Haze had been blanketing Dumai city in Riau for the past couple of days, touching a hazardous level, an official said on Wednesday.
Dumai Environmental Office head Bambang Suriyanto said an Air Pollution Standard Index (ISPU) board owned by a multinational company operating in the city showed air quality had reached 500 parts per minute (ppm) at one point.
"The data was recorded yesterday at 6 a.m. local time, when the haze was quite thick. Fortunately, the haze gradually diminished at noon," said Bambang.
"We have to yet receive data on the air quality this morning. We will inform the public when we do," he added.
Bambang advised residents to avoid outdoor activities.
According to him, the level of 500 ppm indicates that the air is filled with fine particles from remnants of land and forest fires.
"Fine dust in the air is very dangerous for people suffering from asthma and other respiratory disorders. The dust can also cause respiratory infections and heart conditions" said Bambang.
Joko Prabowo, head of the Dumai Forest Police's technical working unit, suspected the haze blanketing Dumai was from neighbouring areas.
"We haven't received any information about land fires reported at the call centre. Direct observations in the field have also not detected fires," Joko said.