AIR quality in October was the worst since the Air Pollution Index started in June and is expected to deteriorate further this month. Industrial areas saw the October average Air Pollution Index climb to 50.8, compared to 48.4 the previous month and 35.8 in June. In densely populated urban areas such as Mongkok, the average rose to 46.1, compared with 42 in September. In areas of new development, such as Tai Po and Sha Tin, the average was 41.6, against the previous month's 35.9. Readings up to 50 reflect 'good' air quality, 50 to 100 are 'moderate', and readings which overshoot 100 are regarded as 'unhealthy', triggering warnings to refrain from exercising outdoors. Senior Environmental Protection Officer Pang Sik-wing is pessimistic about this month's forecast, saying: 'We would expect an average of more than 50 - possibly 60 or more. It will definitely be more than 50.' Air pollution readings this month have soared as high as 90, and the possibility that they may surpass 100 has not been ruled out. During the dry, sunny winter months, the low humidity means dust particles are no longer absorbed by droplets of moisture and 'dampened down'. Diesel-fuelled taxis and minibuses were the main air pollution culprits, Mr Pang said. Diesel exhaust creates sulphur dioxide in minute particles which are inhaled deep into the lung. However, October was a 'moderate' month for pollution, Mr Pang said, pointing to a tropical cyclone at the beginning of the month which meant there was more rainfall than expected.