Twenty-two people were sent to hospital yesterday, two remaining in critical condition last night, after taking part in Hong Kong's biggest marathon amid the worst air pollution since September. Many of the record 40,000 runners complained the 'choking' air affected their performance in the 10th annual Standard Chartered Hong Kong Marathon, half-marathon and 10km events. One competitor, surnamed Chu, 33, is in a critical condition in Ruttonjee Hospital after collapsing near the finish in Wan Chai. Chu stopped breathing and was resuscitated at the scene before being taken to hospital. The second runner in critical condition collapsed in Tsing Yi. The man, surnamed Tsang, 53, is in Princess Margaret Hospital. The air pollution index reached a 'very high' 149 in Causeway Bay, 147 in Central and 134 in Mongkok yesterday morning. Gerry Kipling, who ran the half-marathon, said: 'These are the worst conditions I have ever seen.' Clear the Air chairwoman Annelise Connell said last night: 'The air pollution levels were so high ... there should have been a warning ... to assist the runners to decide if they should risk their health.' The Observatory said the poor air quality was due to insufficient ventilation in the city, which was surrounded by accumulated pollutants coming from the Pearl River Delta. But breezes improved conditions in the afternoon, the observatory said, and the air quality would further improve when a northeast monsoon arrived later this week. The Auxiliary Medical Service said there were 4,825 cases of cramp, nearly double last year's number. About one in eight runners was affected. A spokesman for the marathon organiser said it had reminded runners to undertake sufficient training and to warm up before the race. Hong Kong Medical Association president Choi Kin said: 'When people suddenly perform exercise that aims to test their limits, their bodies may not be able to cope.'