The Khoo Teck Puat Hospital (KTPH) is reporting a 30 per cent increase in patients with asthma this month at its emergency department.
It is also grouping its subsidised patients who are more at risk of unhealthy air, so that they can be in wards with all windows closed and have mechanical fans and air coolers, a spokesman said.
It is also limiting patients to two visitors each.
She said: "This is especially important for the public to note, as the windows in the ward areas are closed or nearly closed and thus having numerous visitors within the wards may compromise the level of comfort for our patients."
Over at the Changi General Hospital, four in five patients with respiratory problems are showing more severe symptoms than normal.
Dr Augustine Tee, head of respiratory medicine at Changi General Hospital had noticed this trend in patients turning up for their normal check-ups.
And this was before the PSI hit the hazardous heights of 371 at noon today.
He said some patients needed extra medicine, or higher doses of their regular drugs.
The hospitals emergency department had also been treating more people for breathing difficulties.