Experts have warned of a threat from the quality of indoor air after two tons of garbage, including quite a number of dead rats and insects, was cleared out of the central air-conditioning system of a high-rise building, Shanghai Morning Post reports.

The air quality of Shanghai has been improving in recent years but the quality of air in high buildings has led to increasing concerns.

As people spend 70-90 per cent of their time indoors, the quality of air is crucial to people's health, according to Fan Cunyang, professor of Tongji University.

Shanghai has about 3,000 office buildings, of which only 15 get their central air-conditioning system cleaned regularly.

If the systems are not up to scratch, people breathing "aged air" for long will suffer many health problems, experts say.

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