfactometry.

“The most important improvement was the definition of the standard European Odour Unit (OUE), making this unit traceable to mass,” explains Dr Ton Harreveld, convenor of the CEN working group responsible for EN 13725 and founder of consultancy Odournet. One OUE is defined as the odour threshold for 0.123 micrograms of n-butanol in a cubic metre.

“We also specified strict performance criteria in EN 13725, applied through ISO/IEC 17025 accreditation, together with independent checking through annual, interlaboratory proficiency testing. This approach has provided a measurement method to quantify odour emissions at a known uncertainty.”

EN 13725 is now applied internationally and is specified in the latest EC Best Available Techniques (BAT) Conclusions for industries where environmental odour is a significant risk. Even before the EU published these documents, regulators such as the Environment Agency specified the use of EN 13725 in permits, together with emission limit values in OUE.

After 10 years, CEN initiated a review of EN 13725. “We now have a lot of international experience with the standard, so we are revising it to improve it further,” Harreveld says. The improvements include better sampling procedures and a robust method to determine odour thresholds for reference materials in addition to n-butanol. In simple terms, the measurement uncertainty will be lower, meaning better monitoring and control. Meanwhile, researchers continue with the quest for the ‘Holy Grail’ – a portable, continuous and instant odour-monitoring instrument that can match the power of the human nose.

The birth of the e-nose

The term ‘electronic nose’ started to appear in scientific papers during the 1980s, when researchers began developing instruments to mimic mammalian noses. These typically employed sensors, such as conducting polymers or metal-oxide semiconductors (MOS). The sensors were designed to detect either classes of odorous chemicals, or single compounds. A company in Arizona, for example, developed an MOS detector with a gold film, specific to hydrogen sulphide (H2S).

E-noses have to be tuned and correlated with specific odorous substances and there have been notable successes with this technology, such as in the food, wine and perfume industries. Engineers have installed e-noses on a space station to detect leaks of hazardous gases, and have been deployed in an odour-monitoring network around the petrochemicals industry in Rotterdam, The Netherlands. However, e-noses are still limited when compared with the human nose, and there are no benchmarks or universal standards. To encourage innovation and credibility, CEN has mandated a working group to develop performance standards and test procedures for e-noses.

“The standard will prescribe a methodology for validating the odour indicator metrics produced by instrumental odour monitoring devices (IOMs), assessing to what degree these metrics are indicative for human odour perception,” explains Harreveld. The validation will focus on metrics for the presence, classification and strength of odours. “IOMs will be tested in the context of the application, using real odour mixtures as a signal, comparing the results with measurements using EN 13725.”

Technological developments in e-nose sensors have reached a plateau in recent years. “There is a lot of promising news, but claims are so far not verifiable,” Harreveld says. “There are some interesting advances using biological sensors in combination with electronics.” In other words, biomimicry. As EN 13725 has shown, nature currently knows best when monitoring odours.

Rick Gould MIEMA CEnv is writing in a personal capacity as a freelance journalist

Image credit: iStock