A foreign tourist died at a coffee shop in Hanoi, Vietnam, on Sunday after he inhaled laughing gas from “funky balloons.”

The 35-year-old man whose nationality was unknown was found unconscious on a couch at a coffee shop on Ly Thai To Street on March 10, VN Express reported. He reportedly had tried inhaling gas from balloons that were filled with nitrous oxide, commonly known as laughing gas.

About 30 minutes later, he died. The news of the incident first surfaced on a Facebook account, which was later confirmed by local police in Hoan Kiem district. However, authorities have yet to disclose the cause of death.

The coffee shop has since been locked as authorities investigate the cause of death. Forensic tests were ongoing, said police. They added that the victim was in the country on a vacation with his family.

Nitrous oxide is known to slow the responses of the brain and body. Side effects can include nausea, headaches, increased sleepiness and excessive sweating.