The World Health Organization is to conduct an examination into the risks of holding the Olympic Games in Brazil because of the Zika virus.
US Senator Jeanne Shaheen has asked WHO to examine whether the Rio Games could accelerate the mosquito-borne virus.
"The Olympics draw people from all over the globe. It's important we understand the health implications," said Shaheen.
On Wednesday, Northern Ireland golfer Rory McIlroy said his concerns about exposure to the virus had eased.
However, former Masters and US PGA champion Vijay Singh and Australia's Marc Leishman are golfers who have already announced their withdrawal from the Olympics because of worries relating to the virus, which is linked to brain defects in newborn babies.