Emissions from two-stroke scooters may dominate vehicular pollution levels in major cities, according to a study published in Nature Communications. The findings indicate that implementing more stringent restrictions on two-stroke scooters, as is already the case in some Chinese cities, could improve air quality in many cities around the globe.

The harmful clouds of smog that hang over many of the World’s largest cities are the result of vehicular emissions - a soup of volatile organic compounds (VOCs) pumped from the exhausts of cars, trucks and motorcycles. Although regulations for passenger cars and trucks are fairly stringent, those for two-stroke scooters are surprising less so, which is a major concern considering projections that scooters will emit more VOCs than all other vehicles combined in Europe by 2020.

Andre Prevot and colleagues chemically analyse the emissions from the exhausts of European two-stroke scooters and show that the VOC emissions are on average 124 times higher from idling two-stroke scooters compared with those from other vehicles, which places many of these scooters in the ‘super-polluting’ category. The team suggests that, despite representing a relatively small fraction of the vehicular fleet, two-stroke scooters could contribute between 60 and 90% of all road-side primary organic aerosols in cities, such as Bangkok, where scooter numbers are high.

China has already recognised the detrimental effect of two-stroke scooters and began banning them from a number of major cities as early as the late 1990s, which has led to significantly lower traffic-related aromatic emissions. The results of this study imply that the rest of the world could benefit from similar restrictions.