Despite the suffocating heat, teachers and students at Good Counsel Catholic Primary School were determined to do their part to lessen global warming.

'It's the fourth time we've taken part in the scheme. By shutting off all the air conditioners in school, we hope to get the message across that everyone's effort makes a difference,' said Eric Cheung, an English teacher at the school.

Having endured the day's blistering temperatures, students from the Kowloon school made a beeline for the tuck shop as the bell rang to buy cold drinks and ice cream.

The school reminded students not to forget their no air-conditioning pledge.

'We appealed to parents and students to leave their air conditioners off for the night. We awarded a certificate to those who kept their pledge,' said Mr Cheung.

With students across the city getting used to air-conditioned classrooms, Footprint hopes the annual event will jolt students out of their environmental indifference and into action to help save the planet.

'Hong Kong people always run their air-conditioners at full throttle, more than overcompensating for the stifling heat.

'Our habit to stay cool is adding to excessive coal burning in the city's power-generating plants, worsening the greenhouse effect,' said Kwok Wing-sai, chairman of Footprint.

'With the city getting hotter, people crank up their air cons. More electricity and greenhouse gases are pumped into the air. This is a vicious cycle. By holding this annual event, we hope people can spare a thought about what's happening to the environment before turning on their air conditioners,' said Mr Kwok.