Hong Kong's air pollution index shot to its highest mark ever on Monday, as the southern metropolis tasted the scourge of northern sandstorms.

The Air Pollution Index (API) in the city, shrouded in a yellow-tinted smog, started to climb early in the morning. By 6 pm, an air pollution index of 500 was recorded in five areas - residential, commercial and rural - across the territories.

Hong Kong's API ranges from 0 to 500 and the air pollution is considered "severe" when the figure exceeds 200.

The environmental protection department said the high APIs are the result of the Northern China sandstorms moving southward with the northeast monsoon.

No improvement is expected until later this afternoon, when the easterly air stream affecting Hong Kong will weaken and a southeast wind will start to reign, said Hilda Lam, assistant director of the Hong Kong Observatory.

The Senior Citizens Home Safety Association, a charitable organization that runs a 24-hour personal emergency service for the elderly, said that, as of 6 pm on Monday, 112 people who contacted them needed to be taken to the hospital and 27 suffered from shortness of breath.

A 4-year-old boy was also taken to the hospital after experiencing shortness of breath.

Yu Wai-cho, a consultant physician in the department of medicine and geriatrics at the Princess Margaret Hospital, said wearing a surgical mask has little effect in blocking fine airborne particles, which can travel deep into the lungs and affect even the healthy.

Cheng Sze-ling, an environmental affairs officer with Friends of the Earth in Hong Kong, said Monday's high APIs are more of a coincidence than a long-term trend.

"Hong Kong's air pollution is mainly caused by local power factories, motor vehicles and the factories in the Pearl River Delta," he said.