Mumbai has 27,000 people per sq km

Mumbai is one of the densest cities in Asia with 27,000 people per square km, according to Asian Green City Index.

The city index study, commissioned by Siemens AG and Economist Intelligence Unit, analysed the environmental sustainability and climate protection of 22 major cities in Asia.

Indian cities taken up for the analysis were Mumbai, Kolkata, Delhi and Bangalore. The study found Singapore to be Asia's greenest metropolis.

The index examines the environmental performance of the 22 major cities in terms of energy and CO ₂, land use and buildings, transport, waste, water, sanitation, air quality and environmental governance. The EIU developed the methodology in cooperation with leading urban experts around the world.

The report said Kolkata benefits from a relatively low level of water consumption, at 138 litres per person a day – better than the average of 278 litres. The low rates might partly be explained by a lack of supply.

Delhi has an extraordinarily low per capita waste generation figure of 147 kg a year. This is attributed to Delhi's traditional culture of careful consumption which economic growth has not yet eroded. The city's advanced policies, including robust strategies to reduce, re-use and recycle waste, also demonstrate how much can be achieved with limited resources.

Bangalore has some of the lowest levels of CO ₂ emissions per capita – 61 per cent of the electricity is generated from mainly hydropower. The city's relatively low income, resulting in a less energy-intensive lifestyle, also plays a part in reducing CO ₂ emissions, as does the shift from heavy industry to IT-related businesses.

There is a variation in green spaces from two square metres per person in Kolkata, to 166 square metres per person in Guangzhou.

On concerns, the progressive rural exodus in Asia is unprecedented. According to the United Nations Population Division, the proportion of Asia's population living in cities has grown in the last 20 years by around a third to over 40 percent. In the last five years alone, the number of inhabitants in Asian cities has been increasing by about one lakh a day.

The increasing urbanisation is having an enormous impact on the infrastructure. The Asian Development Bank estimates that to cope with the influx, the Asian cities must for example build 20,000 new homes and 250 km of road and provide transportation infrastructure and an extra six million litres of drinking water, all on a daily basis, the study said.