The World Health Organisation (WHO) said it was high time Bangalore, Chennai, Mumbai and Kolkata followed New Delhi's way and switched to LNG (liquefied natural gas) as rising pollution levels posed a serious health hazard to these cities. The World Health Organisation (WHO) on Thursday said it was high time Bangalore, Chennai, Mumbai and Kolkata followed New Delhi's way and switched to LNG (liquefied natural gas) as rising pollution levels posed a serious health hazard to these cities.