Passenger cars are intricately designed products. Their impact on the environment is similarly intricate and complex. We are now moving decisively beyond the age when the dominant impact was exhausting burnt fossil fuel directly to the air. For modern internal combustion engine (ICE) vehicles, tailpipe emissions have been dramatically reduced even where fossil fuels are still combusted. Further reductions are potentially within reach with synthetic fuels and increased hybridisation.
In contrast, other emissions are increasing – not just relatively, but absolutely. Vehicles are becoming bigger and heavier, leading to greater manufacturing emissions, in-use tyre wear emissions and end-of-life disposal or recycling costs. Possibly the least understood trend among car buyers is how environmental concerns are impacting the air quality in the vehicle cabin as new and innovative materials, treatments, glues and fragrances are deployed. While tailpipe pollution entering the cabin has been studied by Emissions Analytics and others, the interaction of the volatile organic compounds (VOCs) from these new interior materials is complex. However, doing so is important, as these VOC can hang around in the confined space of the vehicle cabin to be inhaled, and can have impacts on aspects from comfort to human health.
The most obvious manifestation of this problem is bad smells in cars. While Western car buyers tend to like the ‘new car smell’, Asian buyers are less keen. Removing this new car smell has, therefore, been the focus of regulations in Japan, Korea and other countries. The reason for this is that due to different physical sensitivities to certain VOCs commonly found in car materials. But it is not just about the new car smell, as hundreds of VOCs are present in the cabin and they interact in unpredictable ways, which can generate unexpected ‘off odours’. As traditional materials are swapped for new ‘eco’ or other alternatives, or seat covers are treated with less toxic chemicals, or the vehicle is constructed with more glues rather than rivets, the challenge and risks of bad smells grows. Further, deliberate science is put by manufacturers into creating desirable, on-brand, odours.
Emissions Analytics is actively developing new methods to design, describe and manage the olfactory contours of vehicles on sale today, and to understand the interaction with the ventilation system to create maximum consumer comfort and minimise any health impacts. To achieve this, we have taken controlled samples of the air inside a wide range of vehicle cabins and then subjecting the samples to two-dimensional gas chromatography and time-of-flight mass spectrometry analysis to profile the VOCs present in depth. Our laboratory has been provided by Markes International and SepSolve Analytical. This method is significant as it allows almost complete separation of the VOCs present, unlike more basic methods that cannot separate the higher molecular weight compounds, which are often the most potentially deleterious. Even with good separation, identification of the compounds is a challenge as many are not present in the standard spectral libraries. To resolve this, Emissions Analytics has compiled its own specialist library.
Historically, this sort of odour analysis has been performed by highly trained human ‘noses’. This has the advantage of directly gaining the human experience, with all that complexity and subjectivity. This remains important as just knowing all the chemicals present does not necessarily mean the human experience can be predicted. However, where a bad smell is detected, the analytical method provides a way of diagnosing and resolving the problem in a way that a human cannot necessarily do.
It should also be recognised that off odours do not necessarily correlate with a negative health impact on the human occupants of a vehicle, nor does the absence of any bad smell guarantee there is no impact. Even where a known toxic chemical is found in a vehicle, it may not be at a concentration that causes an actual negative effect. Concentration and exposure time are important added dimensions. The levels of VOC exposures were studied in 2019 of taxi drivers in Barcelona – a group of vehicle users for whom prolonged exposures are an issue. Given these considerations, Emissions Analytics has been actively contributing to the CEN Workshop 103 that aims to standardise a method for measuring vehicle interior air quality, which will be published soon. Being able to measure the freshness of air in the cabin will make possible the estimation of VOC exposures.
Looking at the initial testing from a range of interiors, it becomes clear that the VOC soup differs significantly between different manufacturers and of vehicles of different ages. The table below summarises the findings, including a ‘toxicity potential’ rating. This is calculated by combining the concentrations measured by Emissions Analytics and information from the European Chemicals Agency database of compounds and their hazard statements. The rating is a unit-less measure designed for comparative assessment of vehicles. As can be seen, Vehicle #1 has the highest rating, almost four times the average of the five vehicles, and around 28 times greater than the lowest potential toxicity vehicle, Vehicle #4
With an average of over 800 compounds identified, this demonstrates how rich a mix the air in a car can be, and the complexity of the task to separate, identify and quantify them. The total concentration of organic compounds is split into four functional groups, from alcohol (generally the least problematic) to polycyclic aromatic hydrocarbons (PAHs) and nitro-aromatics (with the highest incidence of carcinogenic effects). The alkane and aromatics groups are generally the most prevalent and are often associated with solvents, glues and plastics used in vehicle construction. Vehicle #1 stands out in these respects, whereas Vehicle #3 – low in other respects – is relatively high in PAHs and nitro-aromatics.
Among these compounds, many are common between vehicles. Across these five cars, the most prevalent ten compounds, with their concentrations and a descriptions, are shown in the table below. These pumped samples were taken after the vehicle had been soaked overnight in controlled temperature conditions. The third compound, octanoic acid‚ 2-propenyl ester, is associated with a pineapple aroma in the vehicle.
Beyond these, the individual characteristics of a vehicle tend to be made of some specific, high-concentration compounds and then a long tail of low-concentrations ones. Taking the toxicity potential score, beyond the top ten common compounds above, the highest rated compound in each vehicle is shown below.
This confirms that Vehicle #1 has a greater potential issue than the other vehicles, but the approach demonstrates that the compounds causing this can be identified and their contribution quantified. In this way, the manufacturer can diagnose the problem and consider how to alter the construction of the vehicle to mitigate. This may require substituting materials or methods, but, in doing so, new interactions of chemical compounds need to be assessed as well.
In summary, with ICE vehicles achieving much lower tailpipe emissions and the increased uptake of hybrid and electric vehicles, the impact of non-exhaust emissions is of growing concern for human health and the environment. This means it is now important to obtain a comprehensive view of all possible sources of VOCs from vehicles, including emissions from materials, such as foam, carpeting and seat covers, as well as those generated through tyre wear.
The recent global push towards a circular economy has also meant that automotive manufacturers are being urged to improve the sustainability of their operations by increasing the use of recycled or renewable materials, such as innovative plant-derived plastics. Robust quality control is essential to ensure these novel products will not produce volatile emissions that could be considered harmful or malodourous. Therefore, is not just about vehicle design to create a particular aesthetic, but also for hazard reduction and risk management.
However, the sample complexity, as well as an ever-expanding list of compounds of concern, makes it a challenge for those responsible for performing sampling and analysis. This has led to a need for innovative new methods to be created for complete emissions characterisation. Emissions Analytics is leading through its new laboratory and contribution to standardising measurement methods, with the aim of reducing the overall environmental emissions of future vehicles.