Bad air affects the western reaches of the city more seriously than other areas, a report by an environmental group has found - and it may deteriorate further in the next decade. Green Power urged the government to consider the health risks bad air poses when planning new-town developments. "You cannot consider large-scale developments without implementing mitigation and assessments of health risks to those who will move there," said Cheng Luk-ki, the group's head of scientific research and conservation. "That would be extremely irresponsible." Before planning more housing developments, the administration should investigate the causes of pollution in the area, the report said. In its first year of operation, the government's Air Quality Health Index (AQHI) recorded a total of 4,263 hours, or 177 days, of air that posed a "high risk or above" to health at the city's 12 general monitoring stations. The newly opened monitoring station at Tuen Mun recorded 532 hours of "high risk or above" last year, followed by Yuen Long and Tung Chung, with 515 and 459 hours, respectively. Tung Chung experienced the most number of hours of "serious" AQHI at 56, followed by Tuen Mun with 55 and Yuen Long with 52. The report also warned the situation may get worse in the next decade, with additional emissions coming from projects such as the airport's third runway and the Hong Kong-ZhuhaiMacau Bridge. It accused the government of "downplaying" anticipated pollution in new-town plans, such as the Tung Chung extension and new development areas in Hung Shui Kiu, Yuen Long South and Kam Tin South. A department spokesman said overall air quality over the past five years had in fact improved, but ozone pollution, as a result of higher regional pollution, had worsened. New-town projects will be carried out in accordance with environmental impact assessment regulations, he added. "Proponents of these development projects must propose the mitigation measures needed to achieve air quality objectives and to protect affected local public health," the spokesman said. The Environmental Protection Department replaced its old air pollution index with the AQHI at the end of 2013. The group acknowledged the two data sets could not be compared directly. The AQHI calculates hospital admission risks caused by air pollutants on an 11-tier scale of 1 to 10+, with each corresponding to one of five categories, ranging from "low", "moderate", "high", "very high" to "serious". A "serious" AQHI reading of 10 or 10+ triggers a warning from the department for people with heart disease or respiratory illnesses to avoid outdoor activity, especially in areas with heavy traffic.