Hospitals are closing the windows in their subsidised B2 and C class wards to protect patients against the high level of particles in the air as the haze worsens. These wards are usually naturally ventilated, with windows open.

Mr T. K. Udairam, head of the Eastern Health Alliance, said Changi General Hospital (CGH) has closed all windows and is "blasting" air into the wards to create a positive pressure, with the air going out the through the doors.

At the National University Hospital (NUH), Mr Joe Sim, it's chief executive officer, said priority is being given to wards with the elderly, the young, and those with breathing and heart problems.

It has more than 100 mobile air conditioners which will be placed in these wards. "When we close all the windows, it becomes very stuffy. We're fortunate to have these mobile air conditioners," he told The Straits Times.