It has become an all too common sight in Tsim Sha Tsui East. Traffic comes to a standstill as buses stop to let a stream of tourists alight.

Outside the Mody Road DFS Galleria, which is popular with mainland shopping tours, bus drivers sit in their vehicles waiting for their passengers to finish their hours of shopping.

In addition to violating parking regulations, buses often idle their engines, which environmental groups say highlights weaknesses in the anti-idling law passed last December, which exempts any bus with at least one passenger on board.

Record-high pollution levels last week, which prompted the government to warn vulnerable people to stay indoors, have put a renewed focus on sources of pollution.

Roadside emissions are the second largest contributor to air pollution and pose a particularly dangerous threat since the exhaust comes out at street level, according to the Clean Air Network.

“But when the tour bus is idling, the bad air goes back into the bus. They think they’re breathing in clean air but they’re not.”

Tourists are more interested in getting back to the air-conditioned comfort of the buses than keeping the city’s air clean. Outside the DFS Galleria, tired shoppers often return to sit on their bus and the coaches’ engines idle for fifteen minutes or more.