This is Scientific American 60-Second Science. I'm Christopher Entagliata.

Air pollution causes millions of premature deaths around the world every year, according

to an estimate by the World Health Organization. And one of the most significant drivers of

air pollution in cities is the exhaust that comes out of tailpipes, from cars, trucks,

and buses. In an effort to tame its air pollution, the city of Hong Kong has deployed a system

that can sense when a highly polluting vehicle drives by.

There are more than 150 sites for the monitoring of vehicle emissions in Hong Kong, so that high

immediate vehicles can be identified quickly and you're forced for repair.

John Joe is an environmental engineer at the University of Technology Sydney, and he's co-author

on a new analysis of that program in the journal Science Advances. He says the system works like this.

Sensors at road level shoot beams of light across the road to a reflector. The sensors then

measure how much light is absorbed by pollutants and roughly calculate emissions from passing vehicles.

At the same time, the system's reading license plates, and it sends notices to big polluters.

Enforcement began in 2014. In the following four years, the program pulled 16,000 vehicles off

the road, mostly taxis but also private cars and buses. Government subsidies helped to fix up

96% of those vehicles and ensured they passed smog tests.

You know, hopefully from the publication of this research, we can reservoir this

about the effectiveness of getting the heavy polluting vehicles off the road.

The repairs cut pollutants like carbon monoxide and nitrogen oxides from the city's air,

and that spared people from breathing them in. But the repairs had another effect too.

By reducing the pollution from the tailpipe, where also simultaneously,

increased the performance of fuel combustion in vehicles. So this is indirectly contributing

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