One of the other more likely health risks in the garage experiment was from asphyxiation due to high CO2 levels, arising from the engine combustion. The parallel in the vehicle cabin is elevated CO2 due to respiration of the occupants. Human harm tends to occur when concentrations exceed 15,000 ppm, although cognitive impairment can occur well below that, which might lead to reduced reaction times and increased accident risk. While the garage concentration reached 8,509 ppm after half an hour, concentrations inside the vehicle when tested on the road reached just 1,564 ppm after the same time, even with the ventilation system on the ‘recirculation’ mode. On fresh air mode, concentrations rose by an average of just 13% above the 417 ppm background. As with PN concentrations, there were big variations between vehicle models as to how fresh the air was kept on recirculation: CO2 increased by 103% in the best case and 275% in the worst.
Overall, therefore, the particle exposure inside the cabin is a bigger risk than when locked in a garage with an idling ICE vehicle of the current generation. While CO2 concentrations in the garage were higher than in the cabin, driving a vehicle is operating a complex, mobile machine and, therefore, even a modestly elevated level of CO2 could compromise safety. It should be noted that some relevant pollutants have not been studied here. Particle mass was not chosen due to the relatively low levels being emitted from modern tailpipes and entering the cabin even with low-quality filters and ventilation systems. Nitrogen dioxide (NO2) emissions are extremely low from gasoline vehicles – the dominant powertrain now – and concentrations in the cabin are also very low. A major area of focus in our future work is the role of volatile organic compounds (VOCs). These tend to be low from tailpipes, although some species can be highly toxic even in low concentrations. Inside the cabin, these VOCs arise mainly from interior materials, especially in hot conditions. Some mix of compounds, of varying potential toxicity, evaporate from seats, carpets, dashboards and other plastics. In short, the greatest risks in the cabin are PN, CO2 and VOCs, while in the garage it is PN, CO2 plus carbon monoxide (CO) for gasoline vehicles and NO2 for diesels.
Taking this complex area and turning into something that vehicle owners and buyers can use practically, the AIR Alliance this month is launching its Cabin AIR Index, based on CWA17934. The most immediate action that can be taken, rather than changing the vehicle itself, is to swap the filter in the ventilation system. Changing the filter regularly is important to avoid degradation, and then the choice of filter brand is important. The initial test results – comparing six different filters on the same vehicle – show that the best filter reduced the interior pollution almost three times more than the worst filter. Therefore, this simple component of typically around $40 in value, can make a significant difference in chronic pollution exposure in the cabin.
Of all the vehicles Emissions Analytics has ever tested, the Tesla Model X achieved the best cabin air quality rating, achieving PN concentrations more than 92% below outside levels. Both its bioweapon defence mode and its normal modes achieved excellent protection, thanks to a combination of HEPA (High Efficiency Particulate Air) filters. The downside of this approach is a large physical size (about 1.2 metres wide) and the relatively high replacement cost. The upgrade is around $500 currently. While originally only available on the Models S and X, since late 2021 it was also standard on the Model Y.
In summary, we have shown in previous newsletters that we are thinking about vehicle pollution in the wrong way now. New ICE vehicles emit almost no pollutants from the tailpipe, except CO2. To solve this decarbonisation challenge, we are moving to heavier electric vehicles, and in doing so are creating a tyre emissions problem that dominates anything from the tailpipe, as shown in a previous newsletter. In this newsletter, we have shown that being inside a vehicle can be more hazardous than being outside. In short, apart from replacing older vehicles as soon as possible, we should be concerned with non-exhaust and non-vehicular emissions rather than the tailpipe, focusing particularly on fine particles and VOCs from plastics and tyres. We have a good instinctive grasp of exterior air quality problems, but need to improve our understanding of interior pollution. Tesla is stealing a lead on the competition by acknowledging the issue of cabin air quality, and offering a practical solution today. Let us hope that other manufacturers follow, and the new CWA17934 standard can be used to prove their effectiveness.
