Haze pollution reached hazardous levels in parts of Riau province, including the capital Pekanbaru, with visibility at 200 metres, said Sutopo Nugroho, a spokesman for the National Disaster Management Agency.
"Cases of respiratory infections and other illnesses such as pneumonia, asthma and eye and skin irritation are increasing," he said.
Satellite images showed more than 330 "hotspots" indicating fires in Riau on Monday but plans to use aircraft to drop water on Monday were cancelled because of low visibiility due to the thick smog, Nugroho said.
Police in Riau said they had arrested 28 people suspected of starting some of the fires in recent days.
Smoke from forest fires on Sumatra is an annual dry season hazard that often affects neighbouring Malaysia, Singapore and Thailand.