Ammonia is used in transferring heat from the US segment on the ISS to space. Moderate levels of ammonia are not so dangerous, but exposure to high concentrations of it can be a health hazard.
"Experts have registered an ammonia leak outside the US segment of the ISS. We are speaking about the leak with the speed of some 700 grams [1.5 pounds] per year. But there is no threat to the ISS crew", the source said.
It also indicated that a similar situation occurred in 2017, but the leak was eliminated before the astronauts went into open space.
Currently, Russia cosmonauts Anatoly Ivanishin and Ivan Wagner, and NASA astronaut Christopher Cassidy are working at the ISS.