The API at 11am indicated moderate quality, but visibility has been reduced and there is a slight smell of the smoke.
Prai and Seberang Jaya recorded API readings of 88 and 94 respectively. Seberang Jaya had registered a reading of 64 at 5pm Sunday.
At Universiti Sains Malaysia (USM), the reading was 79, compared with 59 a day earlier.
According to the Meteorological Department, visibility in Penang was at 4km, at 2pm.
An API reading of between 0 and 50 is considered good; 51 to 100, moderate; 101 to 200, unhealthy; 201 to 300, very unhealthy; and 301 and above, hazardous.
A doctor here said five patients suffering from asthma had sought treatment from his clinic in Bayan Baru so far on Monday.
"This is a big recurrent rate since I have not seen them between one and two months.
"They are having breathing problems because of the haze," he said.