A risk assessment is taking place at the Royal Belfast Hospital for Sick Children after the fungus Aspergillus was detected again.
Health chiefs closed the children’s haematology ward for over a month in December after "increased levels of Aspergillus" were found.
It is understood the infectious substance has again been discovered in the cancer and blood disease unit.
Aspergillus can cause breathing difficulties, high temperatures and weight loss in those whose immune systems are already weak but doesn't normally affect healthy people according to the NHS website. It is commonly found in damp buildings, dust and air conditioning systems.
A concerned mum said her child has been transferred elsewhere for treatment as a result of the situation on the haematology/oncology ward at RBHSC.
The woman, who asked to remain anonymous, told Belfast Live: "My family has been affected this time.
"Immunocompromised children are no longer being admitted to the ward and at a time of Covid-19, have to either travel to Dublin or over to the mainland for treatment.
"Thousands of pounds must have been spent trying to fix it in January... but it clearly wasn't fixed very well for it to be back within the air filter system.
"I find it totally unacceptable that newly diagnosed children and family, at such a shocking and devastating time of a cancer diagnosis, have to travel and even be separated from one of their parents."
A Belfast Trust spokesperson said they are carrying out a risk assessment which may lead to existing inpatients being relocated.
They added: "In April this year, a fungus suspected to be Aspergillus was detected in an area of the Children's Hospital.
"A risk assessment is taking place with a view to relocating patients if clinically appropriate.
"Clinical staff continue to work with patients and families in the Children's Hospital with regard to this issue and the Trust is working with a multi-agency team to address it.
"This is not being declared as an outbreak."
