The impact of the various possible population scenarios on Singaporeans' quality of life, and the trade-offs between levels of in-migration, population density and wellbeing, was also discussed at a forum on Singapore's population conundrum yesterday. Centre for Sustainable Asian Cities director Malone-Lee Lai Choo commented that there is room to increase built-up land to 70 per cent and for the population to grow to 6.5 million. She also referred to the net density of Hong Kong's built up land of 26,000 persons per sq km. SMU Professor David Chan, however, was of the view that population density is a derived matrix, and there is "a need to be careful when using it to indicate something as multi-dimensional as quality of life".