The discovery of high levels of bacteria has led the World Expo in Japan's Osaka to suspend daily water shows and use of a shallow play pool, organizers said.
It comes after visitors also complained that swarms of tiny flying insects had invaded the vast waterfront site where Expo 2025 runs until mid-October.
Nearly six million people have visited exhibits from more than 160 countries, regions and organizations since it opened in April.
Although polls showed that public enthusiasm for the Expo was lukewarm before its opening, organizers say crowds have been growing, especially in recent weeks, said AFP.
But concerns were raised over environmental conditions at the reclaimed island site in Osaka Bay, which was once a landfill.
Organizers said Thursday that high levels of legionella bacteria had forced them to close an area with shallow water where visitors, including children, could cool off.
That followed a statement released Wednesday saying daily fountain shows with music and lights at an artificial pond had been suspended for the same reason.
They said they were cleaning the affected areas, adding that a decision would come on Friday on whether the shows could resume.
Days before the Expo opened, a level of methane gas high enough to potentially ignite a fire was detected at the site.
More recently, organizers sprayed insecticide to deter swarms of non-biting midges bothering guests.
Also known as a World's Fair, the Expo phenomenon, which brought the Eiffel Tower to Paris, began with London's 1851 Crystal Palace exhibition.
It is now held every five years in different global locations.
