The five, all experienced runners, collapsed during a relatively easy 10-kilometre race on Sunday. They were treated and released from hospital the same day, the Hospital Authority said.

The incident follows the death of a runner who collapsed along with 21 other competitors in the Hong Kong marathon in February.

Sunday's race organiser Hong Kong Marathon Pro blamed the collapses on high pollution and searing 33-degree Celsius (91 Fahrenheit) temperatures.

"The air quality was very bad," company chairman Li Ho-tung was quoted as saying in the South China Morning Post. "It was tough to run on such a hot day."

The city's official pollution monitor index said smog levels were high but still below the 100-point danger mark when an automatic health warning against engaging in strenuous exercise kicks in.

However, green groups said the monitor was misleading.

"The pollution level is measured according to a base level of safe pollution set by the government -- that level is now outdated and far too high," Cheng Luk-ki of local environmentalist group Green Power told AFP.

"Just because the index is below 100 doesn't mean the air is safe to do vigorous exercise in," Cheng added. "We need a more realistic index standard."

Pollution has become a hot political issue in Hong Kong as smog levels have risen noticeably to often dangerous levels.

Poor air quality reduced visibility to less than one kilometer (about half a mile) on more than 50 days last year.

The government says the problem is mostly due to the industrialisation of southern China's nearby Pearl River Delta region.

Green groups blame local coal-burning power stations and diesel-powered buses.

All rights reserved. © 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.