New science data confirms what travellers in Bangkok and other pollution-hit cities already notice: motor scooters with two-stroke engines are probably the dirtiest vehicles on the roads.
The findings imply that implementing more stringent restrictions on two-stroke scooters, as is already done in some Chinese cities, could improve air quality in many cities around the globe.
The smog that hangs over many of the world's biggest cities is caused by a mix of volatile organic compounds (VOCs) emitted from the exhausts of cars, trucks and motorcycles.
Globally, regulations for passenger cars and trucks are fairly stringent, but not for two-stroke scooters. There are projections that scooters will emit more VOCs than all other vehicles combined in Europe by 2020.
In a study published in Nature Communications, Andre Prevot and colleagues chemically analysed the emissions from the exhausts of European two-stroke scooters and showed the VOC emissions are on average 124 times higher from idling two-stroke scooters compared with those from other vehicles.
They estimated that despite representing a relatively small fraction of the vehicular fleet, two-stroke scooters could contribute between 60 and 90 per cent of all road-side primary organic aerosols in cities such as Bangkok, where scooter numbers are high.
China began banning scooters from a number of major cities as early as the late 1990s, which has led to significantly lower traffic-related aromatic emissions.