The fire and rescue department said it received calls from residents at 1.30am and responded within 20 minutes with 19 firefighters and an ambulance, Berita Harian reported.

“However, the source, type and danger level of the odour have not yet been identified. Monitoring is ongoing, and air readings are being taken using gas detectors by the Hazmat unit.

“Whether the smell originated from the nearby river has also not been confirmed,” the department said in a statement.

Johor environment department director Famey Yusoff also said an investigation is ongoing at the affected locations.

Norhayuda Ahmad, 26, who lives on Jalan Sagu in Taman Daya, said she noticed the smell starting from midnight and it persisted until 2am.

According to her, the odour caused concern for her family, and they had to take her mother, who suffers from asthma, to her sister’s house in Bandar Seri Alam, Pasir Gudang.

“The smell was so strong it gave me a headache, but I was more worried about my mother because of her asthma.

“The smell lessened after 2am but we still don’t know the source. We hope the authorities will quickly identify the source and take action to prevent it from happening again,” she said.

Another resident, Roslan Basiran, 40, said he and his family began coughing after inhaling what seemed like gas.

He said the authorities monitored the area near his home shortly after the incident began.

“Previously, I heard about air pollution complaints from residents in Pasir Gudang, but now it’s happening in this residential area.

“I hope it doesn’t happen again as it could affect the health of the public,” he said.