Hong Kong admits world's largest air purifier choked on debut

Red-faced Hong Kong officials admitted on Tuesday that a record-breaking air filter for a new bypass under the city broke down shortly after it began operating.

Hong Kong opened a long-awaited $4.6 billion tunnel under a 3.7 kilometre (2.3 mile) long section of the main island in late February with much fanfare.

The bypass was greeted by traffic-weary residents with relief and heralded as a technological marvel because of its state-of-the-art air filter system.

The government said it would remove at least 80 percent of harmful particulates and nitrogen dioxide using large fans which suck exhaust into air purification plants in three ventilation buildings along the tunnel.

It was touted as the largest of its kind in the world in terms of volume of air handled -- 5.4 million cubic metres of vehicle exhaust every hour.

But on Tuesday the Highways Department released a statement saying the system's eye-catching eastern filter -- a lemon-squeezer shaped tower which stands on the city's famous harbourside -- had ceased operations "since the end of February".

The statement said contractors discovered 15 fans in the filter system suffered "different degrees of damage including broken bolts and damaged blade edges".

Work to fix the damage was expected to be completed by late April.

The statement said two other filters were operating normally and that the incident had not affected air quality inside the tunnel.

Nonetheless the malfunction will be a source of embarrassment for an infrastructure project already beset by years of delays and cost overruns.

Air quality has decreased around the world in recent decades, especially in Asia where many cities have spent another winter choking under appalling smog levels.

According to the United Nations Environment Programme, some 92 percent of people in the Asia-Pacific region are exposed to levels of air pollution that pose a significant risk to health.

Hong Kongers have long complained about worsening air quality, which is blamed on increased traffic, the city's bustling port and pollution from the neighbouring industry-heavy regions on the Chinese mainland.